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[[Image:IQ curve.svg|thumb|The IQs of a large enough population can be modeled with a [[Normal Distribution]].]]

An '''intelligence quotient''', or '''IQ''', is a score derived from one of several different [[standardized test]]s attempting to measure [[intelligence]]. The term "IQ," from the [[Germany|German]] ''Intelligenz-Quotient'', was coined by the German psychologist [[William Stern]] in 1912 as a proposed method of scoring early modern children's intelligence tests such as those developed by [[Alfred Binet]] and [[Théodore Simon]] in the early 20th Century.<ref>''i.e.'' as a [[quotient]] of "mental age" and "chronological age."</ref> Although the term "IQ" is still in common use, the scoring of modern IQ tests such as the [[Wechsler Adult Intelligence Scale]] is now based on a projection of the subject's [[Bell curve grading|measured rank]] on the [[Normal distribution|Gaussian bell]] curve with a center value (average IQ) of 100, and a [[standard deviation]] of 15, although different tests may have different standard deviations.

IQ scores have been shown to be associated with such factors as [[morbidity]] and [[mortality rate|mortality]],<ref>
{{Cite web
|url=http://www.jnnp.com/cgi/content/abstract/75/8/1100
|title= Premorbid cognitive testing predicts the onset of dementia and Alzheimer's disease better than and independently of APOE genotype
|dateformat=mdy |accessdate=August 6 2006
|year=2004
|author=Cervilla et al.
|publisher=Psychiatry 2004;75:1100-1106.
}}</ref> parental social status,<ref>
[http://www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdf Intelligence: Knowns and Unknowns] (Report of a Task Force established by the Board of Scientific Affairs of the American Psychological Association - Released August 7, 1995 -
A slightly edited version was published in the American Psychologist, Feb 1996. Official Journal of the APA)</ref> and to a substantial degree, parental IQ. While its inheritance has been investigated for nearly a century, controversy remains as to how much is inheritable, and the [[nature versus nurture|mechanisms of inheritance are still a matter of some debate]].<ref name=Devlin97>
{{cite journal
|author=Devlin B, Daniels M, Roeder K
|title=The heritability of IQ
|journal=Nature
|volume=388 |issue=6641 |pages=468–71
|year=1997
|pmid=9242404 |doi=10.1038/41319
|url=
}}<br>The same study suggests that the [[heritable]] component of IQ becomes more significant with age.</ref>

IQ scores are used in many contexts: as predictors of [[education]]al achievement or [[Mental_Retardation|special needs]], by social scientists who study the distribution of IQ scores in populations and the relationships between IQ score and other variables, and as predictors of job performance and [[income]].

The average IQ scores for many populations have been rising at an average rate of three points per decade since the early 20th century with most of the increase in the lower half of the IQ range: a phenomenon called the [[Flynn effect]]. It is disputed whether these changes in scores reflect real changes in intellectual abilities, or merely methodological problems with past or present testing.

== History ==

The modern IQ score is a mathematical transformation of a raw score on an IQ test, based on the rank of that score in a normalization sample.<ref>See: [[quantile]], [[percentile]], [[percentile rank]].</ref> Modern scores are sometimes referred to as "deviance IQ", while older method age-specific scores are referred to as "ratio IQ."

The two methodologies yield similar results near the middle of the bell curve, but the older ratio IQs yielded far higher scores for the intellectually [[gifted]]&mdash; for example, [[Marilyn vos Savant]], who appeared in the [[Guinness World Records|Guinness Book of World Records]], obtained a ratio IQ of 240. While this score could make sense using Binet's formula (and even then, only for a child), on the Gaussian curve model it would be an exceptional 7.9 standard deviations above the mean and hence virtually impossible in a population with a normal IQ distribution (see [[normal distribution]]). In addition, IQ tests like the Wechsler were not intended to discriminate reliably much beyond IQ 145, as [[ceiling effect]]s become a concern.

Since the publication of the Wechsler Adult Intelligence Scale (WAIS), almost all intelligence scales have adopted the normal distribution method of scoring. The use of the normal distribution scoring method makes the term "intelligence [[quotient]]" an inaccurate description, mathematically speaking, of the intelligence measurement, but "I.Q." still enjoys colloquial usage, and is used to describe all of the intelligence scales currently in use.

== Heritability ==
{{main|Inheritance of intelligence}}
The role of genes and environment (nature and nurture) in determining IQ is reviewed in Plomin ''et al.'' (2001, 2003).<ref name="Plomin0103">
Plomin ''et al.'' (2001, 2003)
</ref>{{Failed verification|date=May 2009}} Until recently heritability was mostly studied in children. Various studies find the heritability of IQ between 0.4 and 0.8 in the [[United States]];<ref>
{{cite journal
| author = R. Plomin, N. L. Pedersen, P. Lichtenstein and G. E. McClearn
| year = 1994
| month = May
| title = Variability and stability in cognitive abilities are largely genetic later in life
| journal = Behavior Genetics
| volume = 24
| issue = 3
| url = http://www.springerlink.com/content/t0844nw244473143/
| accessdate = 2006-08-06 | pages = 207
| doi = 10.1007/BF01067188 <!--Retrieved from CrossRef by DOI bot-->
}}</ref><ref name="Neisser95">
{{cite web
|url=http://www.lrainc.com/swtaboo/taboos/apa_01.html
|title=Intelligence: Knowns and Unknowns
|dateformat=mdy |accessdate=August 6 2006
|date=[[August 7]], [[1995]]
|author=Neisser ''et al.''
|publisher=Board of Scientific Affairs of the [[American Psychological Association]]
}}</ref><ref>
{{cite journal
|author=Bouchard TJ, Lykken DT, McGue M, Segal NL, Tellegen A
|title=Sources of human psychological differences: the Minnesota Study of Twins Reared Apart
|journal=Science
|volume=250 |issue=4978 |pages=223–228
|year=1990
|pmid=2218526 |doi=
|url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=2218526
}}</ref>
that is, depending on the study, a little less than half to substantially more than half of the variation in IQ among the children studied was due to variation in their genes. The remainder was thus due to environmental variation and measurement error. A heritability in the range of 0.4 to 0.8 implies that IQ is "substantially" heritable.

The effect of restriction of range on IQ was examined by Matt McGue and colleagues, who wrote that "restriction in range in parent disinhibitory psychopathology and family SES had no effect on adoptive-sibling correlations ... IQ."<ref name="McGue">
{{cite journal
|author=McGue, M. ''et al.''
|title=The Environments of Adopted and Non-adopted Youth: Evidence on Range Restriction From the Sibling Interaction and Behavior Study (SIBS)
|doi=10.1007/s10519-007-9142-7
}}
</ref> On the other hand, a 2003 study by Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, Irving I. Gottesman demonstrated that the proportions of IQ variance attributable to genes and environment vary with socioeconomic status. They found that in impoverished families, 60% of the variance in IQ "in a sample of 7-year-old twins" is accounted for by the shared environment, and the contribution of genes was close to zero.<ref>
{{cite journal
|author=Turkheimer E, Haley A, Waldron M, D'Onofrio B, Gottesman II
|title=Socioeconomic status modifies heritability of IQ in young children
|journal=Psychol Sci
|volume=14 |issue=6 |pages=623–628
|year=2003
|pmid=14629696 |doi= 10.1046/j.0956-7976.2003.psci_1475.x
|url=http://www.blackwell-synergy.com/openurl?genre=article&sid=nlm:pubmed&issn=0956-7976&date=2003&volume=14&issue=6&spage=623
}}</ref>

It is reasonable to expect that genetic influences on traits like IQ should become less important as one gains experiences with age. Surprisingly, the opposite occurs{{Request quotation|date=May 2009}}. Heritability measures in infancy are as low as 20%, around 40% in middle childhood, and as high as 80% in adulthood.<ref name="Plomin0103" />{{Failed verification|date=May 2009}} The ''American Psychological Association's'' 1995 task force on "Intelligence: Knowns and Unknowns" concluded that within the white population the heritability of IQ is "around .75." The ''Minnesota Study of Twins Reared Apart'', a multiyear study of 100 sets of reared-apart twins which was started in 1979, concluded that about 70% of the variance in IQ was found to be associated with genetic variation. Some of the correlation of IQs of twins may be a result of the effect of the maternal environment before birth, shedding some light on why IQ correlation between twins reared apart is so robust.<ref name=Devlin97/>
There are a number of points to consider when interpreting heritability:
* A high heritability does not mean that the environment has no effect on the development of a trait, or that learning is not involved. Vocabulary size, for example, is very substantially heritable (and highly correlated with general intelligence) although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions.<ref name="Neisser95" />
* A common error is to assume that because something is heritable it is necessarily unchangeable. As previously noted, heritable traits can depend on learning, and they may be subject to other environmental effects as well. The value of heritability can change if the distribution of environments (or genes) in the population is substantially altered. For example, an impoverished or suppressive environment could fail to support the development of a trait, and hence restrict individual variation. Differences in variation of heritability are found between developed and developing nations. This could affect estimates of heritability.<ref name="Neisser95" /> Another example is [[Phenylketonuria]] which previously caused mental retardation for everyone who had this genetic disorder. Today, this can be prevented by following a modified diet.
* On the other hand, there can be effective environmental changes that do not change heritability at all. If the environment relevant to a given trait improves in a way that affects all members of the population equally, the mean value of the trait will rise without any change in its heritability (because the differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase.<ref name="Neisser95" />
* Even in developed nations, high heritability of a trait within a given group has no necessary implications for the source of a difference between groups.<ref name="Neisser95" /><ref>
See: ''Ethnic Differences in Children's Intelligence Test Scores: Role of Economic Deprivation, Home Environment, and Maternal Characteristics''
</ref>

=== Environment ===
{{see also|Health and intelligence|environment and intelligence}}
Environmental factors play a role in determining IQ. Proper childhood [[nutrition]] appears critical for [[cognitive development]]; [[malnutrition]] can lower IQ.

