Sex differences in intelligence
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Differences in intelligence or mental power have long been a topic of debate among researchers and scholars. With the advent of the concept of g or general intelligence some form of empiricism was allowed, but results are often inconsistent with studies showing either no differences or advantages for both sexes, with many showing a slight advantage for males. One study did find some advantage for women in later life, while another found that male advantages on some cognitive tests are minimized when controlling for socioeconomic factors. The differences in average IQ between men and women are small in magnitude and inconsistent in direction, although the variability of male scores has been found to be greater than that of females, resulting in more males than females in the top and bottom of the IQ distribution.
There are however differences in the capacity of males and females in performing certain tasks, such as rotation of object in space, often categorized as spatial ability. Other traditionally male advantages, such as in the field of mathematics, are not so clear-cut.
Prior to the 20th century, it was a commonly held view that men were intellectually superior to women. Thomas Gisborne argued (1801) that women were naturally suited to domestic work and not spheres suited to men such as politics, science, or business. He argued that this was because women did not possess the same level of rational thinking that men did and had naturally superior abilities in skills related to family support.
In 1875, Herbert Spencer argued that women were incapable of abstract thought and could not understand issues of justice, and only had the ability to understand issues of care. In 1925, Sigmund Freud also concluded that women were less morally developed in the concept of justice, and, unlike men, were more influenced by feeling than rational thought. Early brain studies comparing mass and volumes between the sexes concluded that women were intellectually inferior because they had smaller and lighter brains. Many believed that the size difference caused women to be excitable, emotional, sensitive, and therefore not suited for political participation.
In the nineteenth century, whether men and women had equal intelligence was seen by many as a prerequisite for the granting of suffrage. Leta Hollingworth argues that women were not permitted to realize their full potential, as they were confined to the roles of child-rearing and housekeeping.
During the early twentieth century, the scientific consensus shifted to the view that gender plays no role in intelligence. In his 1916 study of children's IQs, psychologist Lewis Terman concluded that "the intelligence of girls, at least up to 14 years, does not differ materially from that of boys". He did, however, find "rather marked" differences on a minority of tests. For example, he found boys were "decidedly better" in arithmetical reasoning, while girls were "superior" at answering comprehension questions. He also proposed that discrimination, lack of opportunity, women's responsibilities in motherhood, or emotional factors may have accounted for the fact that few women had careers in intellectual fields.
Current research on general intelligence
According to the 1994 report "Intelligence: Knowns and Unknowns" by the American Psychological Association, "Most standard tests of intelligence have been constructed so that there are no overall score differences between females and males." Differences have been found, however, in specific areas such as mathematics and verbal measures.
When standardized IQ tests were first developed in the early 20th century, girls typically scored higher than boys until age 14, at which time the curve for girls dropped below that for boys. As testing methodology was revised, efforts were made to equalize gender performance.
The mean IQ scores between men and women vary little. The variability of male scores is greater than that of females, however, resulting in more males than females in the top and bottom of the IQ distribution.
Several meta-studies by Richard Lynn between 1994 and 2005 found mean IQ of men exceeding that of women by a range of 3–5 points. Lynn's findings were debated in a series of articles for Nature. Jackson and Rushton found males aged 17–18 years had average of 3.63 IQ points in excess of their female equivalents. A 2005 study by Helmuth Nyborg found an average advantage for males of 3.8 IQ points. One study concluded that after controlling for sociodemographic and health variables, "gender differences tended to disappear on tests for which there was a male advantage and to magnify on tests for which there was a female advantage." A study from 2007 found a 2-4 IQ point advantage for females in later life. One study investigated the differences in IQ between the sexes in relation to age, finding that girls do better at younger ages but that their performance declines relative to boys with age. Colom et al. (2002) found 3.16 higher IQ points for males but no difference on the general intelligence factor (g) and therefore explained the differences as due to non-g factors such as specific group factors and test specificity. A study conducted by Jim Flynn and Lilia Rossi-Case (2011) found that men and women achieved roughly equal IQ scores on Raven's Progressive Matrices after reviewing recent standardization samples in five modernized nations. Irwing (2012) found a 3 point IQ advantage for males in g from subjects aged 16–89 in the United States.
Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier et al. found in a 2004 study that: "Men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance. For men, the gray matter volume in the frontal and parietal lobes correlates with IQ; for women, the gray matter volume in the frontal lobe and Broca's area (which is used in language processing) correlates with IQ.
