This monograph describes research findings linking intelligence and personality traits with health outcomes, including health behaviors, morbidity, and mortality. The field of study of intelligence and health outcomes is called cognitive epidemiology, and the field of study of personality traits and health outcomes is known as personological epidemiology. Intelligence and personality traits are the principal research topics studied by differential psychologists, so the combined field could be called differential epidemiology. This research is important for the following reasons: The findings overviewed are relatively new, and many researchers and practitioners are unaware of them; the effect sizes are on par with better-known, traditional risk factors for illness and death; mechanisms of the associations are largely unknown, so they must be explored further; and the findings have yet to be applied, so we write this to encourage diverse interested parties to consider how applications might be achieved.
To make this research accessible to as many relevant researchers, practitioners, policymakers, and laypersons as possible, we first provide an overview of the basic discoveries regarding intelligence and personality. We describe the nature and structure of the measured phenotypes (i.e., the observable characteristics of an individual) in both fields. Although both areas of study are well established, we recognize that this may not be common knowledge outside of experts in the field. Human intelligence differences are described by a hierarchy that includes general intelligence (g) at the pinnacle, strongly correlated broad domains of cognitive functioning at a lower level, and specific abilities at the foot. The major human differences in personality are described by five personality factors that are widely agreed on with respect to their number and nature: neuroticism, extraversion, openness, agreeableness, and conscientiousness. As a foundation for health-related findings, we provide a summary of research showing that intelligence and personality differences can be measured reliably and validly and are stable across many years (even decades), substantially heritable, and related to important life outcomes. Cognitive and personality traits are fundamental aspects of a person, and they have relevance to life chances and outcomes, including health outcomes.
We provide an overview of major and recent research on the associations between intelligence and personality traits and health outcomes. These outcomes include mortality from all causes, specific causes of death, specific illnesses, and others, such as health-related behaviors. Intelligence and personality traits are significantly and substantially (by comparison with traditional risk factors) related to all of these outcomes. The studies we describe are unusual in psychology: They have large sample sizes (typically thousands of subjects, sometimes ~1 million), the samples are more representative of the background population than in most studies, the follow-up times are long (sometimes many decades, almost the whole human life span), and the outcomes are objective health measures (including death), not just self-reports. In addition to the associations, possible mechanisms for the associations are described and discussed, and some attempts to test these mechanisms are illustrated. It is relatively early in this research field, so a significant amount of work remains to be done.
Finally, we make some preliminary remarks about possible applications, with the knowledge that the psychological predictors addressed are somewhat stable aspects of the person, with substantial genetic causes. Nevertheless, we believe differential epidemiology can be a useful component of interventions to improve individual and public health. Intelligence and personality differences are possible causes of later health inequalities; the eventual aim of cognitive and personological epidemiology is to reduce or eliminate these inequalities, to the extent that it is possible, and provide information to help people toward their own optimal health through the life course. We present these findings to a wider audience so that more associations will be explored, a better understanding of the mechanisms of health inequalities will be produced, and inventive applications will follow on the basis of what we hope will be seen as practically useful knowledge.