How intelligence and education contribute to substance use: Hints from the Minnesota Twin family study

Abstract

In old and even middle age, there are associations between physical health and both intelligence and education. This may occur because intelligence and/or education exert effects on lifestyle choices that, in turn, affect later health. Substance use is one aspect of lifestyle choice in young adulthood that could play such a role. The effects of intelligence and/or education on substance use could be direct and environmental, or indirect due to the presence of confounding genetic and shared family influences. We used the Minnesota Twin Family Study to distinguish these effects in males and females at age 24. In contrast to prevailing expectations, there were moderately negative direct nonshared environmental effects of both IQ and education on both smoking and drinking in both males and females. That is, controlling for family background effects in the form of both genetic and shared environmental influences, both higher IQ and greater education were associated with greater alcohol and nicotine use. These effects were accounted for by alcohol and nicotine use at age 17. Our results suggest that genetic and family-culture variables confound the associations between intelligence and education and substance use in young adults, rendering them indirect. Further research is needed to understand the roles of IQ and education in alcohol and nicotine use and their relative impacts on physical health throughout the lifespan.

Introduction

In middle and old age, there are associations between physical health and both intelligence and education (Carlson et al., 1999, Deary et al., 2006, Gottfredson and Deary, 2004, Malmstrom et al., 2005, Tabbarah et al., 2002), but how these associations develop is not understood. Physical health problems are major sources of economic expense, anxiety, and deterioration in quality of life. Understanding the ways in which intelligence and education contribute to physical health would be an important epidemiological advance, and might lead to new approaches to help people better manage their own health, leading to more effective disease prevention and reduced medical care costs.

One possible reason for the associations between physical health and intelligence and education is that intelligence and/or education may act to support lifestyle choices and the development of habits that over time maintain or undermine physical health. Education is generally the variable more readily available to researchers, and many epidemiological studies include adjustment for it as a possible confounding, causally prior environmental factor in studies of the determinants of health outcomes. There is, however, evidence that intelligence functions in this manner as well and may lie earlier on the causal pathways (Gottfredson, 2004, Hart et al., 2003). The effects of the two variables are rarely compared head to head in epidemiological studies. Moreover, if intelligence and/or education do have their effects on physical health through the emergence of lifestyle choices and habits such as smoking and drinking that maintain or undermine health in later life, we should see their effects on these choices and habits in earlier life. Few studies have examined the influences of intelligence or education on specific lifestyle behaviors such as smoking and drinking in younger adults that might contribute to their later physical health. One study that did, however, found an association between higher IQ and reduced smoking behavior in young adults that was attenuated by education (Batty, Deary, Schoon, & Gale, 2007).

The authors of this study (Batty et al., 2007) were careful not to draw causal conclusions about the nature of the association between earlier measured intelligence and later smoking behavior, but they did note that the range of other risk factors for which they were able to control helped to establish the specificity of the association to IQ. They also pointed out that adjusting for education may have overcorrected since IQ also contributes to educational attainment. But however long, any list of statistical controls is necessarily arbitrary and incomplete. There is always the possibility that some unmeasured variable confounds the association. For example, Batty et al. (2007) controlled for parental and participant social class and participant annual gross earnings. The operative variable, however, could actually be a personality trait like conscientiousness, which could lead to more careful completion of the survey's IQ test and thus higher scores. Conscientiousness has also been associated with both better health-related behaviors and higher educational attainment (Roberts, Kuncel, Shiner, Caspi, & Goldberg, 2007).

The variables at issue here, particularly IQ but also education, suggest an additional potentially confounding complication. Traditional epidemiological studies are incapable of controlling for genetic or family cultural selection. This is the tendency for genetic or family cultural influences on one trait to influence another as well, so that the association between risk and outcome reflects at least some origin of the risk in the individual rather than merely effect of the risk in its own right (Rutter, 2007). IQ is well known to be subject to such familial influences, particularly genetic ones (e.g., Bouchard & McGue, 1981), as are health-related behaviors such as substance use (e.g., Prescott, Madden, & Stallings, 2006) and physical health itself (e.g., Reed & Dick, 2003). But education is subject to these kinds of familial influences as well (e.g., Behrman et al., 1995, Johnson et al., submitted for publication). We are not aware of any studies investigating whether these familial influences on IQ and/or education overlap with those on health-related behaviors such as smoking and drinking, but evidence that intelligence and/or education may be involved in lifestyle choices that affect physical health suggests that they might be. To the extent that they are, we would not consider the effects of intelligence and/or education on health-related behaviors to be direct because these effects would not hold up after controlling for the genetic and family cultural variables that distinguish among but not within families; instead these effects would be indirect because confounded by genetic and shared environmental familial selection processes.¹ It makes sense to distinguish direct from indirect familial selection effects to the extent possible, as the policy options for dealing with them differ.

It is thus important that epidemiological research include study designs capable of distinguishing direct effects of intelligence and/or education on health-related behaviors from indirect effects operating through familial selection processes. Behavior genetic twin designs can provide an important element of quasi-experimental control because, to the extent that monozygotic twins differ in intelligence and/or education, associated differences in their health-related behaviors such as smoking and drinking cannot be attributed either to genetic confounds or to any aspect of the family environment that they share such as socioeconomic status, family structure, or family relationships, whether such variables are assessed in the analysis or not. It is still not possible to draw strictly causal conclusions of course, as even differences within pairs may result from factors lther than those contributing to the variables actually considered, but the range of such possibilities is much more limited. In addition, articulation of the kinds of selection processes involved in common genetic and/or family environmental influences can help to develop hypotheses about the specific influences they entail.

In young adulthood, smoking and drinking behaviors may be particularly good examples of the kinds of lifestyle choices and personal habits that may affect later physical health outcomes. Smoking is a well-known risk factor for a host of cardiovascular and respiratory diseases, and excessive alcohol consumption also contributes to later health problems. Moreover, the disinhibitory behavior involved in smoking and drinking behaviors may indicate a lack of personal conscientiousness that may be associated with other lifestyle choices that impact later health. Before any effects of smoking and drinking on physical health can be manifest, however, the choices to drink to excess and to smoke must be made and the habits of doing so instilled. In this study, we focused on the emergence of these health-related behavior choices and habits in young adulthood. We explored the associations between intelligence and education and alcohol use and smoking in young adult male and female twin samples in order to distinguish direct environmental effects of education and intelligence from indirect effects involving genetic and family environmental selection processes. Our study design also allowed us to distinguish the effects of intelligence from those of education.