A recent study found that the [[FADS2]] gene, along with [[breastfeeding]], adds about seven IQ points to those with the "C" version of the gene. Those with the "G" version of the FADS2 gene see no advantage.<ref>
[http://www.physorg.com/news113505546.html Gene governs IQ boost from breastfeeding]
</ref><ref name="pmid17984066">
{{cite journal
|author=Caspi A, Williams B, Kim-Cohen J, ''et al.''
|title=Moderation of breastfeeding effects on the IQ by genetic variation in fatty acid metabolism
|journal= Proceedings of the National Academy of Sciences
|volume=104 |issue= |pages=18860
|year=2007
|pmid=17984066 |doi=10.1073/pnas.0704292104
}}</ref>

[[Music]]al training in childhood also increases IQ.<ref>
Schellenberg, E. G. (2004). "Music lessons enhance IQ." Psychol Sci 15(8): 511-4.
</ref> Recent studies have shown that training in using one's [[working memory]] may increase IQ.<ref>
(Klingberg ''et al.,'' 2002)
</ref><ref>
{{cite journal
|author=Jaeggi SM, Buschkuehl M, Jonides J, Perrig WJ
|title=Improving fluid intelligence with training on working memory
|journal=Proc. Natl. Acad. Sci. U.S.A.
|volume=105 |issue=19 |pages=6829–6833
|year=2008
|pmid=18443283 |doi=10.1073/pnas.0801268105
|url=http://www.pnas.org/cgi/content/abstract/0801268105v1
}}</ref>

=== Family environment ===
In the developed world, [[Wiktionary:personality|personality]] traits in some studies show that, contrary to expectations, environmental effects actually can cause non-related children raised in the same family ("adoptive siblings") to be as different as children raised in different families.<ref name="Plomin0103"/>{{Failed verification|date=May 2009}}<ref>
Harris (1998)
</ref> There are some family effects on the IQ of children, accounting for up to a quarter of the variance, however, by adulthood this [[correlation]] approaches zero.<ref>
{{cite journal
|author=Bouchard TJ
|title=Genetic and environmental influences on adult intelligence and special mental abilities
|journal=Hum. Biol.
|volume=70 |issue=2 |pages=257–279
|year=1998
|pmid=9549239 |doi=
|url=
}}</ref> For IQ, adoption studies show that, after adolescence, adoptive siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: [[twin|monozygotic (identical) twins]] raised separately are highly similar in IQ (0.86), more so than [[Twin|dizygotic (fraternal) twins]] raised together (0.6) and much more than adoptive siblings (~0.0).<ref name="Plomin0103" />{{Failed verification|date=May 2009}}

=== Biased older studies? ===
Stoolmiller (1999)<ref name="Stoolmiller99">
{{cite journal
|author=Stoolmiller M
|title=Implications of the restricted range of family environments for estimates of heritability and nonshared environment in behavior-genetic adoption studies
|journal=Psychol Bull
|volume=125 |issue=4 |pages=392–409
|year=1999
|pmid=10414224 |doi= 10.1037/0033-2909.125.4.392
|url=http://content.apa.org/journals/bul/125/4/392
}}</ref> found that the range restriction of family environments that goes with adoption, that adopting families tend to be more similar on for example socio-economic status than the general population, means that the role of the shared family environment has been underestimated in previous studies. Corrections for range correction applied to adoption studies indicate that socio-economic status could account for as much as 50% of the variance in IQ.<ref name="Stoolmiller99" /> However, the effect of restriction of range on IQ for adoption studies was examined by Matt McGue and colleagues, who wrote that "restriction in range in parent disinhibitory psychopathology and family socio-economic status had no effect on adoptive-sibling correlations [in] IQ".<ref name="McGue"/>

Eric Turkheimer and colleagues (2003),<ref>
Eric Turkheimer and colleagues (2003)
</ref> not using an adoption study, included impoverished US families. Results demonstrated that the proportions of IQ variance attributable to genes and environment vary nonlinearly with socio-economic status. The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared family environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.<ref>
[http://www.blackwell-synergy.com/doi/abs/10.1046/j.0956-7976.2003.psci_1475.x?cookieSet=1 Socioeconomic status modifies heritability of iq in young children] Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, Irving I. Gottesman. Psychological Science 14 (6), 623–628. 2003
</ref> They suggest that the role of shared environmental factors may have been underestimated in older studies which often only studied affluent middle class families.<ref>
[http://www.connectforkids.org/node/516 New Thinking on Children, Poverty & IQ] November 10, 2003 Connect for Kids
</ref>

=== Maternal (fetal) environment ===
A meta-analysis, by Devlin and colleagues in ''[[Nature (journal)|Nature]]'' (1997),<ref name=Devlin97/> of 212 previous studies evaluated an alternative model for environmental influence and found that it fits the data better than the 'family-environments' model commonly used. The shared maternal (fetal) environment effects, often assumed to be negligible, account for 20% of covariance between twins and 5% between siblings, and the effects of genes are correspondingly reduced, with two measures of heritability being less than 50%.

Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different from previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports.<ref name="Bouchard02">
{{cite journal
|title=Genetic and environmental influences on human psychological differences
|author=Bouchard, T.J.; McGue, M.
|doi=10.1002/neu.10160
}}
</ref> They write that: <blockquote>Chipuer et al. and Loehlin conclude that the postnatal rather than the prenatal environment is most important. The Devlin et al. conclusion that the prenatal environment contributes to twin IQ similarity is especially remarkable given the existence of an extensive empirical literature on prenatal effects. Price (1950), in a comprehensive review published over 50 years ago, argued that almost all MZ twin prenatal effects produced differences rather than similarities. As of 1950 the literature on the topic was so large that the entire bibliography was not published. It was finally published in 1978 with an additional 260 references. At that time Price reiterated his earlier conclusion. Research subsequent to the 1978 review largely reinforces Price’s hypothesis.</blockquote>

=== The Dickens and Flynn model ===
Dickens and Flynn<ref name="DickensFlynn2001">
Dickens and Flynn (2001)
</ref> postulate that the arguments regarding the disappearance of the shared family environment should apply equally well to groups separated in time. This is contradicted by the [[Flynn effect]]. Changes here have happened too quickly to be explained by genetic heritable adaptation. This [[paradox]] can be explained by observing that the measure "heritability" includes both a direct effect of the [[genotype]] on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues (the model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects). The Flynn effect can be explained by a generally more stimulating environment for all people. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they taught children how to replicate outside the program the kinds of cognitively demanding experiences that produce IQ gains while they are in the program and motivate them to persist in that replication long after they have left the program.<ref name="DickensFlynn2001">
William T. Dickens and James R. Flynn, [http://www.apa.org/journals/features/rev1082346.pdf Heritability Estimates Versus Large Environmental Effects:The IQ Paradox Resolved], ''Psychological Review'' 2001. Vol. 108, No. 2. 346-369.
</ref><ref>
William T. Dickens and James R. Flynn, "[http://www.brookings.edu/views/papers/dickens/20020205.pdf The IQ Paradox: Still Resolved]," ''Psychological Review'' 109, no. 4 (2002).
</ref>

== IQ and the brain ==
{{main|Neuroscience and intelligence}}

In 2004, Richard Haier, professor of psychology in the Department of Pediatrics and colleagues at [[University of California, Irvine]] and the [[University of New Mexico]] used [[MRI]] to obtain structural images of the brain in 47 normal adults who also took standard IQ tests. The study demonstrated that general human intelligence appears to be based on the volume and location of [[gray matter]] tissue in the brain also demonstrated that, of the brain's gray matter, only about 6 percent appeared to be related to IQ.<ref>
{{cite web
|url=http://today.uci.edu/news/release_detail.asp?key=1187
|title=Human Intelligence Determined by Volume and Location of Gray...dick Matter Tissue in Brain
|dateformat=mdy |accessdate=August 6 2006
|date=July 19, 2004
|author=Richard Haier
|publisher=Brain Research Institute, UC Irvine College of Medicine
}}</ref>

Many different sources of information have converged on the view that the frontal lobes are critical for [[fluid intelligence]]. Patients with damage to the [[frontal lobe]] are impaired on fluid intelligence tests (Duncan et al. 1995). The volume of frontal grey (Thompson et al. 2001)<!--expand this reference and add <ref> --> and white matter (Schoenemann et al. 2005)<!--expand this reference and add <ref> --> have also been associated with general intelligence. In addition, recent neuroimaging studies have limited this association to the lateral prefrontal cortex. Duncan and colleagues (2000) showed using [[Positron Emission Tomography]] that problem-solving tasks that correlated more highly with IQ also activate the lateral [[prefrontal cortex]]. More recently, Gray and colleagues (2003) used functional magnetic resonance imaging (fMRI) to show that those individuals that were more adept at resisting distraction on a demanding working memory task had both a higher IQ and increased prefrontal activity. For an extensive review of this topic, see Gray and Thompson (2004).<ref>
{{cite web
|url=http://www.loni.ucla.edu/~thompson/PDF/nrn0604-GrayThompson.pdf
|format=PDF|title= Neurobiology of Intelligence: Science and Ethics
|accessdate=2006-08-06
|year=2004
|author=Jeremy R. Gray, Psychology Department, Yale University, and Paul M. Thompson, Laboratory of Nero Imaging, Department of Neurology, University of California, Los Angeles School of Medicine
|publisher=Nature Publishing Group, Volume 5
}}</ref>

