Impact of health on intelligence
Health can affect intelligence in various ways. Conversely, intelligence can affect health. Health effects on intelligence have been described as being among the most important factors in the origins of human group differences in IQ test scores and other measures of cognitive ability. 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 of the child is less effective. Such impairment may sometimes be permanent, sometimes be partially or wholly compensated for by later growth.
Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring 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.
Improvements in nutrition (often involving specific micronutrients) due to in public policy changes have been implicated in IQ increases in many nations (as part of the overall Flynn effect), such as efforts fighting iodine deficiency in the U.S.
- 1 Nutrition
- 2 Toxins
- 3 Healthcare during pregnancy and childbirth
- 4 Stress
- 5 Infectious diseases
- 6 Effects of other diseases
- 7 Other associations
- 8 See also
- 9 References
Malnutrition may occur during several periods of growth, such as pregnancy, during breastfeeding, infancy, or childhood. It may also happen due to deficiencies of different nutrients, such as micronutrients, protein or energy. This may cause different effects.
Some observers have argued that malnutrition during the first six months of life harms cognitive development much more than malnutrition later in life. However, a study from the Philippines argues that malnutrition in the second year of life may have a larger negative impact than malnutrition in the first year of life.
Intrauterine growth retardation
Undernutrition during pregnancy, and other factors, may cause intrauterine growth retardation (IUGR), which is one cause of low birth weight. However, it has been suggested that in IUGR the brain may be selectively spared. Brain growth is usually less affected than whole body weight or length. Several studies from developed nations have found that with the exception of extreme intrauterine growth retardation also affecting brain growth, and hypoxic injury, IUGR seems to have little or no measurable effect on mental performance and behavior in adolescence or adulthood. For example, acute undernutrition for a few months during the Dutch famine of 1944 caused a decrease in mean birthweight in certain areas. This was later associated with a change in performance on IQ tests for 18–19 years old Dutch males draftees from these areas compared to control areas. The subjects were exposed to famine prenatally but not after birth. During the famine, births decreased more among those with lower Socioeconomic status (SES), whereas after the famine, there was a compensatory increase in births among those with lower SES. Since SES correlates with IQ, this may have hidden an effect caused by the undernutrition.
Studies often find higher IQ in children and adults who were breastfed. It has also been proposed that the omega-3 fatty acids that are found in high doses in breast milk, and that are known to be essential constituents of brain tissues, could at least partially account for an increase in IQ.
Recently, however, the longstanding belief that breastfeeding causes an increase in the IQ of offspring was challenged in a 2006 paper published in the British Medical Journal. The results indicated that mother's IQ, not breastfeeding, explained the differences in the IQ scores of offspring measured between ages 5 and 14. The results of this study argued that prior studies had not controlled for the mother's IQ. Since mother's IQ was predictive of whether a child was breastfed, the study concluded that "breast feeding [itself] has little or no effect on intelligence in children." Instead, it was the mother's IQ that had a significant correlation with the IQ of her offspring, whether the offspring was breastfed or was not breastfed. Another study found that breastfeeding had a positive effect on cognitive development at 24 months of age even after controlling for parental IQ.
A potential resolution to these different interpretations was proposed in a study showing that breastfeeding was linked to raised IQ (as much as 7 points when not controlling for maternal IQ) if the infants had an SNP coding for a "C" rather than G base within the FADS2 gene. Those with the "G" version showed no IQ advantage, suggesting a biochemical interaction of child's genes on the effect of breast feeding. Other studies have failed to replicate any correlation between the FADS2 gene, breastfeeding and IQ, while others show a negative effect on IQ when combining bottledfeeding, and the "G" version of FADS2 .
Micronutrients and vitamin deficiencies
Micronutrient deficiencies (e.g. in iodine and iron) influence the development of intelligence and remain a problem in the developing world. For example, iodine deficiency causes a fall, in average, of 12 IQ points.
Policy recommendations to increase availability of micronutrient supplements have been made and justified in part by the potential to counteract intelligence-related developmental problems. For example, the Copenhagen consensus, states that lack of both iodine and iron has been implicated in impaired brain development, and this can affect enormous numbers of people: it is estimated that 2 billion people (one-third of the total global population) are affected by iodine deficiency, including 285 million 6- to 12-year-old children. In developing countries, it is estimated that 40% of children aged four and under suffer from anaemia because of insufficient iron in their diets.