Some studies have identified the degree of IQ variance as a difference between males and females. Males tend to show greater variability on many traits including tests of cognitive abilities, though this may differ between countries. A 2005 study by Ian Deary, Paul Irwing, Geoff Der, and Timothy Bates, focusing on the ASVAB showed a significantly higher variance in male scores, resulting in more than twice as many men as women scoring in the top 2%. The study also found a very small (d' ≈ 0.07, less than 7%, of a standard deviation) average male advantage in g. A 2006 study by Rosalind Arden and Robert Plomin focused on children aged 2, 3, 4, 7, 9 and 10 and stated that there was greater variance "among boys at every age except age two despite the girls’ mean advantage from ages two to seven. Girls are significantly over-represented, as measured by chi-square tests, at the high tail and boys at the low tail at ages 2, 3 and 4. By age 10 the boys have a higher mean, greater variance and are over-represented in the high tail."
A psychological study was conducted where about 1200 high school graduates were recruited to take tests looking at each of their verbal, reasoning, spatial abilities, and general scholastic knowledge. Male and female performances were compared through these tests. As a result of this testing, it was discovered that males had a higher mean score on all four tests than the mean score of the females who participated in the study. In 1995, it was suggested by Charles Lewis and Warren W. Willingham that patterns of gender differences on IQ scores can change because of the selectivity of the sample itself. They argued two factors played in giving the males an advantage: the greater male variability and the sampling of a greater proportion of women.
Another study on intelligence came up with similar findings. Young adolescents were asked to volunteer in this study and completed various assessments including ones looking at language, math, and sciences skills as well as the Toulous-Pieron test of attention and the Dominoes test. While one sample of children completed these assessments, another sample completed these plus another handful. The test results from both samples show a null sex difference in general intelligence in young adolescents. Researchers concluded that since g does not differ through academic and cognitive abilities in young adolescents, male or female, and that some other factor must be responsible for the variance between the sexes.
It was believed at one point that Gf, or fluid intelligence, can be used to systematically detect sex differences in general intelligence if there are any. The PMA Inductive Reasoning Test, Cattell’s Culture-Fair Intelligence Test, and the Advanced Progressive Matrices were used to test a group of about 4000 high school graduates. Through the results of these tests, researchers discovered that females perform better in the PMA Inductive Reasoning Test and males perform better in the Advanced Progressive Matrices assessment. There was no sex difference noted from the results of the Culture-Fair Test. Sex difference in fluid intelligence was proven to be non-existent in this study.
While research has shown that males and females do indeed each excel in different abilities, math and science might be an exception to this.
Large, representative studies of US students show that no sex differences in mathematics performance exist before secondary school. During and after secondary school, historic sex differences in mathematics enrollment account for nearly all of the sex differences in mathematics performance. However, a performance difference in mathematics on the SAT exists favoring males, though differences in mathematics course performance measures favor females. In 1983, Benbow concluded that the study showed a large sex difference by age 13 and that it was especially pronounced at the high end of the distribution. However, Gallagher and Kaufman criticized Benbow's and other reports finding males overrepresented in the highest percentages as not ensuring representative sampling.
In a 2008 study paid for by the National Science Foundation in the United States, researchers found that "girls perform as well as boys on standardized math tests. Although 20 years ago, high school boys performed better than girls in math, the researchers found that is no longer the case. The reason, they said, is simple: Girls used to take fewer advanced math courses than boys, but now they are taking just as many." However, the study indicated that, while on average boys and girls performed similarly, boys were overrepresented among the very best performers as well as among the very worst.
Kiefer and Sekaquaptewa proposed that a source of some women's underperformance and lowered perseverance in mathematical fields is these women's underlying "implicit" sex-based stereotypes regarding mathematical ability and association, as well as their identification with their gender. Some psychologists believe that many historical and current sex differences in mathematics performance may be related to boy's higher likelihood of receiving math encouragement than girls. Parents were, and sometimes still are, more likely to consider a son's mathematical achievement as being a natural skill while a daughter's mathematical achievement is more likely to be seen as something she studied hard for. This difference in attitude may contribute to girls and women being discouraged from further involvement in mathematics-related subjects and careers. Stereotype threat has been shown to affect performance and confidence in mathematics of both males and females. However, a review of stereotype threat literature found most studies couldn't be replicated or suffered methodological problems and concluded "that although stereotype threat may affect some women, the existing state of knowledge does not support the current level of enthusiasm for this as a mechanism underlying the gender gap in mathematics."
Two cross-country comparisons have found great variation in the gender differences regarding the degree of variance in mathematical ability. In most nations males have greater variance. In a few females have greater variance. Hyde and Mertz argue that boys and girls differ in the variance of their ability due to sociocultural factors.
Some studies investigating the spatial abilities of men and women have found no significant differences, though metastudies show a male advantage in mental rotation and assessing horizontality and verticality, and a female advantage in spatial memory.
