Cognitive epidemiology is a field of research that examines the associations between intelligence test scores (IQ scores or extracted g-factors) and health, more specifically morbidity (mental and physical) and mortality. Typically, test scores are obtained at an early age, and compared to later morbidity and mortality. In addition to exploring and establishing these associations, cognitive epidemiology seeks to understand causal relationships between intelligence and health outcomes. Researchers in the field argue that intelligence measured at an early age is an important predictor of later health and mortality differences.
Overall mortality and morbidity
A strong inverse correlation between early life intelligence and mortality has been shown across different populations, in different countries, and in different epochs." Various explanations for these findings have been proposed:
"First, ...intelligence is associated with more education, and thereafter with more professional occupationsthat 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..."
People with higher IQ test scores tend to be less likely to smoke or drink alcohol heavily. They also eat better diets, and they are more physically active. So they have a range of better behaviours that may partly explain their lower mortality risk.—-Dr. David Batty
The strong correlation between intelligence and mortality has raised questions as to how better public education could delay mortality.
There is a known inverse correlation between socioeconomic position and health. A 2006 study found that controlling for IQ caused a marked reduction in this association.
Research in Scotland has shown that a 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.
Coronary heart disease
Among the findings of cognitive epidemiology is that men with a higher IQ have less risk of dying from coronary heart disease. The association is attenuated, but not removed, when controlling for socio-economic variables, such as educational attainment or income. This suggests that IQ may be an independent risk factor for mortality.
Post-Traumatic Stress Disorder, severe depression, and schizophrenia are less prevalent in higher IQ bands. On the other hand, higher IQ persons show a higher prevalence of Obsessive Compulsive Disorder.
The Archive of General Psychiatry published a longitudinal study of a randomly selected sample of 713 study participants (336 boys and 377 girls), from both urban and suburban settings. Of that group, nearly 76 percent had suffered through at least one traumatic event. Those participants were assessed at age 6 years and followed up to age 17 years. In that group of children, those with an IQ above 115 were significantly less likely to have Post-Traumatic Stress Disorder as a result of the trauma, less likely to display behavioral problems, and less likely to experience a trauma. The low incidence of Post-Traumatic Stress Disorder among children with higher IQs was true even if the child grew up in an urban environment (where trauma averaged three times the rate of the suburb), or had behavioral problems.
A decrease in IQ has also been shown as an early predictor of late-onset Alzheimer's Disease and other forms of dementia. In a 2004 study, Cervilla and colleagues showed that tests of cognitive ability provide useful predictive information up to a decade before the onset of dementia.
However, when diagnosing individuals with a higher level of cognitive ability, a study of those with IQ's of 120 or more, 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.
In 2000, Whalley and colleagues published a paper in the journal Neurology, which examined links between childhood mental ability and late-onset dementia. The study showed that mental ability scores were significantly lower in children who eventually developed late-onset dementia when compared with other children tested.
In terms of the effect of one's intelligence on health, in one British study, high childhood IQ was shown to correlate with one's chance of becoming a vegetarian in adulthood. In another British study, high childhood IQ was shown to inversely correlate with the chances of smoking.
A study of 11,282 individuals in Scotland who took intelligence tests at ages 7, 9 and 11 in the 1950s and 1960s, 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 even after accounting for factors such as the child's socioeconomic background.
Proposed general fitness factor of both cognitive ability and health, the f-factor
Because of the above mentioned findings, some researchers have proposed a general factor of fitness analogous to the g-factor for general mental ability/intelligence. This factor is supposed to combine fertility factors, health factors, and the g-factor. For instance, one study found a small but significant correlation between three measures of sperm quality and intelligence.
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