Overall, studies investigating whether cognitive function in already iron-deficient children can be improved with iron supplements have produced mixed results, possibly because deficiency in critical growth periods may cause irreversible damage. However, several studies with better design have shown substantial benefits. One way to prevent iron deficiency is to give specific supplementation to children, for example as tablets. However, this is costly, distribution mechanisms are often ineffective, and compliance is low. Fortification of staple foods (cereals, flour, sugar, salt) to deliver micronutrients to children on a large scale is probably the most sustainable and affordable option, even though commitment from governments and the food industry is needed. Developed nations fortify several foods with various micronutrients.
Additional vitamin-mineral supplementation may have an effect also in the developed world. A study giving such supplementation to "working class," primarily Hispanic, 6–12-year-old children in the United States for 3 months found an average increase of 2 to 3 IQ points. Most of this can be explained by the very large increase of a subgroup of the children, presumably because these were not adequately nourished unlike the majority. The study suggests that parents of schoolchildren whose academic performance is substandard would be well advised to seek a nutritionally oriented physician for assessment of their children's nutritional status as a possible etiology.
More speculatively, other nutrients may prove important in the future. Vitamin B12 and folate may be important for cognitive function in old age. Fish oil supplement to pregnant and lactating mothers has been linked to increased cognitive ability in one study. Nonetheless, fish oil supplement studies have failed to support claims of preventing heart attacks or strokes.
Another study found that pregnant women who consumed 340 grams of low-mercury containing fish with fatty acids per week have benefits that outweigh the risks for mercury poisoning. They were less likely to have children with low verbal IQ, motor coordination and behavioral problems. However, foods containing high amounts of mercury, such as shark, swordfish, king mackerel and tilefish, might cause mental retardation.
Protein and energy malnutrition
One study from a developing country, Guatemala, found that poor growth during infancy, rather than low birth weight, was negatively related to adolescent performance on cognitive and achievement tests. A later related very long term study looked at the effect of giving 6–24-month-old children in Guatemala a high protein-energy drink as a dietary supplement. A significantly positive and fairly substantial effects was found on increasing the probability of attending school and of passing the first grade, increasing the grade attained by age 13, increasing completed schooling attainment, and for adults aged 25–40 increasing IQ test scores.
31% of children under the age of 5 in the developing world are moderately (height-for-age is below minus 2 standard deviations) or severely stunted (below minus 3 standard deviations). The prevalence was even higher previously since the worldwide prevalence of stunting is declining by about half of a percentage point each year. A study on stunted children aged 9–24 months in Jamaica found that when aged 17–18 years they had significantly poorer scores than a non-stunted group on cognitive and educational tests and psychosocial functioning. Giving a nutritional supplementation (1 kg milk based formula each week) to these already stunted children had no significant effect on later scores, but psychosocial stimulation (weekly play sessions with mother and child) had a positive effect.
Certain toxins, such as lead, mercury, arsenic, toluene, and PCB are well-known causes of neuro-developmental disorders. Recognition of these risks has led to evidence-based programmes of prevention, such as elimination of lead additives in petrol. Although these prevention campaigns are highly successful, most were initiated only after substantial delays.
Policies to manage lead differ between nations, particularly between the developed and developing world. Use of leaded gasoline has been reduced or eliminated in most developed nations, and lead levels in US children have been substantially reduced by policies relating to lead reduction. Even slightly elevated lead levels around the age of 24 months are associated with intellectual and academic performance deficits at age 10 years.
A Lancet review identified 201 chemicals with the ability to cause clinical neurotoxic effects in human adults, as described in the peer-reviewed scientific literature. Most of them are commonly used. Many additional chemicals have been shown to be neurotoxic in laboratory models. The article notes that children are more vulnerable and argues that new, precautionary approaches that recognise the unique vulnerability of the developing brain are needed for testing and control of chemicals in order to avoid the previous substantial before starting restrictions on usage. An appendix listed further industrial chemicals considered to be neurotoxic.
Alcohol and drugs
Current cannabis use was found to be significantly correlated in a dose-dependent manner with a decline in IQ scores, during the effect of the use. However, no such decline was seen in subjects who had formerly been heavy cannabis users and had stopped taking the drug. The authors concluded that cannabis does not have a long-term effect on intelligence. However this is contradicted by the long term longitudinal study, carried out by Otago and Duke universities,which found that regular use of marijuana in teenage years affects IQ in adulthood even when the use stops. The drop in IQ was 8 points. Adults smoking marijuana had no lasting effect on IQ. Effects on fetal development are minimal when compared with the well-documented adverse effects of tobacco or alcohol use.