A study involving 307 children (age between six to nineteen) measuring the size of brain structures using magnetic resonance imaging (MRI) and measuring verbal and non-verbal abilities has been conducted (Shaw et al. 2006). The study has indicated that there is a relationship between IQ and the structure of the cortex—the characteristic change being the group with the superior IQ scores starts with thinner cortex in the early age then becomes thicker than average by the late teens.<ref>
{{cite web
|url=http://www.bri.ucla.edu/bri_weekly/news_060330.asp
|title=Scans Show Different Growth for Intelligent Brains
|accessdate=2006-08-06
|year=2006
|author=Nicholas Wade
|publisher=Brain Research Institute, UCLA..
}}</ref>

There is "a highly significant association" between the [[CHRM2]] [[gene]] and intelligence according to a 2006 Dutch family study. The study concluded that there was an association between the CHRM2 gene on [[chromosome 7]] and Performance IQ, as measured by the Wechsler Adult Intelligence Scale-Revised. The Dutch family study used a sample of 667 individuals from 304 families.<ref>
{{cite journal
|author=Gosso MF, van Belzen M, de Geus EJ, ''et al.''
|title=Association between the CHRM2 gene and intelligence in a sample of 304 Dutch families
|journal=[[Genes, Brain and Behavior]]
|volume=5 |issue=8 |pages=577–584
|year=2006
|pmid=17081262 |doi=10.1111/j.1601-183X.2006.00211.x
|url=
}}</ref> A similar association was found independently in the ''Minnesota Twin and Family Study'' (Comings et al. 2003) and by the Department of Psychiatry at the Washington University.<ref>
{{cite journal
|author=Dick DM, Aliev F, Kramer J, ''et al.''
|title=Association of CHRM2 with IQ: converging evidence for a gene influencing intelligence
|journal=Behav. Genet.
|volume=37 |issue=2 |pages=265–272
|year=2007
|pmid=17160701 |doi=10.1007/s10519-006-9131-2
|url=
}}</ref>

Significant injuries isolated to one side of the brain, especially those occurring at a young age, may not significantly affect IQ.<ref>
{{cite journal
|author=Bava S, Ballantyne AO, Trauner DA
|title=Disparity of verbal and performance IQ following early bilateral brain damage
|journal=Cogn Behav Neurol
|volume=18 |issue=3 |pages=163–170
|year=2005
|pmid=16175020 |doi= 10.1097/01.wnn.0000178228.61938.3e
|url=http://meta.wkhealth.com/pt/pt-core/template-journal/lwwgateway/media/landingpage.htm?issn=1543-3633&volume=18&issue=3&spage=163
}}</ref>

Studies reach conflicting conclusions regarding the controversial idea that brain size correlates positively with IQ. Jensen and Reed claim no direct correlation exists in nonpathological subjects.<ref>
Reed, T.E., & Jensen, A.R. 1993. Cranial capacity: new Caucasian data and comments on Rushton's claimed Mongoloid-Caucasoid brain-size differences. Intelligence, 17, 423-431
</ref> A more recent meta-analysis suggests otherwise.<ref>
McDaniel, M.A. (2005) Big-brained people are smarter: A meta-analysis of the relationship between ''in vivo'' brain volume and intelligence. ''Intelligence, 33'', 337-346. [http://www.people.vcu.edu/~mamcdani/Big-Brained%20article.pdf PDF]
</ref>

An alternative approach has sought to link differences in neural plasticity with intelligence,<ref>
{{cite journal
|author=Garlick D
|title=Understanding the nature of the general factor of intelligence: the role of individual differences in neural plasticity as an explanatory mechanism
|journal=Psychol Rev
|volume=109 |issue=1 |pages=116–136
|year=2002
|pmid=11863034 |doi= 10.1037/0033-295X.109.1.116
|url=http://content.apa.org/journals/rev/109/1/116
}}</ref> and this view has recently received some empirical support.<ref>
{{cite journal
|author=Shaw P, Greenstein D, Lerch J, ''et al.''
|title=Intellectual ability and cortical development in children and adolescents
|journal=Nature
|volume=440 |issue=7084 |pages=676–679
|year=2006
|pmid=16572172 |doi=10.1038/nature04513
|url=}}</ref>

== Trends in IQ ==
{{main|Flynn effect}}

Since the twentieth century, IQ scores have increased at an average rate of around three IQ points per decade in most parts of the world.<ref>
(Flynn, 1999)
</ref> This phenomenon has been named the ''Flynn effect'' (aka the "Lynn-Flynn effect") named after [[Richard Lynn]] and [[James R. Flynn]]. Attempted explanations have included improved nutrition, a trend towards smaller families, better education, greater environmental complexity, and [[heterosis]]. Some factions believe that modern education has become more geared toward IQ tests, thus rendering higher scores, but not necessarily higher intelligence.<ref>
{{citation
|last=Mingroni |first=Michael A.
|title=Resolving the IQ Paradox: Heterosis as a Cause of the Flynn Effect and Other Trends
|year=2007
|pages=806–829
}}</ref> Tests are therefore renormalized occasionally to obtain mean scores of 100, for example [[Wechsler Intelligence Scale for Children|WISC]]-R (1974), WISC-III (1991) and WISC-IV (2003). This adjustment specifically addresses the variation over time, allowing us to compare scores from different times.

Some researchers argue that the Flynn effect may have ended in some developed nations, starting in the 1980s in the [[United Kingdom]]<ref>[http://www.telegraph.co.uk/education/educationnews/4548943/British-teenagers-have-lower-IQs-than-their-counterparts-did-30-years-ago.html British teenagers have lower IQs than their counterparts did 30 years ago]. ''The Telegraph''. Feb 7, 2009.</ref>, and in the mid-1990s in Denmark<ref>
Teasdale, Thomas W., and David R. Owen. (2005). "A long-term rise and recent decline in intelligence test performance: The Flynn Effect in reverse." ''Personality and Individual Differences.'' 39(4):837-843.
</ref> and in Norway.<ref>
{{cite web
|url=http://72.14.207.104/search?q=cache:sLUpdtOiKmoJ:www.missouri.edu/~aab2b3/LFE_GNXP/sdarticle.pdf+%2B%22Jon+Martin+Sundet%22+%2BIQ&hl=en&gl=us&ct=clnk&cd=9
|title=The end of the Flynn Effect. A study of secular trends in mean intelligence scores of Norwegian conscripts during half a century.
|accessdate=2006-08-06
|date=
|author=
|publisher=
}}</ref>

== Mutability ==
Though generally believed to be immutable, recent research suggests that certain mental activities can change the brain's raw ability to process information, leading to the conclusion that intelligence can be altered or changed over time. Studies into the neuroscience of animals indicate that challenging activities can produce changes in gene expression patterns of the brain. (Training Degus to Use Rakes <ref name=Okanoya>
{{cite journal
|author=Okanoya K, Tokimoto N, Kumazawa N, Hihara S, Iriki A
|title=Tool-use training in a species of rodent: the emergence of an optimal motor strategy and functional understanding
|journal=PLoS ONE
|volume=3 |issue=3 |pages=e1860
|year=2008
|pmid=18365015 |pmc=2268009 |doi=10.1371/journal.pone.0001860
|url=
}}</ref> and Iriki's earlier research with macaque monkeys indicating brain changes.)

A study on young adults published in April 2008 by a team from the Universities of Michigan and Bern supports the possibility of the transfer of fluid intelligence from specifically designed [[working memory]] training.<ref>
{{cite journal
|author=Jaeggi SM, Buschkuehl M, Jonides J, Perrig WJ
|title=Improving fluid intelligence with training on working memory
|journal=Proc. Natl. Acad. Sci. U.S.A.
|volume=105 |issue=19 |pages=6829–6833
|year=2008
|pmid=18443283 |doi=10.1073/pnas.0801268105
|url=
}}</ref>
Further research will be needed to determine nature, extent and duration of the proposed transfer:<ref>
{{cite journal
|author=R.J. Sternberg
|title=Increasing fluid intelligence is possible after all (Commentary)
|journal=Proc. Natl. Acad. Sci. U.S.A.
|volume=105
|issue=19
|pages=6791-6792
|year=2008
}}</ref>
Among other questions, it remains to be seen whether the results extend to other kinds of fluid intelligence tests than the matrix test used in the study, and if so, whether, after training, fluid intelligence measures retain their correlation with educational and occupational achievement or if the value of fluid intelligence for predicting performance on other tasks changes. It is also unclear whether the training is durable of extended periods of time.

== Group differences ==
Among the most controversial issues related to the study of intelligence is the observation that intelligence measures such as IQ scores vary between populations. While there is little scholarly debate about the ''existence'' of some of these differences, the ''reasons'' remain highly controversial both within academia and in the public sphere.

