Intelligence as longevity predictor
Inverse correlation between intelligence, mortality and morbidity
People differ with respect to the efficiency with which their brains operate, and this is the domain of psychologists interested in intelligence differences, showing considerable stability across the life course.
The general rule is that people who do well on one type of mental task tend to do well on all of the others. This is the phenomenon known as general intelligence—or general mental ability.
There are separable domains of cognitive ability—such as reasoning, spatial ability, memory, processing speed, and vocabulary—but they are highly correlated.
With respect to its effects on health, it is general mental ability that seems to be the important factor.
It has been known for many years that intelligence—especially general intelligence— strongly predicts people’s success at work, in education, and in their social lives; and in everyday practical decision making.
General intelligence declines with age in the cognitive domains of memory and processing speed.
These are aspects of fluid intelligence – active engagement with novel and abstract information under time pressure. On the other hand, crystallized intelligence, formed by knowledge and experience, shows little age-related decline.
Cognitive epidemiology examines the associations between intelligence test scores and health. It seeks to understand causal relationships between intelligence and health outcomes.
A strong inverse correlation between early life intelligence and mortality has been shown across different populations, in different countries, and in different epochs.
There is also link between intelligence and adult morbidity. Long term sickness or disability pension in adulthood has been shown to be related to lower cognitive abilities.
Well-characterized cohort studies typically reveal an association between low intelligence test scores and the risk of hospital admission for any psychological disorder by middle age.
Among the findings of cognitive epidemiology is that men with a higher IQ have less risk of dying from coronary heart disease. One study found that low verbal, visuospatial and arithmetic scores were particularly good predictors of coronary heart disease. Atherosclerosis or thickening of the artery walls due to fatty substances is a major factor in heart disease and some forms of stroke. It has also been linked to lower IQ.
Lower intelligence in childhood and adolescence correlates with an increased risk of obesity.
Higher intelligence in childhood and adulthood has been linked to lower blood pressure and a lower risk of hypertension.
Strong evidence has been found in support of a link between intelligence and stroke, with those with lower intelligence being at greater risk of stroke.
A New Zealand study of 1,037 males and females suggests that lower childhood intelligence scores were associated with an increased risk of developing schizophrenia spectrum disorders, major depression, and generalized anxiety disorder in adulthood; whereas higher childhood IQ predicted an increased likelihood of mania. These findings suggest that men of extremely high intelligence are at a higher risk of experiencing bipolar in its purest form. Bipolar disorder is characterized by periods of immense pain and suffering, self-destructive behaviours, and has one of the highest mortality rates of all mental illnesses.
A decrease in IQ has also been shown as an early predictor of late-onset Alzheimer’s Disease. Researchers 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. The investigators reported that lower childhood intelligence was a risk factor for late-onset vascular dementia, but not Alzheimer’s-type dementia, suggesting that vascular processes are likely mediators in the pathway between early life intelligence and later cognitive decline. This is consistent with an inverse association of intelligence and later cardiovascular disease, coronary heart disease and stroke, both of which have vascular origins.
The System integrity hypothesis posits that childhood intelligence is just one aspect of a well wired and well-functioning body and suggests that there is a latent trait that encompasses intelligence, health and many other factors. According to the system integrity hypothesis lower IQ does not cause mortality but instead poor system integrity causes lower intelligence and poorer health.
The model of sexual selection is proposed by Geoffrey Miller who argues that human intelligence is unnecessarily sophisticated for the needs of hunter-gatherers to survive. He argues that the manifestations of intelligence such as language, music and art did not evolve because of their utilitarian value to the survival of ancient hominids. Rather, intelligence may have been a fitness indicator. Hominids would have been chosen for greater intelligence as an indicator of healthy genes.
Sexual selection for intelligence and judging ability can act on indicators of success, such as highly visible displays of wealth.
When people search for mates based on their success, wealth, reputation, disease-free body appearance, or psychological traits such as benevolence or confidence; the effect is to select for superior intelligence that results in superior disease resistance.