Maternal tobacco smoking during pregnancy is associated with increased activity, decreased attention, and diminished intellectual abilities. However, a recent study finds that maternal tobacco smoking has no direct causal effect on the child's IQ. Adjusting for maternal cognitive ability as measured by IQ and education eliminated the association between lower IQ and tobacco smoking. But another study instead looking at the relationship between environmental tobacco smoke exposure, measured with a blood biomarker, and cognitive abilities among U.S. children and adolescents 6–16 years of age, found an inverse association between exposure and cognitive ability among children even at extremely low levels of exposure. The study controlled for sex, race, region, poverty, parent education and marital status, ferritin, and blood lead concentration.
Healthcare during pregnancy and childbirth
Healthcare during pregnancy and childbirth, access to which is often governed by policy, also influences cognitive development. Preventable causes of low intelligence in children include infectious diseases such as meningitis, parasites, and cerebral malaria, prenatal drug and alcohol exposure, newborn asphyxia, low birth weight, head injuries, and endocrine disorders. A direct policy focus on determinants of childhood cognitive ability has been urged.
A recent theory suggests that early childhood stress may affect the developing brain and cause negative effects. Exposure to violence in childhood has been associated with lower school grades and lower IQ in children of all races. A group of largely African American urban first-grade children and their caregivers were evaluated using self-report, interview, and standardized tests, including IQ tests. The study reported that exposure to violence and trauma-related distress in young children were associated with substantial decrements in IQ and reading achievement. Exposure to Violence or Trauma lead to a 7.5-point (SD, 0.5) decrement in IQ and a 9.8-point (SD, 0.66) decrement in reading achievement.
Violence may have a negative impact on IQ, or IQ may be protective against violence. The causal mechanism and direction of causation is unknown. Neighborhood risk has been related to lower school grades for African-American adolescents in another study from 2006.
A 2010 study by Eppig, Fincher and Thornhill found a close correlation between the infectious disease burden in a country and the average IQ of its population. The researchers found that when disease was controlled for, IQ showed no correlation with other variables such as educational and nutritional levels. Since brain development requires a very high proportion of all the body's energy in newborns and children, the researchers argue that fighting infection reduces children's IQ potential. The Eppig research may help to explain the Flynn effect, the rise in intelligence noted in rich countries. They also tested other hypotheses as well, including genetic explanations, concluding that infectious disease was "the best predictor". Christopher Hassall and Thomas Sherratt repeated the analysis, and concluded "that infectious disease may be the only really important predictor of average national IQ".
In order to mitigate the effects of education on IQ, Eppig, Fincher & Thornhill (2010) repeated their analysis across the United States where standardized and compulsory education exists. The correlation between infectious disease and average IQ was confirmed, and they concluded that the "evidence suggests that infectious disease is a primary cause of the global variation in human intelligence".
Tropical infectious diseases
Malaria affects 300–500 million persons each year, mostly children under age five in Africa, causing widespread anemia during a period of rapid brain development and also direct brain damage from cerebral malaria to which children are more vulnerable. A 2006 systematic review found that Plasmodium falciparum infection causes cognitive deficits in both the short- and long-term. Policies aimed at malaria reduction may have cognitive benefits. It has been suggested that the future economic and educational development of Africa critically depends on the eradication of malaria.
Roundworms infect hundreds of millions of people. There is evidence that high intensities of worms in the intestines can affect mental performance, but a systematic review in 2000 and a 2009 update found that there was insufficient evidence to show that deworming treatments improve cognitive performance or school performance in children.
Effects of other diseases
There are numerous diseases affecting the central nervous system which can cause cognitive impairment. Many of these are associated with aging. Some common examples include Alzheimer's disease and Multi-infarct dementia. Many diseases may be neurological or psychiatric and may primarily affect the brain. Others may affect many other organs, like HIV, Hashimoto's thyroiditis causing hypothyroidism, or cancer.
Major depression, affecting about 16% of the population on at least one occasion in their lives and the leading cause of disability in North America, may give symptoms similar to dementia. Patients treated for depression score higher on IQ tests than before treatment.