=== Health ===
{{main|Health and intelligence}}

Persons with a higher IQ have generally lower adult [[morbidity]] and [[mortality rate|mortality]]. [[Post-Traumatic Stress Disorder]],<ref name=Breslau>
{{cite journal
|author=Breslau N, Lucia VC, Alvarado GF
|title=Intelligence and other predisposing factors in exposure to trauma and posttraumatic stress disorder: a follow-up study at age 17 years
|journal=Arch. Gen. Psychiatry
|volume=63 |issue=11 |pages=1238–1245
|year=2006
|pmid=17088504 |doi=10.1001/archpsyc.63.11.1238
|url=
}}</ref> severe [[clinical depression|depression]]<ref>
{{cite journal
|author=Sackeim HA, Freeman J, McElhiney M, Coleman E, Prudic J, Devanand DP
|title=Effects of major depression on estimates of intelligence
|journal=J Clin Exp Neuropsychol
|volume=14 |issue=2 |pages=268–288
|year=1992
|pmid=1572949 |doi= 10.1080/01688639208402828
|url=http://www.ncbi.nlm.nih.gov/pubmed/1572949
}}</ref><ref name=Mandelli>
{{cite journal
|author=Mandelli L, Serretti A, Colombo C, ''et al.''
|title=Improvement of cognitive functioning in mood disorder patients with depressive symptomatic recovery during treatment: an exploratory analysis
|journal=Psychiatry Clin. Neurosci.
|volume=60 |issue=5 |pages=598–604
|year=2006
|pmid=16958944 |doi=10.1111/j.1440-1819.2006.01564.x
|url=http://www.blackwell-synergy.com/doi/abs/10.1111/j.1440-1819.2006.01564.x
}}</ref>
and [[schizophrenia]]<ref>
{{cite journal

|author=Zinkstok JR, de Wilde O, van Amelsvoort TA, Tanck MW, Baas F, Linszen DH
|title=Association between the DTNBP1 gene and intelligence: a case-control study in young patients with schizophrenia and related disorders and unaffected siblings
|journal=Behav Brain Funct
|volume=3 |issue= |pages=19
|year=2007
|pmid=17445278 |pmc=1864987 |doi=10.1186/1744-9081-3-19
|url=http://www.behavioralandbrainfunctions.com/content/3/1/19
}}</ref><ref>
{{cite journal
|author=Woodberry KA, Giuliano AJ, Seidman LJ
|title=Premorbid IQ in schizophrenia: a meta-analytic review
|journal=Am J Psychiatry
|volume=165 |issue=5 |pages=579–587
|year=2008
|pmid=18413704 |doi=10.1176/appi.ajp.2008.07081242
|url=http://www.ncbi.nlm.nih.gov/pubmed/18413704 }}</ref> are less prevalent in higher IQ bands.

A study of 11,282 individuals in [[Scotland]] who took intelligence tests at ages 7, 9 and 11 in the 1950s and 1960s, found an "inverse linear association" between childhood IQ scores and hospital admissions for injuries in adulthood. The association between childhood IQ and the risk of later injury remained even after accounting for factors such as the child's socioeconomic background.<ref>
{{Cite web
|url=http://www.ajph.org/cgi/content/abstract/AJPH.2005.080168v1
|title=Associations Between Childhood Intelligence and Hospital Admissions for Unintentional Injuries in Adulthood: The Aberdeen Children of the 1950s Cohort Study
|dateformat=mdy |accessdate=January 10 2007
|year=2006
|author= Debbie A. Lawlor, University of Bristol, Heather Clark, University of Aberdeen, David A. Leon, London School of Hygiene & Tropical Medicine
|publisher=American Journal of Public Health, December 2006
}}</ref>
Research in Scotland has also shown that a 15-point lower IQ meant people had a fifth less chance of living to 76, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long.<ref name=Whalley>
{{cite journal
|author=Whalley LJ, Deary IJ
|title=Longitudinal cohort study of childhood IQ and survival up to age 76
|journal=BMJ
|volume=322 |issue=7290 |pages=819
|year=2001
|pmid=11290633 |pmc=30556 |doi= 10.1136/bmj.322.7290.819
|url=http://bmj.com/cgi/pmidlookup?view=long&pmid=11290633
}}</ref>

A decrease in IQ has also been shown as an early predictor of late-onset [[Alzheimer's Disease]] and other forms of [[dementia]]. In a 2004 study, Cervilla and colleagues showed that tests of cognitive ability provide useful predictive information up to a decade before the onset of dementia.<ref>
{{cite journal
|author=Cervilla J, Prince M, Joels S, Lovestone S, Mann A
|title=Premorbid cognitive testing predicts the onset of dementia and Alzheimer's disease better than and independently of APOE genotype
|journal=J. Neurol. Neurosurg. Psychiatr.
|volume=75 |issue=8 |pages=1100–1106
|year=2004
|pmid=15258208 |pmc=1739178 |doi=10.1136/jnnp.2003.028076
|url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1739178
}}</ref> However, when diagnosing individuals with a higher level of cognitive ability, in this study those with IQs of 120 or more,<ref>
{{cite web
|url=http://laboratory-manager.advanceweb.com/common/editorial/editorial.aspx?CC=27318
|title=More Sensitive Test Norms Better Predict Who Might Develop Alzheimer's Disease
|accessdate=2006-08-06
|date=
|author=Dorene Rentz, Brigham and Women's Hospital's Department of Neurology and Harvard Medical School
|publisher=Neuropsychology, published by the American Psychological Association
}}</ref>
patients should not be diagnosed from the standard norm but from an adjusted high-IQ norm that measured changes against the individual's higher ability level. In 2000, Whalley and colleagues published a paper in the journal ''Neurology,'' which examined links between childhood mental ability and late-onset dementia. The study showed that mental ability scores were significantly lower in children who eventually developed late-onset dementia when compared with other children tested.<ref name=Whalley00>
{{cite journal
|author=Whalley LJ, Starr JM, Athawes R, Hunter D, Pattie A, Deary IJ
|title=Childhood mental ability and dementia
|journal=Neurology
|volume=55 |issue=10 |pages=1455–1459
|year=2000
|pmid=11094097 |doi=
|url=http://www.neurology.org/cgi/pmidlookup?view=long&pmid=11094097
}}</ref>

Several factors can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the [[blood-brain barrier]] is less effective. Such impairment may sometimes be permanent, or may sometimes be partially or wholly compensated for by later growth. Several harmful factors may also combine, possibly causing greater impairment.

Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring [[food fortification|fortification]] of certain food products and laws establishing safe levels of pollutants (e.g. lead, mercury, and organochlorides). Comprehensive policy recommendations targeting reduction of cognitive impairment in children have been proposed.<ref name="Olness">
{{cite journal
|author=Olness K
|title=Effects on brain development leading to cognitive impairment: a worldwide epidemic
|journal=J Dev Behav Pediatr
|volume=24 |issue=2 |pages=120–130
|year=2003
|pmid=12692458 |doi=
|url=http://meta.wkhealth.com/pt/pt-core/template-journal/lwwgateway/media/landingpage.htm?issn=0196-206X&volume=24&issue=2&spage=120
}}</ref>

In terms of the effect of one's intelligence on health, high childhood IQ correlates with one's chance of becoming a vegetarian in adulthood,<ref>
{{cite journal
|last=Gale
|first=CR
|title=IQ in childhood and vegetarianism in adulthood: 1970 British cohort study
|journal=British Journal of Medicine
|volume=334 |issue=7587 |pages=245
|year=
|pmid=17175567 |doi=10.1136/bmj.39030.675069.55
|url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1790759
}}
</ref> and inversely correlates with the chances of smoking,<ref>
{{cite journal
|last=Taylor
|first=MD
|title=Childhood IQ and social factors on smoking behaviour, lung function and smoking-related outcomes in adulthood: linking the Scottish Mental Survey 1932 and the Midspan studies
|journal=British Journal of Health Psychology
|volume=10 |issue=3 |pages=399–401
|year=
|doi=10.1348/135910705X25075
}}</ref> becoming obese, and having serious [[traumatic]] accidents in adulthood.

=== Sex ===
{{main|Sex and intelligence}}
Men and women have statistically significant differences in average scores on tests of particular abilities.<ref>
Douglas N. Jackson and J. Philippe Rushton, Males have greater g: Sex differences in general mental ability from 100,000 17- to 18-year-olds on the Scholastic Assessment Test, Intelligence, Volume 34, Issue 5, September-October 2006, Pages 479-486.
</ref><ref>
Lynn, R., & Irwing, P. (2004). Sex differences on the Progressive Matrices: A meta-analysis. Intelligence, 32, 481−498
</ref> Studies also illustrate consistently greater variance in the performance of men compared to that of women (i.e., men are more represented at the extremes of performance)<ref>
Deary, I.J., Irwing, P., Der, G., & Bates, T.C. (2007). "Brother–sister differences in the g factor in intelligence: Analysis of full, opposite-sex siblings from the NLSY1979." Intelligence, '''35'''(5): 451-456.
</ref>.

IQ tests are weighted on these sex differences so there is no bias on average in favor of one sex, however the consistent difference in variance is not removed. Because the tests are defined so there is no average difference it is difficult to put any meaning on a statement that one sex has a higher intelligence than the other. However some people have made claims like this even using unbiased IQ tests. For instance claims that men tend to outperform women on average by 3-4 IQ points based on tests of medical students where the greater variance of mens' IQ can be expected to contribute to the result,<ref>
{{cite journal
|author=Stumpf, H. and Jackson, D. N.
|title=Gender-related differences in cognitive abilities: evidence from a medical school admissions program
|journal=Personality and Individual Differences
|year=1994 |volume=17 |pages=335–344
|doi=10.1016/0191-8869(94)90281-X
}}</ref> or where a 'correction' is made for different maturation ages.<ref>
[http://www.rlynn.co.uk/pages/publications.asp Publications<!-- Bot generated title -->]
</ref>

=== Race ===
{{main|Race and intelligence}}

The 1996 Task Force investigation on Intelligence sponsored by the American Psychological Association concluded that there are significant variations in I.Q. across races.<ref name="Neisser95" /> The problem of determining the causes underlying this variation relates to the question of the contributions of "[[Nature versus nurture|nature and nurture]]" to I.Q. Most scientists believe there is insufficient [[data]] to resolve the contributions of heredity and environment. One of the most notable researchers arguing for a strong hereditary basis is [[Arthur Jensen]]. In contrast, [[Richard Nisbett]] , the long-time director of the Culture and Cognition program at the University of Michigan, argues that intelligence is a matter of environment and biased standards that praise a certain type of “intelligence” (success on standardized tests) over another.

