Health and intelligence
Health can affect intelligence in various ways.
There is a well-known association between lower socioeconomic status and poorer health. Socioeconomic status is usually measured by factors such as social class, education, income, and residential area. However, after controlling for IQ this association was markedly reduced.
Similarly, many of the differences in overall health between different races that are usually blamed on socioeconomic differences and discrimination may actually to a large degree be due to race and intelligence differences.
Overall mortality and morbidity
"First, ...intelligence is associated with more education, and thereafter with more professional occupations
that might place the person in healthier environments. ...Second, people with higher intelligence might engage in more healthy behaviours. ...Third, mental test scores from early life might act as a record of insults to the brain that have occurred before that date. ...Fourth, mental test scores obtained in youth might be an indicator of a well-put-together system. It is hypothesized that a well-wired body is more able to respond effectively to environmental insults..."
A study in Scotland found that 15-point lower IQ meant people had a fifth less chance of seeing their 76th birthday, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long.
One study found an "inverse linear association" between childhood intelligence and hospital admissions for injuries in adulthood. The association between childhood IQ and the risk of later injury remained after accounting for factors such as the child's socioeconomic background.
Mental ability scores were significantly lower in children who eventually developed late-onset dementia when compared with other children tested. Blacks in the US and the UK have a higher prevalence of dementia.
A decrease in IQ has also been shown as an early predictor of late-onset Alzheimer's Disease and other forms of dementia. A 2004 study found that tests of cognitive ability provided useful predictive information up to a decade before the onset of dementia. However, when diagnosing individuals with a higher level of cognitive ability,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.
Stress, such as caused by exposure to violence and psychological trauma, during childhood has been associated with lower IQ. It is unclear if it is the stress that causes the lower IQ or if having lower IQ and lower IQ parents causes increased risk of exposure to violence and psychological trauma. High IQ has also been found to protect post-traumatic stress disorder and behavorial problems after a trauma.
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. According to the two most recent studies in the review, IQ may be associated with myopia in schoolchildren, independent of books read per week. Epidemiological ethnic studies show large differences in the prevalence of myopia, for example often very high prevalence among East Asian children, but the results have been argued to be inconsistent and largely reflect environmental factors.
Long working hours (55 vs. 40) was associated with reduced scores on cognitive tests in a 5-year study on midlife British civil servants.
Higher intelligence was associated with higher quality sperm in one study.
A large number of factors have been argued, with varying empirical support, to lower intelligence. Such factors may be particularly or only important during pregnancy and childhood when the brain is growing and the blood-brain barrier is less effective. Such impairment during development may sometimes be permanent and irreversible, sometimes be partially or wholly compensated for by later growth. Such factors include:
- Micronutrient deficiency, in particular iodine and iron deficiency.
- Protein-calorie malnutrition.
- Essential fatty acids malnutrition.
- Lack of breastfeeding.
- Intrauterine growth retardation.
- Untreated endocrine disorders in mother or child.
- Infectious diseases including parasites such as malaria, HIV, intestinal worms, and meningitis.
- Environmental toxins such as lead, mercury, arsenic, and organochlorides.
- Humans are being exposed a wide range of new chemicals, such as medicines, food additives, and industrial chemicals, with poorly understood long-term effects on intelligence, especially during childhood.
- Recreational drugs such as tobacco, alcohol, and cannabis.
- Birth complications such as asphyxia
- Head injuries
- Diseases related to aging such as stroke, cancer, and Alzheimer's disease.
- Other diseases such as endocrine disorders or depression.
Improvements of such factors is one of the proposed explanations for the Flynn effect and may play some part regarding countries and intelligence differences. However, developed countries have usually developed policies regarding many of these factors such as laws requiring fortification of foods with micronutrients, health controls of pregnant mothers and infants, eradication of malaria and other infectious diseases, and banning of environmental toxins. This makes such factors relatively less important as an explanation for race and intelligence differences in the same country.
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