Myopia and hyperopia
A 2008 literature review writes that studies in several nations have found a relationship between myopia and higher IQ and between myopia and school achievement. Several, but not all, studies have found hyperopia to be associated with lower IQ and school achievements. A common explanation for myopia is near-work. Regarding the relationship to IQ, several explanations have been proposed. One is that the myopic child is better adapted at reading, and reads and studies more, which increases intelligence. The reverse explanation is that the intelligent and studious child reads more which causes myopia. Another is that the myopic child have an advantage at IQ testing which is near work because of less eye strain. Still another explanation is that pleiotropic gene(s) affect the size of both brain and eyes simultaneously. A study of Chinese schoolchildren found that after controlling for age, gender, school, parental myopia, father’s education, and books read per week, myopia was still associated with high nonverbal IQ. Nonverbal IQ was a more important explanation than books read per week.
Long working hours (55 vs. 40) was associated with decreased scores on cognitive tests in a 5-year study on midlife British civil servants.
- I. J. Deary. (2008). Why do intelligent people live longer? Nature, 456, 175-6. 10.1038/456175a
- Olness K (April 2003). "Effects on brain development leading to cognitive impairment: a worldwide epidemic". J Dev Behav Pediatr 24 (2): 120–30. doi:10.1097/00004703-200304000-00009. PMID 12692458.
- Perlmutter, David; Carol Colman (2006). Raise a Smarter Child By Kindergarten: Raise Iq Points By Up to 30 Points and Turn on Your Child's Smart Genes Points. Morgan Road Books. ISBN 978-0-7679-2301-9.
- Glewwe P, King EM (2001). "The Impact of Early Childhood Nutritional Status on Cognitive Development: Does the Timing of Malnutrition Matter?". World Bank Economic Review 15 (1): 81–113. doi:10.1093/wber/15.1.81.
- "Causes and consequences of intrauterine growth retardation. Proceedings of an IDECG Workshop. Baton Rouge, Louisiana, USA. November 11–15, 1996". Eur J Clin Nutr 52 (Suppl 1): S1–103. January 1998. PMID 9547065.[dead link] Neisser; et al. (August 7, 1995). "Intelligence: Knowns and Unknowns". Board of Scientific Affairs of the American Psychological Association. Retrieved August 6, 2006.
- Masters R (1997). "Brain biochemistry and social status: The neurotoxicity hypothesis". In White, Elliott. Intelligence, political inequality, and public policy. New York, N.Y: Prager. pp. 141–183. ISBN 0-275-95655-5.
- Der G, Batty GD, Deary IJ; Batty; Deary (November 2006). "Effect of breast feeding on intelligence in children: prospective study, sibling pairs analysis, and meta-analysis". BMJ 333 (7575): 945. doi:10.1136/bmj.38978.699583.55. PMC 1633819. PMID 17020911.
- Gómez-Sanchiz M, Cañete R, Rodero I, Baeza JE, González JA; Cañete; Rodero; Baeza; González (October 2004). "Influence of breast-feeding and parental intelligence on cognitive development in the 24-month-old child". Clin Pediatr (Phila) 43 (8): 753–61. doi:10.1177/000992280404300811. PMID 15494884.
- Baby's IQ Raised by Breastmilk and Genes
- Caspi A, Williams B, Kim-Cohen J, et al. (2007). "Moderation of breastfeeding effects on the IQ by genetic variation in fatty acid metabolism". Proceedings of the National Academy of Sciences 104 (47): 18860–5. doi:10.1073/pnas.0704292104. PMC 2141867. PMID 17984066.
- N. W. Martin, B. Benyamin, N. K. Hansell, G. W. Montgomery, N. G. Martin, M. J. Wright and T. C. Bates. (2011). Cognitive function in adolescence: testing for interactions between breast-feeding and FADS2 polymorphisms. Journal of the American Academy of Child and Adolescent Psychiatry, 50, 55-62 e4. 10.1016/j.jaac.2010.10.010
- Steer CD, Davey Smith G, Emmett PM, Hibbeln JR, Golding J (2010). Penha-Goncalves, Carlos, ed. "FADS2 polymorphisms modify the effect of breastfeeding on child IQ". PLoS ONE 5 (7): e11570. doi:10.1371/journal.pone.0011570. PMC 2903485. PMID 20644632.