In a recent editorial in the New York Times entitled, “All Brains Are the Same Color“, Dr. Nisbett argues against the hypothesis that IQ differences between blacks and whites are genetic. He notes that decades of research have not supported the assertion that one of the races in the [[United States]] is biologically inferior in terms of innate intelligence. Rather, he argues, “Whites showed better comprehension of sayings, better ability to recognize similarities and better facility with analogies — when solutions required knowledge of words and concepts that were more likely to be known to whites than to blacks. But when these kinds of reasoning were tested with words and concepts known equally well to blacks and whites, there were no differences. Within each race, prior knowledge predicted learning and reasoning, but between the races it was prior knowledge only that differed.”

== Positive correlations with IQ ==

While IQ is sometimes treated as an end unto itself, scholarly work on IQ focuses to a large extent on IQ's [[validity (psychometric)|validity]], that is, the degree to which IQ correlates with outcomes such as job performance, social pathologies, or academic achievement. Different IQ tests differ in their validity for various outcomes. Traditionally, correlation for IQ and outcomes is viewed as a means to also predict performance; however readers should distinguish between [[prediction]] in the [[hard sciences]] and the [[social sciences]].

=== Other tests ===
One study found a correlation of .82 between ''g'' ([[general intelligence factor]]) and [[SAT]] scores;<ref>[http://www.blackwell-synergy.com/links/doi/10.1111%2Fj.0956-797.2004.00687.x]</ref> another has found correlation of .81 between ''g'' and [[GCSE]] scores.<ref>[http://www.sciencedirect.com/science/article/B6W4M-4JDN6DP-1/2/850d67264b9588a28059387bca359ff7]</ref>

Correlations between IQ scores (general cognitive ability) and achievement test scores are reported to be .81 by Deary and colleagues, with the percentage of variance accounted for by general cognitive ability ranging "from 58.6% in Mathematics and 48% in English to 18.1% in Art and Design".<ref>
Ian J. Deary, Steve Strand, Pauline Smith and Cres Fernandes, Intelligence and educational achievement, Intelligence, Volume 35, Issue 1, January-February 2007, Pages 13-21.
</ref>

=== Job performance ===
According to Schmidt and Hunter, "for hiring employees without previous experience in the job the most valid predictor of future performance is general mental ability."<ref name="Schmidt98">
Schmidt, F. L. and Hunter, J. E. (1998). The validity and utility of selection methods in psychology: practical and theoretical implications of 85 years of research findings. Psychological Bulletin, 124, 262–274.
</ref> The validity depends on the type of job and varies across different studies, ranging from 0.2 to 0.6.<ref name="Hunter84">
Hunter, J. E. and Hunter, R. F. (1984). Validity and utility of alternative predictors of job performance. Psychological Bulletin, 96, 72–98.
</ref> However IQ mostly correlates with cognitive ability only if IQ scores are below average, making it less useful for predicting performance of higher scorers.<ref name="DiazAsper1">
{{cite journal
|author=Diaz-Asper CM, Schretlen DJ, Pearlson GD
|title=How well does IQ predict neuropsychological test performance in normal adults?
|journal=J Int Neuropsychol Soc
|volume=10 |issue=1 |pages=82–90
|year=2004
|pmid=14751010 |doi=10.1017/S1355617704101100
|url=
}}</ref> Also, IQ is related to the "academic tasks" (auditory and linguistic measures, memory tasks, academic achievement levels) and much less related to tasks where precise hand work ("motor functions") is required.<ref>
{{cite journal
|author=Warner MH, Ernst J, Townes BD, Peel J, Preston M
|title=Relationships between IQ and neuropsychological measures in neuropsychiatric populations: within-laboratory and cross-cultural replications using WAIS and WAIS-R
|journal=J Clin Exp Neuropsychol
|volume=9 |issue=5 |pages=545–562
|year=1987
|pmid=3667899 |doi= 10.1080/01688638708410768
|url=
}}</ref>
For highly qualified activities (research, management) high IQ scores are very relevant, whereas for less qualified activities, physical ability (body speed, hand-eye coordination) is more important.
According to Marley Watkins and colleagues, IQ is a causal influence on future academic achievement, whereas academic achievement does not substantially influence future IQ scores.<ref>
Marley W. Watkins, Pui-Wa Lei and Gary L. Canivez, Psychometric intelligence and achievement: A cross-lagged panel analysis, Intelligence, Volume 35, Issue 1, January-February 2007, Pages 59-68.
</ref> Treena Eileen Rohde and Lee Anne Thompson write that general cognitive ability but not specific ability scores predict academic achievement, with the exception that processing speed and spatial ability predict performance on the SAT math beyond the effect of general cognitive ability.<ref>
Treena Eileen Rohde and Lee Anne Thompson, Predicting academic achievement with cognitive ability, Intelligence, Volume 35, Issue 1, January-February 2007, Pages 83-92.
</ref>

The [[American Psychological Association]]'s report ''Intelligence: Knowns and Unknowns''<ref name="Neisser95" /> states that other individual characteristics such as interpersonal skills, aspects of personality, etcetera, are probably of equal or greater importance, but at this point we do not have equally reliable instruments to measure them <ref name="Neisser95" />, although, more recently, others argue that since most of professional tasks are now standardized or automated, and ranked IQ is a stable measurement over time with high correlation with many positive personal traits from the general population, it is the best tool to help determining the best hiring and job placement at any stage in a [[career]], independently of experience, personality bias or any formal training one may acquire.

=== Income ===
<!--
{| class="wikitable"
|-
|+ '''Relation between IQ and earnings in the U.S.'''
! IQ || <75 || 75&ndash;90 || 90&ndash;110 || 110&ndash;125 || >125
|-
| Age 18 || 2,000 || 5,000 || 8,000 || 8,000 || 21,000
|-
| Age 26 || 3,000 || 10,000 || 16,000 || 20,000 || 42,000
|-
| Age 32 || 5,000 || 12,400 || 20,000 || 27,000 || 48,000
|-
| colspan="6" | Values are the average earnings (1993 US Dollars) of each IQ sub-population.<ref>
Murray, C. (1997). IQ and economic success. Public Interest, 128, 21–35.
</ref>
|}
-->
Some researchers claim that "in economic terms it appears that the IQ score measures something with decreasing marginal value. It is important to have enough of it, but having lots and lots does not buy you that much."<ref>
Detterman and Daniel, 1989.
</ref><ref>
{{cite web
|url=http://www.americanscientist.org/template/AssetDetail/assetid/24538/page/4
|title=The Role of Intelligence in Modern Society
|pages=4 (Nonlinearities in Intelligence)
|dateformat=mdy|accessdate=August 6 2006
|datemonth=July|dateyear=1995
|author=Earl Hunt
|publisher=American Scientist
}}</ref>

Other studies show that ability and performance for jobs are linearly related, such that at all IQ levels, an increase in IQ translates into a concomitant increase in performance.<ref>
Coward, W.M. and Sackett, P.R. (1990). Linearity of ability-performance relationships: A reconfirmation. ''Journal of Applied Psychology,'' 75:297–300.
</ref> Charles Murray, coauthor of ''[[The Bell Curve]],'' found that IQ has a substantial effect on income independently of family background.<ref>
Murray, Charles (1998). Income Inequality and IQ, AEI Press [http://www.aei.org/docLib/20040302_book443.pdf PDF]
</ref>

The [[American Psychological Association]]'s report ''Intelligence: Knowns and Unknowns''<ref name="Neisser95" /> states that IQ scores account for about one-fourth of the social status variance and one-sixth of the income variance. Statistical controls for parental SES eliminate about a quarter of this predictive power. Psychometric intelligence appears as only one of a great many factors that influence social outcomes.<ref name="Neisser95" />

One reason why some studies claim that IQ only accounts for a sixth of the variation in income is because many studies are based on young adults (many of whom have not yet completed their education). On pg 568 of ''[[The g Factor]]'', [[Arthur Jensen]] claims that although the correlation between IQ and income averages a moderate 0.4 (one sixth or 16% of the variance), the relationship increases with age, and peaks at middle age when people have reached their maximum career potential. In the book, ''[[A Question of Intelligence]]'', [[Daniel Seligman]] cites an IQ income correlation of 0.5 (25% of the variance).

A 2002 study<ref>
[http://www.ingentaconnect.com/content/aea/jep/2002/00000016/00000003/art00001 The Inheritance of Inequality] Bowles, Samuel; Gintis, Herbert. The Journal of Economic Perspectives. Volume 16, Number 3, 1 August 2002, pp. 3-30(28)
</ref> further examined the impact of non-IQ factors on income and concluded that an offspring's inherited wealth, race, and schooling are more important as factors in determining income than IQ. For example, in 2004 African-American workers had the second-highest [[median]] earnings of American [[minority group]]s after Asian Americans<ref>{{cite web |url=http://www.census.gov/prod/2005pubs/acs-01.pdf |title=Incomes, Earnings, and Poverty from the 2004 American Community Survey |publisher=[[United States Census Bureau]]|month=August | year=2005 |dateformat=mdy |accessdate=October 24 2006 |format=PDF}}</ref> and among minority groups, only Asian Americans were more likely to hold [[white-collar worker|white-collar]] occupations (management, professional, and related fields) despite the significant IQ gap between African and Asian Americans.<ref>{{cite web |url=http://www.census.gov/prod/2003pubs/c2kbr-25.pdf|title=Occupations: 2000 |author=Peter Fronczek and Patricia Johnson |publisher=[[United States Census Bureau]] |month=August | year=2003 |dateformat=mdy |accessdate=October 24 2006 |format=PDF}}</ref>

