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.
[1] 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.
[2][3] Improvements in nutrition (often involving specific micronutrients) due to 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.[4] Malnutrition may occur during several periods of growth, such as pregnancy, during breastfeeding, infancy, or childhood.
While it is debatable whether or not as an infant or after two years is the worst time for malnourishment, the bottom line in these studies is that not having enough nutrients at a young age negatively effects learning.
[3][7] It has been proposed that omega-3 fatty acids in breast milk, 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.
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."
[12] Two studies in Chile on 18-year-old high-school graduates found that nutritional status during the first year of life affected IQ, scholastic achievement, and brain volume.
[15] 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.
[17] 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.
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.
[22] 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.
However, foods containing high amounts of mercury, such as shark, swordfish, king mackerel and tilefish, might cause mental retardation.
[23][24][25][26][27][28] 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.
[29] 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.
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.
[37] Maternal tobacco smoking during pregnancy is associated with increased activity, decreased attention, and diminished intellectual abilities.
The study controlled for sex, race, region, poverty, parent education and marital status, ferritin, and blood lead concentration.
[44] 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.
[2] A recent theory suggests that early childhood stress may affect the developing brain and cause negative effects.
The study reported that exposure to violence and trauma-related distress in young children were associated with substantial decrements in IQ and reading achievement.
[50] 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.
[52] Christopher Hassall and Thomas Sherratt repeated the analysis, and concluded "that infectious disease may be the only really important predictor of average national IQ".
According to a 2015 report in The American Scholar, an assortment of neglected tropical diseases as well as some recently identified pathogens such as Pseudo-nitzschia have also been found to erode human intelligence.
[59] 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.
[63] Long working hours (55 vs. 40) was associated with decreased scores on cognitive tests in a 5-year study on midlife British civil servants.