A proposed hypothesis is that men and women evolved different mental abilities to adapt to their different roles in society. This explanation suggests that men may have evolved greater spatial abilities as a result of certain behaviors, such as navigating during a hunt. Similarly, this hypothesis suggests that women may have evolved to devote more mental resources to remembering locations of food sources in relation to objects and other features in order to gather food.
A number of studies have shown that women tend to rely more on visual information than men in a number of spatial tasks related to perceived orientation. However, 'visual dependence' has been found to be task specific and not a general characteristic of spatial processing that differs between the sexes. Here an alternative hypothesis suggests that heightened visual dependence in females does not generalize to all aspects of spatial processing but is probably attributable to task-specific differences in how male and females brains process multisensory spatial information.
Results from studies conducted in the physical environment are not conclusive about sex differences, with various studies on the same task showing no differences. For example, there are studies that show no difference in 'wayfinding'. One study found men more likely to report having a good sense of direction and are more confident about finding their way in a new environment, but evidence does not support men having better map reading skills. Women have been found to use landmarks more often when giving directions and when describing routes. Additionally, a study concludes that women are better at recalling where objects are located in a physical environment. Women show greater proficiency and reliance on distinctive landmarks for navigation while males rely on an overall mental map.
Performance in mental rotation and similar spatial tasks is affected by gender expectations. For example, studies show that being told before the test that men typically perform better, or that the task is linked with jobs like aviation engineering typically associated with men versus jobs like fashion design typically associated with women, will negatively affect female performance on spatial rotation and positively influence it when subjects are told the opposite. Experiences such as playing video games also increase a person's mental rotation ability. A study from the University of Toronto showed that differences in ability get reduced after playing video games requiring complex mental rotation. The experiment showed that playing such games creates larger gains in spatial cognition in females than males.
The possibility of testosterone and other androgens as a cause of sex differences in psychology has been a subject of study. Adult women who were exposed to unusually high levels of androgens in the womb due to congenital adrenal hyperplasia score significantly higher on tests of spatial ability. Many studies find positive correlations between testosterone levels in healthy males and measures of spatial ability. However, the relationship is complex.
A study was done to compare the relationship between mental rotation ability and gender difference specifically with the SAT-Math. Cognitive gender differences are apparent and findings of a male advantage in certain mathematical domains have been demonstrated cross-culturally. These gender differences found are largely in geometry and word problems and tend to be in countries with the highest achieving students and with the largest gender gap in experience. Smaller differences were noted in countries with lower achieving students in mathematics which includes the United States. Moore and Smith state that within the United States, poorly educated female students outperform their male peers, but as the level of education increases, the male advantage in mathematics emerges.
Spatial ability may be responsible in part for facilitating gender differences in math aptitude. Casey et al. (1995) looked at the relationship of mental rotation ability and the SAT-M among four samples. The four samples were: (1) undergraduates at two liberal arts colleges in the Northeast that were tested on their mental rotation ability in groups of 10-20, (2) a group of mathematically talented preadolescents participating in a summer math and science training in the Midwest which included seventh to ninth graders who were either recruited from a national talent search program or statewide teacher selection program, (3) a high ability group of college bound students who were enrolled in a middle-income suburban high school in the Northeast and elected to take the SAT, and (4) a low ability group of college bound students who were enrolled in a middle-income suburban high school in the Northeast and elected to take the SAT. The data used were SAT math and verbal scores and mental rotation scores. Mental rotation was assessed using the Vandenberg Test of Mental Rotation. Students were asked to match two out of four choices to a standard figure.
The study found that that when mental rotation is used as a predictor of Math aptitude for female students, the correlations between mental rotation and SAT-Math scores ranged from 0.35 to 0.38 whereas males showed no consistent pattern. Male correlations ranged from -0.03 to 0.54. However, an interesting finding was that in the three high ability samples, there was a significant gender difference in SAT-Math scores alone. This difference favored males. In the three high ability samples, males scored higher than females in mental rotation ability. Interesting enough, for the verbal aptitude test on SAT, there was a significant difference in verbal ability for the low ability college bound sample favoring girls.
Dyslexia is a learning disability that impairs a person’s fluency or comprehension accuracy in being able to read. The cause of this disability is associated with abnormal brain anatomy and function. Gray matter deficits have been demonstrated in dyslexics using structural magnetic resonance imaging. This deficit has been found in specific regions within the left hemisphere involved in language.
There is higher prevalence of dyslexia in males than in females. However, different abnormalities are found in female brains as opposed to male brains. In a study that examined gray matter volume in dyslexic females, it was found that there was less gray matter volume in the right precuneus and paracentral lobule/medial frontal gyrus. In males, there was less gray matter volume in the left inferior parietal cortex. This study shows that dyslexia in females does not involve the left hemisphere regions involved in language as it does in males. Instead, it affects the sensory and motor cortices such as the motor and premotor cortex and primary visual cortex.
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