=== Other correlations with IQ ===
<!--
{| class="wikitable"
|-
|+ '''Economic and social correlates of IQ'''
! Factors || Correlation
|-
| School grades and IQ || 0.5
|-
| Total years of education and IQ || 0.55
|-
| IQ and parental socioeconomic status || 0.33
|-
| Job performance and IQ || 0.54
|-
| Negative social outcomes and IQ || −0.2
|-
| IQs of identical twins || 0.86
|-
| IQs of husband and wife || 0.4
|-
| GDP per capita and average national IQ || 0.7
|}
-->
In addition, IQ and its correlation to health, [[violent crime]], [[gross state product]], and government effectiveness are the subject of a 2006 paper in the publication ''Intelligence''. The paper breaks down IQ averages by U.S. states using the federal government's [[National Assessment of Educational Progress]] math and reading test scores as a source.<ref>
{{cite web
|url=http://www.people.vcu.edu/~mamcdani/Publications/McDaniel%20(2006)%20Estimating%20state%20IQ.pdf
|format=PDF|title=Estimating state IQ: Measurement challenges and preliminary correlates
|accessmonthday= |accessyear=
|date=accepted for publication August 2006
|author= Michael A. McDaniel, Virginia Commonwealth University
|publisher=Intelligence
}}</ref>

There is a correlation of -0.19 between IQ scores and number of juvenile offences in a large Danish sample; with social class controlled, the correlation dropped to -0.17. Similarly, the correlations for most "negative outcome" variables are typically smaller than 0.20, which means that test scores are associated with less than 4% of their total variance. It is important to realize that the causal links between psychometric ability and social outcomes may be indirect. Children with poor scholastic performance may feel alienated. Consequently, they may be more likely to engage in delinquent behavior, compared to other children who do well.<ref name="Neisser95" />

IQ is also negatively correlated with [[Health and intelligence#Association with other diseases|certain diseases]].

Tambs ''et al.''<ref>
Tambs K, Sundet JM, Magnus P, Berg K. "Genetic and environmental contributions to the covariance between occupational status, educational attainment, and IQ: a study of twins." Behav Genet. 1989 Mar;19(2):209&ndash;22. PMID 2719624.
</ref> found that occupational status, educational attainment, and IQ are individually heritable; and further found that "genetic variance influencing educational attainment ... contributed approximately one-fourth of the genetic variance for occupational status and nearly half the genetic variance for IQ". In a sample of U.S. siblings, Rowe ''et al.''<ref>
Rowe, D. C., W. J. Vesterdal, and J. L. Rodgers, "The Bell Curve Revisited: How Genes and Shared Environment Mediate IQ-SES Associations," University of Arizona, 1997
</ref> report that the inequality in education and income was predominantly due to genes, with shared environmental factors playing a subordinate role.

== Public policy ==
{{globalize/USA}}
{{main|Intelligence and public policy}}

In the [[United States]], certain [[Public policy (law)|public policies]] and [[law]]s regarding military service,<ref>
{{cite web
|url=http://www.rand.org/pubs/technical_reports/2005/RAND_TR193.pdf
|format=PDF|title=RAND_TR193.pdf
}}</ref><ref>
{{cite web
|url=http://www.rand.org/pubs/monograph_reports/MR818/MR818.ch2.pdf
|format=PDF|title=MR818.ch2.pdf
}}</ref>
education, public benefits,<ref>
{{cite web
|url=http://www.ssa.gov/disability/professionals/bluebook/12.00-MentalDisorders-Adult.htm
|title=Social Security Administration
|accessmonthday=
|date=
|author=
|publisher=
}}</ref>
crime,<ref>
{{cite web
|url=http://www.isteve.com/2002_IQ_Supreme_Court_Death_Penalty.htm
|title=IQ Defenders Feel Vindicated by Supreme Court
|accessdate=2006-08-06
|date=Steve Sailer
|author=June 24, 2002
|publisher=UPI
}}</ref> and employment incorporate an individual's IQ or similar measurements into their decisions. However, in 1971, for the purpose of minimizing employment practices that disparately impacted racial minorities, the U.S. Supreme Court banned the use of IQ tests in employment, except in very rare cases<ref>
Nicholas Lemann. The IQ Meritocracy. Time 100 [http://www.time.com/time/time100/scientist/other/iq.html link]
</ref>. Internationally, certain public policies, such as improving nutrition and prohibiting [[neurotoxins]], have as one of their goals raising, or preventing a decline in, intelligence.

== Criticism and views ==
=== Binet ===
[[Alfred Binet]], a French psychologist, did not believe that IQ test scales qualified to measure intelligence. He neither invented the term "intelligence quotient" nor supported its numerical expression. He stated:

{{quote|The scale, properly speaking, does not permit the measure of intelligence, because intellectual qualities are not superposable, and therefore cannot be measured as linear surfaces are measured.|Binet, ''1905''}}

Binet had designed the Binet-Simon intelligence scale in order to identify students who needed special help in coping with the school curriculum. He argued that with proper remedial education programs, most students regardless of background could catch up and perform quite well in school. He did not believe that intelligence was a measurable fixed entity.

Binet cautioned:
{{quote|Some recent thinkers seem to have given their moral support to these deplorable verdicts by affirming that an individual's intelligence is a fixed quantity, a quantity that cannot be increased. We must protest and react against this brutal pessimism; we must try to demonstrate that it is founded on nothing.<ref>
Rawat, R. [http://www.rso.cornell.edu/scitech/archive/95sum/bell.html The Return of Determinism?] </ref>}}

=== The Mismeasure of Man ===
Some scientists dispute [[psychometrics]] entirely. In ''[[The Mismeasure of Man]]'', Harvard professor and paleontologist [[Stephen Jay Gould]] argued that intelligence tests were based on faulty assumptions and showed their history of being used as the basis for [[scientific racism]], although did not at any point attempt to scientifically refute intelligence tests. He wrote:

{{quote|…the abstraction of intelligence as a single entity, its location within the brain, its quantification as one number for each individual, and the use of these numbers to rank people in a single series of worthiness, invariably to find that oppressed and disadvantaged groups—races, classes, or sexes—are innately inferior and deserve their status.(pp. 24–25)}}

He spent much of the book criticizing the concept of IQ, including a historical discussion of how the IQ tests were created and a technical discussion of why [[General intelligence factor|''g'']] is simply a mathematical artifact. Later editions of the book included criticism of ''[[The Bell Curve]].''

Gould did not dispute the stability of test scores, nor the fact that they predict certain forms of achievement. He did argue, however, that to base a concept of intelligence on these test scores ''alone'' is to ignore many important aspects of mental ability.

=== Relation between IQ and intelligence ===
{{see also|Intelligence}}
According to Dr. C. George Boeree of [[Shippensburg University]], intelligence is a person's capacity to (1) acquire knowledge (i.e. learn and understand), (2) apply knowledge (solve problems), and (3) engage in abstract reasoning. It is the power of one's intellect, and as such is clearly a very important aspect of one's overall well-being. Psychologists have attempted to measure it for well over a century.

Several other ways of measuring intelligence have been proposed. [[Daniel Schacter]], [[Daniel Gilbert (psychologist)|Daniel Gilbert]], and others have moved beyond general intelligence and IQ as the sole means to describe intelligence.<ref>
''[http://select.nytimes.com/2007/09/14/opinion/14brooks.html The Waning of I.Q.]'' by [[David Brooks]], [[The New York Times]]
</ref>

=== Test bias ===
{{Seealso|Stereotype threat}}
The [[American Psychological Association]]'s report ''Intelligence: Knowns and Unknowns''<ref name="Neisser95" /> states that IQ tests as predictors of social achievement are not biased against people of African descent since they predict future performance, such as school achievement, similarly to the way they predict future performance for European descent.<ref name="Neisser95" />

However, IQ tests may well be biased when used in other situations. A 2005 study stated that "differential validity in prediction suggests that the WAIS-R test may contain cultural influences that reduce the validity of the WAIS-R as a measure of cognitive ability for Mexican American students,"<ref>
[http://asm.sagepub.com/cgi/content/abstract/12/3/303 Culture-Fair Cognitive Ability Assessment] Steven P. Verney Assessment, Vol. 12, No. 3, 303-319 (2005)
</ref> indicating a weaker positive correlation relative to sampled white students. Other recent studies have questioned the culture-fairness of IQ tests when used in South Africa.<ref>
{{cite journal
|author=Shuttleworth-Edwards AB, Kemp RD, Rust AL, Muirhead JG, Hartman NP, Radloff SE
|title=Cross-cultural effects on IQ test performance: a review and preliminary normative indications on WAIS-III test performance
|journal=J Clin Exp Neuropsychol
|volume=26 |issue=7 |pages=903–920
|year=2004
|pmid=15742541 |doi= 10.1080/13803390490510824
|url=}}</ref><ref>
[http://www.blackwell-synergy.com/doi/abs/10.1111/j.1468-2389.2006.00346.x Case for Non-Biased Intelligence Testing Against Black Africans Has Not Been Made: A Comment on Rushton, Skuy, and Bons (2004)] 1*, Leah K. Hamilton1, Betty R. Onyura1 and Andrew S. Winston International Journal of Selection and Assessment Volume 14 Issue 3 Page 278 - September 2006
</ref> Standard intelligence tests, such as the Stanford-Binet, are often inappropriate for children with [[autism]] and dyslexia; the alternative of using developmental or adaptive skills measures are relatively poor measures of intelligence in autistic children, and have resulted in incorrect claims that a majority of children with autism are mentally retarded.<ref>
{{cite journal
|author= Edelson, MG
|year=2006
|title= Are the majority of children with autism mentally retarded? a systematic evaluation of the data
|journal= Focus Autism Other Dev Disabl
|volume=21 |issue=2 |pages=66–83
|url=http://www.willamette.edu/dept/comm/reprint/edelson/
|accessdate=2007-04-15
|doi= 10.1177/10883576060210020301
}}</ref>

=== Outdated methodology ===
A 2006 paper argues that mainstream contemporary test analysis does not reflect substantial recent developments in the field and "bears an uncanny resemblance to the psychometric state of the art as it existed in the 1950s."<ref>
[http://users.fmg.uva.nl/dborsboom/papers.htm The attack of the psychometricians]. Denny Borsboom. Psychometrika Vol. 71, No. 3, 425–440. September 2006.
</ref> It also claims that some of the most influential recent studies on group differences in intelligence, in order to show that the tests are unbiased, use outdated methodology.