No summary available
- Ivanovic DM, Leiva BP, Pérez HT, et al. (January 2002). "Nutritional status, brain development and scholastic achievement of Chilean high-school graduates from high and low intellectual quotient and socio-economic status". Br. J. Nutr. 87 (1): 81–92. doi:10.1079/BJN2001485. PMID 11895316.
- Ivanovic DM, Leiva BP, Pérez HT, et al. (2004). "Head size and intelligence, learning, nutritional status and brain development. Head, IQ, learning, nutrition and brain". Neuropsychologia 42 (8): 1118–31. doi:10.1016/j.neuropsychologia.2003.11.022. PMID 15093150.
- Qian M, Wang D, Watkins WE, et al. (2005). "The effects of iodine on intelligence in children: a meta-analysis of studies conducted in China". Asia Pacific Journal of Clinical Nutrition 14 (1): 32–42. PMID 15734706.
- Behrman, J.R., Alderman, H., and Hoddinott, J., "Hunger and Malnutrition," Copenhagen Consensus 2004.
- UNICEF and The Micronutrient Initiative, "Vitamin & Mineral Deficiency: A Global Progress Report," March 2004.
- Saloojee H, Pettifor JM (December 2001). "Iron deficiency and impaired child development". BMJ 323 (7326): 1377–8. doi:10.1136/bmj.323.7326.1377. PMC 1121846. PMID 11744547.
- FOOD FORTIFICATION TECHNOLOGY Food Fortification: Technology and Quality Control. (FAO Food And Nutrition Paper - 60)
- Schoenthaler SJ, Bier ID, Young K, Nichols D, Jansenns S (February 2000). "The effect of vitamin-mineral supplementation on the intelligence of American schoolchildren: a randomized, double-blind placebo-controlled trial". J Altern Complement Med 6 (1): 19–29. doi:10.1089/acm.2000.6.19. PMID 10706232.
- Duthie SJ, Whalley LJ, Collins AR, Leaper S, Berger K, Deary IJ (May 2002). "Homocysteine, B vitamin status, and cognitive function in the elderly". Am. J. Clin. Nutr. 75 (5): 908–13. PMID 11976166. Erratum in: <Please add first missing authors to populate metadata.> (February 2003). Am. J. Clin. Nutr. 77 (2): 523. Missing or empty
- Helland IB, Smith L, Saarem K, Saugstad OD, Drevon CA (January 2003). "Maternal supplementation with very-long-chain n-3 fatty acids during pregnancy and lactation augments children's IQ at 4 years of age". Pediatrics 111 (1): e39–44. doi:10.1542/peds.111.1.e39. PMID 12509593.
- Zimmer, Carl (September 17, 2015). "Inuit Study Adds Twist to Omega-3 Fatty Acids’ Health Story". New York Times. Retrieved October 11, 2015.
- O'Connor, Anahad (March 30, 2015). "Fish Oil Claims Not Supported by Research". New York Times. Retrieved October 11, 2015.
- Grey, Andrew; Bolland, Mark (March 2014). "Clinical Trial Evidence and Use of Fish Oil Supplements". JAMA Internal Medicine 174 (3): 460–462. doi:10.1001/jamainternmed.2013.12765. Retrieved October 11, 2015.
- Lyketsos CG (May 2003). "Should pregnant women avoid eating fish? Lessons from the Seychelles". Lancet 361 (9370): 1667–8. doi:10.1016/S0140-6736(03)13379-X. PMID 12767728.
- Pregnant Women: Eat More Fish or Not? - To Your Health - MSNBC.com
- Fish Diet in Pregnancy May Hone Kids' IQ
- "The omega point". The Economist. January 19, 2006.
- Medical News: Eating Fish During Pregnancy Provides 'Brain Food' for Child - in OB/GYN, Pregnancy from MedPage Today
- Pregnant? Omega-3 Essential for Baby's Brain
- Pollitt E, Gorman KS, Engle PL, Martorell R, Rivera J (1993). "Early supplementary feeding and cognition: effects over two decades". Monogr Soc Res Child Dev (Monographs of the Society for Research in Child Development, Vol. 58, No. 7) 58 (7): 1–99; discussion 111–8. doi:10.2307/1166162. JSTOR 1166162. PMID 8272081.
- Stein AD, Behrman JR, DiGirolamo A, et al. (June 2005). "Schooling, educational achievement, and cognitive functioning among young Guatemalan adults". Food Nutr Bull 26 (2 Suppl 1): S46–54. PMID 16060211.
