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. stress further information: psychological trauma, historical trauma, and neuroplasticity 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. infectious diseases 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) harvtxt error: no target: citerefeppigfincherthornhill2010 (help) 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. hiv infection in children in sub-saharan africa affects their motor development, but there is insufficient evidence to show a slowing of language development. 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. 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. 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 has 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. other associations long working hours (55 vs. 40) was associated with decreased scores on cognitive tests in a 5-year study on midlife british civil servants.