Some argue that IQ scores are used as an excuse for not trying to reduce poverty or otherwise improve living standards for all. Claimed low intelligence has historically been used to justify the [[feudal system]] and unequal treatment of women (see [[sex and intelligence]]). In contrast, others claim that the refusal of "high-IQ elites" to take IQ seriously as a cause of inequality is itself immoral.<ref>
{{cite web
|url=http://www.isteve.com/How_to_Help_the_Left_Half_of_the_Bell_Curve.htm
|title=How to Help the Left Half of the Bell Curve
|accessdate=2006-08-06
|year=2000
|author=Steve Sailer
|publisher=VDARE.com
}}</ref>

=== The view of the American Psychological Association ===
In response to the controversy surrounding ''[[The Bell Curve]]'', the [[American Psychological Association]]'s Board of Scientific Affairs established a task force in 1995 to write a consensus statement on the state of intelligence research which could be used by all sides as a basis for discussion. The full text of the report is available through several websites.<ref name="Neisser95" /><ref>
[http://www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdf]
</ref>

In this paper the representatives of the association regret that IQ-related works are frequently written with a view to their political consequences: "research findings were often assessed not so much on their merits or their scientific standing as on their supposed political implications".

The task force concluded that IQ scores do have high predictive validity for individual differences in school achievement. They confirm the predictive validity of IQ for adult occupational status, even when variables such as education and family background have been statistically controlled. They agree that individual differences in intelligence are substantially influenced by genetics and that both genes and environment, in complex interplay,
are essential to the development of intellectual competence.

They state there is little evidence to show that childhood diet influences intelligence except in cases of severe malnutrition. The task force agrees that large differences do exist between the average IQ scores of blacks and whites, and that these differences cannot be attributed to biases in test construction. The task force suggests that explanations based on social status and cultural differences are possible, and that environmental factors have raised mean test scores in many populations. Regarding genetic causes, they noted that there is not much direct evidence on this point, but what little there is fails to support the genetic hypothesis.

The APA journal that published the statement, ''[[American Psychologist]],'' subsequently published eleven critical responses in January 1997, several of them arguing that the report failed to examine adequately the evidence for partly-genetic explanations.

== High IQ societies ==
{{main| High IQ society}}
A high IQ society is an organization that limits membership to people who are within a certain high percentile of IQ test results. (For example, [[Mensa International]])

== Pop culture usage ==
Many websites and magazines use the term IQ to refer to technical or popular knowledge in a variety of subjects not related to intelligence, including [[sex]],<ref>
{{cite web
|url=http://www.plannedparenthood.org/health-topics/sex-101/your-sex-iq-4328.htm
|title=Planned Parenthood Sex IQ
|accessdate=2008-08-10
}}</ref>
[[poker]],<ref>
{{cite web
|url=http://www.testyourpoker.com
|title=NL Holdem Poker IQ Test
|accessdate=2008-08-10
}}</ref> and [[American football]],<ref>
{{Cite web
|url=http://www.footballiqtest.com
|title=American Football IQ
|accessdate=2008-08-10
}}</ref> among a wide variety of other topics. These tests are generally not [[standardized]] and do not fit within the normal definition of [[intelligence]]. Intelligence tests such as the [[Wechsler Adult Intelligence Scale]], [[Wechsler Intelligence Scale for Children]], [[Stanford-Binet]], Woodcock-Johnson III Tests of Cognitive Abilities, or the Kaufman Assessment Battery for Children-II, to name some of the best constructed, are not merely placing a test taker's score within the norm, as presumably are the thousands of alleged "IQ Tests" found on the internet, but they are also testing factors (e.g., [[fluid and crystallized intelligence]], working memory, and the like) that were previously found to represent pure measures of intelligence using [[factor analysis]]. This claim may not be made for the hundreds of online tests marketing themselves as IQ Tests, a distinction that may be unfortunately lost upon the public taking them.

== Reference charts ==
{{main|IQ reference chart}}
IQ reference charts are tables suggested by psychologists to divide intelligence ranges in various categories.

== See also ==
{{col-begin}}{{col-break}}
* [[Academia]]
* [[Autodidacticism]]
* [[Child prodigy]]
* [[Cultural intelligence]]
* [[Curiosity quotient]]
* [[CHC Theory]]
* [[Developmental disability]]
* [[Emotional intelligence]]
* [[EQ SQ theory]]
* [[Genetics of intelligence]] &nbsp;&nbsp;&nbsp;&nbsp;
{{col-break}}
* [[Graduate Record Examination]] &nbsp;&nbsp;&nbsp;&nbsp;
* [[Intellectual giftedness]]
* ''[[IQ and Global Inequality]]''
* [[IQ and the wealth of nations]]
* [[IQ reference chart]]
* [[Late bloomer]]
* [[List of admission tests to colleges and universities]]
* [[Mental disorder]]
* [[Mental retardation]]
* [[Nature versus nurture]]
* [[Neurology]]
* [[Psychiatry]]
* [[Psychology]]
{{col-break}}
* ''[[Race Differences in Intelligence]]''
* [[Savant syndrome]]
* [[SAT]]
* [[Scholarly method]]
* [[Sentience quotient]]
* [[Social IQ]]
* [[Sociology]]
* [[Spiritual intelligence]]
* [[Theory of multiple intelligences]]
* [[Triarchic theory of intelligence]]
* [[Triple Nine Society]]
* [[Wechsler Individual Achievement Test]] (WIAS)
{{col-end}}