- Children's Health: Stunting in children under 5-moderate and severe
- Stunted growth affects almost 40 percent of the developing world's infants, Cornell study reports
- Effects of early childhood psychosocial stimulation and nutritional supplementation on cognition and education in growth-stunted Jamaican children: prospective cohort study Lancet (British edition), 2005 (Vol. 966) (No. 9499) 1804-1807. Walker, S. P., Chang, S. M., Powell, C. A., Grantham-McGregor, S. M.
- Walker SP, Chang SM, Powell CA, Simonoff E, Grantham-McGregor SM (September 2006). "Effects of psychosocial stimulation and dietary supplementation in early childhood on psychosocial functioning in late adolescence: follow-up of randomised controlled trial". BMJ 333 (7566): 472. doi:10.1136/bmj.38897.555208.2F. PMC 1557928. PMID 16877454.
- Grandjean P, Landrigan PJ (December 2006). "Developmental neurotoxicity of industrial chemicals". Lancet 368 (9553): 2167–78. doi:10.1016/S0140-6736(06)69665-7. PMID 17174709.
- Meyer PA, McGeehin MA, Falk H (August 2003). "A global approach to childhood lead poisoning prevention". Int J Hyg Environ Health 206 (4–5): 363–9. doi:10.1078/1438-4639-00232. PMID 12971691.
- Bellinger DC, Stiles KM, Needleman HL (December 1992). "Low-level lead exposure, intelligence and academic achievement: a long-term follow-up study". Pediatrics 90 (6): 855–61. PMID 1437425.
- Ribas-Fitó N, Torrent M, Carrizo D, et al. (November 2006). "In utero exposure to background concentrations of DDT and cognitive functioning among preschoolers". Am. J. Epidemiol. 164 (10): 955–62. doi:10.1093/aje/kwj299. PMID 16968864.
- Grandjean P, Landrigan PJ (December 2006). "Developmental neurotoxicity of industrial chemicals". Lancet 368 (9553): 2167–78. doi:10.1016/S0140-6736(06)69665-7. PMID 17174709.
- Potentials for exposure to industrial chemicals suspected of causing developmental neurotoxicity Philippe Grandjean, MD, PhD, Adjunct Professor Marian Perez, MPH, Project Coordinator Department of Environmental Health, Harvard School of Public Health, Boston, MA, USA
- Abel EL, Sokol RJ (January 1987). "Incidence of fetal alcohol syndrome and economic impact of FAS-related anomalies". Drug Alcohol Depend 19 (1): 51–70. doi:10.1016/0376-8716(87)90087-1. PMID 3545731.
- Nature Journal. August 2012 and 30mt Radio discussion on Radio NZ with authors
- Iversen L (February 2005). "Long-term effects of exposure to cannabis". Curr Opin Pharmacol 5 (1): 69–72. doi:10.1016/j.coph.2004.08.010. PMID 15661628.
- Weitzman M, Byrd RS, Aligne CA, Moss M (2002). "The effects of tobacco exposure on children's behavioral and cognitive functioning: implications for clinical and public health policy and future research". Neurotoxicol Teratol 24 (3): 397–406. doi:10.1016/S0892-0362(02)00201-5. PMID 12009494.
- Breslau N, Paneth N, Lucia VC, Paneth-Pollak R (October 2005). "Maternal smoking during pregnancy and offspring IQ". Int J Epidemiol 34 (5): 1047–53. doi:10.1093/ije/dyi163. PMID 16085682.
- Yolton K, Dietrich K, Auinger P, Lanphear BP, Hornung R (January 2005). "Exposure to environmental tobacco smoke and cognitive abilities among U.S. children and adolescents". Environ. Health Perspect. 113 (1): 98–103. doi:10.1289/ehp.7210. PMC 1253717. PMID 15626655.
- Blair C (April 2006). "How similar are fluid cognition and general intelligence? A developmental neuroscience perspective on fluid cognition as an aspect of human cognitive ability". Behav Brain Sci 29 (2): 109–25; discussion 125–60. doi:10.1017/S0140525X06009034. PMID 16606477. Multiple comments can be seen on Google Scholar.
- Delaney-Black V, Covington C, Ondersma SJ, et al. (March 2002). "Violence exposure, trauma, and IQ and/or reading deficits among urban children". Arch Pediatr Adolesc Med 156 (3): 280–5. doi:10.1001/archpedi.156.3.280 (inactive 2015-01-01). PMID 11876674.
