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.
A study of one million Swedish men found "a strong link between cognitive ability and the risk of death."
A similar study of 4,289 former US soldiers showed a similar relationship between IQ and mortality.
The strong inverse correlation between intelligence and mortality has raised questions as to how better public education could delay mortality.
There is a known positive 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.
Another Scottish study found that once individuals had reached old age (79 in this study), it was no longer childhood intelligence or current intelligence scores that best predicted mortality but the relative decline in cognitive abilities from age 11 to age 79. They also found that fluid abilities were better predictors of survival in old age than crystallized abilities.
The relationship between childhood intelligence and mortality has even been found to hold for gifted children, those with an intelligence over 135. A 15-point increase in intelligence was associated with a decreased risk of mortality of 32%. This relationship was present until an intelligence score of 163 at which point there was no further advantage of a higher intelligence on mortality risk.
A meta-analysis of the relationship between intelligence and mortality found that there was a 24% increase in mortality for a 1SD (15 point) drop in IQ score. This meta-analysis also concluded that the association between intelligence and mortality was similar for men and women despite sex differences in disease prevalence and life expectancies.
A whole population follow-up over 68 years showed that the association with overall mortality was also present for most major causes of death. The exceptions were cancers unrelated to smoking and suicide.
There is also a strong inverse correlation between intelligence and adult morbidity. Long term sick leave in adulthood has been shown to be related to lower cognitive abilities, as has likelihood of receiving a disability pension.
Physical illness 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. 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.
Obesity Lower intelligence in childhood and adolescence correlates with an increased risk of obesity. One study found that a 15-point increase in intelligence score was associated with a 24% decrease in risk of obesity at age 51. The direction of this relationship has been greatly debated with some arguing that obesity causes lower intelligence, however, recent studies have indicated that a lower intelligence increases the chances of obesity.
Blood pressure Higher intelligence in childhood and adulthood has been linked to lower blood pressure and a lower risk of hypertension.
Stroke 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. One study found visuospatial reasoning was the best predictor of stroke compared to other cognitive tests. Further this study found that controlling for socioeconomic variables did little to attenuate the relationship between visuospatial reasoning and stroke.
Cancer Studies exploring the link between cancer and intelligence have come to varying conclusions. A few studies, which were mostly small have found an increased risk of death from cancer in those with lower intelligence. Other studies have found an increased risk of skin cancer with higher intelligence. However, on the whole most studies have found no consistent link between cancer and intelligence.
Psychiatric Bipolar disorder and intelligence Bipolar disorder is a mood disorder characterized by periods of elevated mood known as mania or hypomania and periods of depression. Anecdotal and biographical evidence popularized the idea that those with bipolar disorder are tormented geniuses that are uniquely equipped with high levels of creativity and superior intelligence. Bipolar disorder is relatively rare, affecting only 2.5% of the population, as it is also the case with especially high intelligence. The uncommon nature of the disorder and rarity of high IQ pose unique challenges in sourcing large enough samples that are required to conduct a rigorous analysis of the association between intelligence and bipolar disorder. Nevertheless, there has been much progress starting from the mid-90s, with several studies beginning to shed a light on this elusive relationship.
One such study examined individual compulsory school grades of Swedish pupils between the ages of 15 and 16 to find that individuals with excellent school performance had a nearly four times increased rate to develop a variation of bipolar disorder later in life than those with average grades. The same study also found that students with lowest grades were at a moderately increased risk of developing bipolar disorder with nearly a twofold increase when compared to average-grade students.
A New Zealand study of 1,037 males and females from the 1972–1973 birth cohort of Dunedin suggests that lower childhood IQs 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. This study only included eight cases of mania and thus should only be used to support already existing trends.
In the largest study yet published analyzing the relationship between bipolar disorder and intelligence, Edinburgh University researchers looked at the link between intelligence and bipolar disorder in a sample of over one million men enlisted in the Swedish army during a 22-year follow-up period. Regression results showed that the risk of hospitalization for bipolar disorder with comorbidity to other mental health illnesses decreased in a linear pattern with an increase in IQ. However, when researchers restricted the analysis to men without any psychiatric comorbidity, the relationship between bipolar disorder and intelligence followed a J-curve. These findings suggest that men of extremely high intelligence are at a higher risk of experiencing bipolar in its purest form, and demands future investigation of the correlation between extreme brightness and pure bipolar.
Additional support of a potential association between high intelligence and bipolar disorder comes from biographical and anecdotal evidence, and primarily focus on the relationship between creativity and bipolar disorder. Doctor Kay Redfield Jamison has been a prolific writer on the subject publishing several articles and an extensive book analyzing the relationship between the artistic temperament and mood disorders. Although a link between bipolar disorder and creativity has been established, there is no confirming evidence suggesting any significant relationship between creativity and intelligence. Additionally, even though some of these studies suggest a potential benefit to bipolar disorder in regards to intelligence, there is significant amount of controversy as to the individual and societal cost of this presumed intellectual advantage. Bipolar disorder is characterized by periods of immense pain and suffering, self-destructive behaviors, and has one of the highest mortality rates of all mental illnesses.
Schizophrenia and cognition Schizophrenia is chronic and disabling mental illness that is characterized by abnormal behavior, psychotic episodes and inability to distinguish between reality and fantasy. Even though schizophrenia can severely impair those with the disorder, there has been a great interest in the relationship of this disorder and intelligence. Interest in the association of intelligence and schizophrenia has been widespread partly stems from the perceived connection between schizophrenia and creativity, and posthumous research of famous intellectuals that have been insinuated to have had the illness. Hollywood played a pivotal role popularizing the myth of the schizophrenic genius with the movie A Beautiful Mind that depicted the life story of Nobel Laureate, John Nash and his struggle with the illness.
Although stories of extremely bright individuals with schizophrenia such as that of John Nash do exist, they are the outliers and not the norm. Studies analyzing the association between schizophrenia and intelligence overwhelmingly suggest that schizophrenia is linked to lower intelligence and decreased cognitive functioning. Since the manifestation of schizophrenia is partly characterized by cognitive and motor declines, current research focuses on understanding premorbid IQ patterns of schizophrenia patients.
In the most comprehensive meta-analysis published since the groundbreaking study by Aylward et al. in 1984, researchers at Harvard University found a medium-sized deficit in global cognition prior to the onset of schizophrenia. The mean premorbid IQ estimate for schizophrenia samples was 94.7 or 0.35 standard deviations below the mean, and thus at the lower end of the average IQ range. Additionally, all studies containing reliable premorbid and post-onset IQ estimates of schizophrenia patients found significant decline in IQ scores when comparing premorbid IQ to post-onset IQ. However, while the decline in IQ over the course of the onset of schizophrenia is consistent with theory, some alternative explanations for this decline suggested by the researchers include the clinical state of the patients and/or side effects of antipsychotic medications.