== References ==
=== Notes ===
{{reflist|2}}

=== Bibliography ===
{{refbegin}}

*{{cite book
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|title=Human cognitive abilities: A survey of factor-analytical studies
|location=New York
|publisher=[[Cambridge University Press]]
|isbn=0-521-38275-0
|year=1993
|page=
}}
*{{cite journal
|author=Coward, W.M.; Sacket, P.R.
|title=Linearity of ability-performance relationships: A reconfirmation.
|journal=[[Journal of Applied Psychology]]
|volume=75
|issue=
|year=1990
|pages=297–300
|doi=10.1037/0021-9010.75.3.297
}}
* {{cite journal
|author=Duncan, J. ''et al.''
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|journal=[[Science (journal)|Science]]
|volume=289 |issue=5478 |pages=457–60
|year=2000
|pmid=10903207 |doi=10.1126/science.289.5478.457
}}
* {{cite journal
|author=Duncan, J.; Burgess, P.; Emslie, H.
|title=Fluid intelligence after frontal lobe lesions
|journal=[[Neuropsychologia]]
|volume=33 |issue=3 |pages=261–8
|year=1995
|pmid=7791994 |doi=10.1016/0028-3932(94)00124-8
}}
*{{cite journal
|author=Flynn, J.R.
|title=Searching for Justice: The discovery of IQ gains over time.
|journal=[[American Psychologist]]
|volume=54 |issue= |pages=5–20
|year=1999
|url=http://www.stat.columbia.edu/~gelman/stuff_for_blog/flynn.pdf
|format=PDF|doi=10.1037/0003-066X.54.1.5
}}
*{{cite journal
|author=Frey, M.C.; Detterman, D.K.
|title=Scholastic assessment or ''g''? The relationship between the scholastic assessment test and general cognitive ability.
|journal=[[Psychological Science]]
|volume=15 |issue=6 |page=373–378
|year=2003
|url=
|pmid=15147489 |doi=10.1111/j.0956-7976.2004.00687.x
}}
* {{cite journal
|author=Gale, C.R.; Deary, I.J.; Schoon, I.; Batty, G.D.
|title=IQ in childhood and vegetarianism in adulthood: 1970 British Cohort Study
|journal=[[British Medical Journal]]
|volume=334 |issue=7587 |pages=245
|year=2007
|pmid=17175567 |doi=10.1136/bmj.39030.675069.55
}}
*{{cite journal
|author=Gottfredson, L.S.
|title=Why g matters: The complexity of everyday life.
|journal=[[Intelligence (journal)|Intelligence]]
|volume=24 |issue=1 |pages=79–132
|year=1997
|url=http://www.udel.edu/educ/gottfredson/reprints/1997whygmatters.pdf
|format=PDF|doi=10.1016/S0160-2896(97)90014-3
}}
*{{cite journal
|author=Gottfredson, L.S.
|title=The general intelligence factor.
|journal=[[Scientific American Presents]]
|volume=9 |issue=4 |pages=24–29
|year=1998
|url=http://www.udel.edu/educ/gottfredson/reprints/1998generalintelligencefactor.pdf
|format=PDF}}
*{{cite book
|author=Gottfredson, L.S.
|chapter=Suppressing intelligence research: Hurting those we intend to help.
|title=Destructive trends in mental health: The well-intentioned path to harm
|editor=Wright, R.H. and Cummings, N.A (Eds.)
|location=New York
|publisher=[[Taylor and Francis]]
|year=2005
|pages=155–186
|url=http://www.udel.edu/educ/gottfredson/reprints/2005suppressingintelligence.pdf
|format=PDF|isbn=0-415-95086-4
}}
*{{cite book
|author=Gottfredson, L.S.
|chapter=Social consequences of group differences in cognitive ability (Consequencias sociais das diferencas de grupo em habilidade cognitiva)
|title=Introdução à psicologia das diferenças individuais
|editor=Flores-Mendoza, C.E. and Colom, R. (Eds.)
|location=Porto Alegre, Brazil
|publisher=[[ArtMed Publishers]]
|year=2006
|pages=155–186
|url=http://www.udel.edu/educ/gottfredson/reprints/2004socialconsequences.pdf
|format=PDF|isbn=8-536-30621-1
}}
* {{cite journal
|author=Gray, J.R.; Chabris, C.F.; Brave,r T.S.
|title=Neural mechanisms of general fluid intelligence
|journal=[[Nat. Neurosci.]]
|volume=6 |issue=3 |pages=316–22
|year=2003
|pmid=12592404 |doi=10.1038/nn1014
}}
* {{cite journal
|author=Gray JR, Thompson PM
|title=Neurobiology of intelligence: science and ethics
|journal=[[Nat. Rev. Neurosci.]]
|volume=5 |issue=6 |pages=471–82
|year=2004
|pmid=15152197 |doi=10.1038/nrn1405
}}
* {{cite journal
|author=Haier, R.J.; Jung, R.E.; Yeo, R.A.; Head, K.; Alkire, M.T.
|title=The neuroanatomy of general intelligence: Sex matters
|journal=[[NeuroImage]]
|volume=25 |issue=1 |pages=320–327
|year=2005
|pmid=15734366 |doi=10.1016/j.neuroimage.2004.11.019
}}
*{{cite book
|author=Harris, J.R.
|title=The nurture assumption : why children turn out the way they do
|location=New York (NY)
|publisher=[[Free Press]]
|isbn=0-684-84409-5
|year=1998
|page=
}}
* {{cite journal
|author=Hunt, E
|title=Multiple views of multiple intelligence. [Review of Intelligence Reframed: Multiple Intelligences for the 21st Century.]
|journal=[[Contemporary Psychology]]
|volume=46 |issue= |pages=5–7
|year=2001
}}
*{{cite book
|author=Jensen, A.R.
|title=Bias in mental testing
|location=New York (NY)
|publisher=[[Free Press]]
|isbn=0-029-16430-3
|year=1979
|page=
}}
*{{cite book
|author=Jensen, A.R.
|title=The g Factor: The Science of Mental Ability |location=Wesport (CT)
|publisher=[[Praeger Publishers]]
|isbn=0-275-96103-6
|year=1979
|page=
}}
*{{cite book
|author=Jensen, A.R.
|title=Clocking the Mind: Mental Chronometry and Individual Differences.
|location=
|publisher=[[Elsevier]]
|isbn=0-080-44939-5
|year=2006
|page=
}}
* {{cite journal
|author=Klingberg, T.; Forssberg, H.; Westerberg, H.
|title=Training of working memory in children with ADHD
|journal=[[J Clin Exp Neuropsychol]]
|volume=24 |issue=6 |pages=781–91
|year=2002
|pmid=12424652 |doi=10.1076/jcen.24.6.781.8395
}}
* {{cite journal
|author=McClearn, G.E. ''et al.''
|title=Substantial genetic influence on cognitive abilities in twins 80 or more years old
|journal=[[Science (journal)|Science]]
|volume=276 |issue=5318 |pages=1560–1563
|year=1997
|pmid=9171059 |doi=10.1126/science.276.5318.1560
}}
*{{cite journal
|author=Mingroni, M.A.
|title=The secular rise in IQ: Giving heterosis a closer look
|journal=[[Intelligence (journal)|Intelligence]]
|volume=32 |issue= |pages=65–83
|year=2004
|doi=10.1016/S0160-2896(03)00058-8
}}
*{{cite book
|author=Murray, C.
|title=Income Inequality and IQ
|location=Washington (DC)
|publisher=[[AEI Press]]
|isbn=0-8447-7094-9
|year=1998
|page=
|url=http://www.aei.org/docLib/20040302_book443.pdf
|format=PDF}}
*{{cite journal
|author=Noguera, P.A
|title=Racial politics and the elusive quest for excellence and equity in education.
|journal=Motion Magazine
|volume=
|issue=
|year=2001
|page=
|id=Article # ER010930002
|url=http://www.inmotionmagazine.com/er/pnrp1.html
}}
*{{cite book
|author=Plomin, R.; DeFries, J.C.; Craig, I.W.; McGuffin, P
|title=Behavioral genetics in the postgenomic era
|location=Washington (DC)
|publisher=[[American Psychological Association]]
|isbn=1-557-98926-5
|year=2003
|page=
}}
*{{cite book
|author=Plomin, R.; DeFries, J.C.; McClearn, G.E.; McGuffin, P
|title=Behavioral genetics
|edition=4th
|location=New York (NY)
|publisher=[[Worth Publishers]]
|isbn=0-716-75159-3
|year=2000
|page=
}}
*{{citation
|author=Rowe, D.C.; Vesterdal, W.J.; Rodgers, J.L.
|title=The Bell Curve Revisited: How Genes and Shared Environment Mediate IQ-SES Associations
|year=1997
}}<!-- What is this? I can't find anything on google -->
* {{cite journal
|author=Schoenemann, P.T.; Sheehan, M.J.; Glotzer, L.D.
|title=Prefrontal white matter volume is disproportionately larger in humans than in other primates
|journal=[[Nat. Neurosci.]]
|volume=8 |issue=2 |pages=242–252
|year=2005
|pmid=15665874 |doi=10.1038/nn1394
}}
* {{cite journal
|author=Shaw, P. ''et al.''
|title=Intellectual ability and cortical development in children and adolescents
|journal=[[Nature (journal)|Nature]]
|volume=440 |issue=7084 |pages=676–679
|year=2006
|pmid=16572172 |doi=10.1038/nature04513
}}
* {{cite journal
|author=Tambs, K.; Sundet, J.M.; Magnus, P.; Berg, K.
|title=Genetic and environmental contributions to the covariance between occupational status, educational attainment, and IQ: a study of twins
|journal=[[Behav. Genet.]]
|volume=19 |issue=2 |pages=209–22
|year=1989
|pmid=2719624| doi=10.1007/BF01065905
}}
* {{cite journal
|author=Thompson, P.M. ''et al.''
|title=Genetic influences on brain structure
|journal=[[Nat. Neurosci.]]
|volume=4 |issue=12 |pages=1253–1258
|year=2001
|pmid=11694885 |doi=10.1038/nn758
}}
*{{cite book
|author=Wechsler, D.
|title=Wechsler Adult Intelligence Scale
|edition=3rd
|location=San Antonia (TX)
|publisher=The Psychological Corporation
|isbn=
|year=1997
|page=
}}
*{{cite book
|author=Wechsler, D.
|title=Wechsler Intelligence Scale for Children
|edition=4th
|location=San Antonia (TX)
|publisher=The Psychological Corporation
|isbn=
|year=2003
|page=
}}
*{{cite journal
|author=Weiss, V.
|title=National IQ means transformed from Programme for International Student Assessment (PISA) Scores
|journal=[[The Journal of Social, Political and Economic Studies]]
|volume=31 |issue=1 |pages=71-94
|year=2009
|url=http://mpra.ub.uni-muenchen.de/14600/
}}

{{refend}}

==External links==
{{wikiversity}}
* [http://www.cerebrals.org/index.php?go=tests Ability and above average intelligence tests]
* [http://www.psychpage.com/learning/library/intell/mainstream.html Mainstream Science on Intelligence]
* [http://aceviper.net/IQ_TEST/iq_test_2005.php Official Ace Viper IQ Test]
* [http://www.udel.edu/educ/gottfredson/reprints/1997mainstream.pdf PDF Reprint - Mainstream science on intelligence: An editorial with 52 signatories, history, and bibliography.]
* [http://www.psych.utoronto.ca/~reingold/courses/intelligence/cache/1198yam.html Scientific American: Intelligence Considered]
*[http://www.qi-inteligencia.com/index.php?option=com_content&view=category&id=35&Itemid=59 Articles in English at "Intelligence and IQ Tests"]

[[Category:Psychometrics]]
[[Category:Intelligence]]
[[Category:Intelligence by type]]

[[ar:نسبة الذكاء]]
[[az:İntellekt nisbəti]]
[[ca:Quocient intel·lectual]]
[[cs:Inteligenční kvocient]]
[[da:Intelligenskvotient]]
[[de:Intelligenzquotient]]
[[et:Intelligentsuskvoot]]
[[el:Δείκτης Νοημοσύνης]]
[[es:Coeficiente intelectual]]
[[eo:Intelekta kvociento]]
[[fa:ضریب هوشی]]
[[fr:Quotient intellectuel]]
[[gl:Cociente intelectual]]
[[ko:지능 지수]]
[[is:Greindarvísitala]]
[[it:Quoziente d'intelligenza]]
[[he:מנת משכל]]
[[lt:Intelekto koeficientas]]
[[hu:IQ]]
[[arz:نسبة الذكاء]]
[[nl:Intelligentiemeting]]
[[ja:知能指数]]
[[no:IQ]]
[[pl:Iloraz inteligencji]]
[[pt:Quociente de inteligência]]
[[ro:Coeficient de inteligenţă]]
[[ru:Коэффициент интеллекта]]
[[simple:Intelligence quotient]]
[[sk:Inteligenčný kvocient]]
[[sl:Inteligenčni količnik]]
[[sr:Коефицијент интелигенције]]
[[fi:Älykkyysosamäärä]]
[[sv:IQ]]
[[th:ระดับเชาวน์ปัญญา]]
[[tr:IQ]]
[[uk:Коефіцієнт інтелекту]]
[[vi:IQ]]
[[yi:איי-קיו]]
[[zh:智商]]

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