- Saltzman KM, Weems CF, Carrion VG (2006). "IQ and posttraumatic stress symptoms in children exposed to interpersonal violence". Child Psychiatry Hum Dev 36 (3): 261–72. doi:10.1007/s10578-005-0002-5. PMID 16362242.
- Gonzales NA, Cauce AM, Friedman RJ, Mason CA (June 1996). "Family, peer, and neighborhood influences on academic achievement among African-American adolescents: one-year prospective effects". Am J Community Psychol 24 (3): 365–87. doi:10.1007/BF02512027. PMID 8864209.
- MacKenzie, Debora. "Link found between infectious disease and IQ". New Scientist. Retrieved 4 July 2010.
- Eppig, Christopher (2011). "Why Is Average IQ Higher in Some Places?". Scientific American.
- Boivin MJ (October 2002). "Effects of early cerebral malaria on cognitive ability in Senegalese children". J Dev Behav Pediatr 23 (5): 353–64. doi:10.1097/00004703-200210000-00010. PMID 12394524.
- Kihara, Michael; Carter, J. A.; Newton, C. R. J. C. (2006). "The effect of Plasmodium falciparum on cognition: a systematic review". Tropical Medicine & International Health 11 (4): 386–97. doi:10.1111/j.1365-3156.2006.01579.x.
- Watkins WE, Pollitt E (March 1997). ""Stupidity or worms": do intestinal worms impair mental performance?". Psychol Bull 121 (2): 171–91. doi:10.1037/0033-2909.121.2.171. PMID 9100486.
- Dickson, Rumona; Shally Awasthi; Paula Williamson; Colin Demellweek; Paul Garner (2000). "Effects of treatment for intestinal helminth infection on growth and cognitive performance in children: systematic review of randomised trials". BMJ 320 (7251): 1697–1701. doi:10.1136/bmj.320.7251.1697. PMC 27412. PMID 10864543.
- Taylor-Robinson, David; Ashley Jones; Paul Garner (2009). Yamey, Gavin, ed. "Does Deworming Improve Growth and School Performance in Children?". PLoS Neglected Tropical Diseases 3 (1): e358. doi:10.1371/journal.pntd.0000358.
- Abubakar, Amina; Anneloes Van Baar, Fons J. R. Van de Vijver, Penny Holding, Charles R. J. C. Newton (2008). "Paediatric HIV and neurodevelopment in sub-Saharan Africa: a systematic review". Tropical Medicine & International Health 13 (7): 880–7. doi:10.1111/j.1365-3156.2008.02079.x.
- Sackeim HA, Freeman J, McElhiney M, Coleman E, Prudic J, Devanand DP; Freeman; McElhiney; Coleman; Prudic; Devanand (March 1992). "Effects of major depression on estimates of intelligence". J Clin Exp Neuropsychol 14 (2): 268–88. doi:10.1080/01688639208402828. PMID 1572949.
- Mandelli L, Serretti A, Colombo C, et al. (October 2006). "Improvement of cognitive functioning in mood disorder patients with depressive symptomatic recovery during treatment: an exploratory analysis". Psychiatry Clin. Neurosci. 60 (5): 598–604. doi:10.1111/j.1440-1819.2006.01564.x. PMID 16958944.
- Czepita, D.; Lodygowska, E.; Czepita, M. (2008). "Are children with myopia more intelligent? A literature review". Annales Academiae Medicae Stetinensis 54 (1): 13–16; discussion 16. PMID 19127804.
- Saw, S. -M.; Tan, S. -B.; Fung, D.; Chia, K. -S.; Koh, D.; Tan, D. T. H.; Stone, R. A. (2004). "IQ and the Association with Myopia in Children". Investigative Ophthalmology & Visual Science 45 (9): 2943–2948. doi:10.1167/iovs.03-1296.
- Virtanen M., A. Singh-Manoux, J.E. Ferrie, D. Gimeno, M.G. Marmot, M. Elovainio, M. Jokela, J. Vahtera, and M. Kivimäki (2009). "Long Working Hours and Cognitive Function: The Whitehall II Study". American Journal of Epidemiology 169 (5): 596–605. doi:10.1093/aje/kwn382. PMC 2727184. PMID 19126590.