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Intelligence and Health Outcomes
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Intelligence and Health Outcomes

The importance of cognitive ability to disparities in human health is being overlooked.

· 9 min read

“Health disparities,” states the US Center for Disease Control and Prevention (CDC), “are differences in the incidence, prevalence, and mortality of a disease and the related adverse health conditions that exist among specific population groups. These groups may be characterized by gender, age, race or ethnicity, education, income, social class, disability, geographic location, or sexual orientation.” 

Research going back many years has found that socioeconomic status (SES)—based on educational attainment, income, and occupational status—has a strong association with health outcomes. Lower SES is associated with lower life expectancy, higher rates of morbidity from many diseases, higher overall mortality rates, and higher rates of infant and perinatal mortality.

Fundamental Cause Theory (FCT) was first proposed by Bruce Link of Columbia University and Jo Phalen of UCLA in 1995 as a way to explain disparities in health outcomes. Link and Phalen argued that:

[S]ocial factors such as socioeconomic status and social support are likely “fundamental causes” of disease that, because they embody access to important resources, affect multiple disease outcomes through multiple mechanisms, and consequently maintain an association with disease even when intervening mechanisms change.

They observed that populations with lower SES had greater morbidity and mortality rates during the era when infection was a common cause of disease and death. The relationship between lower SES and worse health outcomes has persisted, even as infectious causes of disease and death have been surpassed in recent years by chronic conditions such as heart disease, diabetes, and cancer. 

FCT is based on the assumption that populations with higher SES have more “flexible resources”at their disposal—“knowledge, money, power, prestige, and beneficial social connections”—to avoid many of the worse health outcomes common in lower SES populations. These flexible resources are considered in FCT to be a sort of “toolkit” that allows higher SES populations to have more favorable health outcomes, even when new diseases such as COVID-19 enter the picture.

A related approach to understanding health disparities in a population are the “Social determinants of health (SDOH),” which are described by the CDC as “the nonmedical factors that influence health outcomes. They are the conditions in which people are born, grow, work, live, and age, and the wider set of forces and systems shaping the conditions of daily life.” SDOH appear to be included within FCT in its explanation of health disparities in a population. 

The relatively new field of Cognitive Epidemiology, however, proposes a different approach to understanding health disparities. It attempts to understand the role that intelligence plays in helping populations manage their health and become less susceptible to preventable disease and premature death. Developments in this field began in the late 1990s with a seminal article by David Lubinski of Iowa State University and Lloyd Humphries of the University of Illinois.

In 2001, Lawrence Whalley and Ian Deary in Scotland reported that high scores on an intelligence test taken at age 11 predicted longevity at age 76 for both men and women. Their research, however, could not explain how intelligence influenced longevity. Three years later, Linda Gottfredson of the University of Delaware published an article with the provocative title: “Intelligence: Is It the Epidemiologists’ Elusive ‘Fundamental Cause’ of Social Class Inequalities in Health?” Gottfredson argued that differences in g, the general factor of intelligence, satisfied all requirements to be considered a fundamental cause of differing health outcomes.

Proponents of FCT resisted the prospect that intelligence differences within a population could lead to health disparities. “A central feature of fundamental cause theory,” they stated, “is the idea of flexible resources, and, just as clearly, intelligence is not one of the flexible resources mentioned or even considered in extant discussions of that theory.” Nevertheless, they tested the possibility that intelligence was one of the “flexible resources” affecting health disparities in 2008 by examining two data sets from the United States. Each data set resulted from a longitudinal study in which a test of intelligence or mental ability was obtained early in an individual’s life, and then health outcomes later in life (including mortality) were assessed.

The researchers found that intelligence in early life was positively correlated with various measures of health outcomes, including mortality, when evaluated by itself. The authors concluded that the “significant effects of intelligence were eliminated by controls for education and income (in four of five tests) or sharply diminished in magnitude (for the effect of intelligence on self-reported health).” Satisfied that intelligence did not pose a challenge to FCT, the authors of a major review of the theory in 2021 did not consider intelligence as one of the flexible resources affecting health disparities.

Meanwhile, researchers in the field of Cognitive Epidemiology continued to make important discoveries regarding the influence of intelligence on a variety of health outcomes. These discoveries were summarized in a 2021 review article by Ian Deary and colleagues, which reported that intelligence had positive predictive value in outcomes such as lower all-cause mortality as well as lower rates of mortality from heart disease, stroke, smoking-related cancers, respiratory diseases, digestive diseases, diabetes, dementia, and accidents. A positive predictive effect was also found in lower rates of non-fatal conditions, such as arthritis, dementia, depression, schizophrenia, hypertension, and diabetes. In some—but not all—cases, the results comparing these outcomes with intelligence measured earlier in life were statistically significant even after adjusting for education and adult SES.

Deary and his co-authors argued that the use of statistical controls for education and adult SES in studies of the effects of intelligence on health outcomes may be a case of “overcorrection” because these variables are themselves highly correlated with intelligence. This falsely tends to diminish the apparent importance of intelligence for the outcomes being studied. For example, the authors stated that “educational attainments are substantially heritable and highly genetically correlated with intelligence.”

Could health disparities be explained by differences in health literacy, defined as the “degree to which individuals have the capacity to obtain, process and understand basic health information and services needed to make appropriate health decisions”? This report stated that “Low health literacy is associated with more hospitalizations, greater use of emergency care, decreased use of preventive services, poorer ability to interpret labels and health messages, poorer health status, higher mortality, and higher health care costs.” However, health literacy is itself a manifestation of intelligence. Researchers have found that “health literacy scores appear to be associated with health outcomes largely because they denote basic cognitive abilities.”

Charlie Reeve and Debra Basalik of the University of North Carolina-Charlotte tested Linda Gottfredson’s theory that intelligence was an important factor in disparities in health outcomes. They found that “IQ has consistent, significant, and practically meaningful unique effects on both positive and negative state health indicators independent of wealth, health care expenditures and racial composition.”

Gary Marks, a researcher at the University of Melbourne, analyzed two large data sets from the United States, the 1979 National Longitudinal Study of Youth and the 1997 National Longitudinal Study of Youth (NLSY79 and NLSY97). NLSY79 involved individuals born from 1957 through 1964; NLSY97 involved individuals born from 1980 through 1984. For each cohort, extensive background information was collected regarding the individuals’ families, including parental education level, occupational status, and income and wealth (although there were some differences between the cohorts in the exact data collected). The participants in each study took the Armed Forces Qualification Test (AFQT), either during their teenage years or as young adults, the results of which correlate very highly with the results of a standard intelligence test. The participants were then followed at one- or two-year intervals for many years, and data on educational attainment, occupation, income, wealth, and many more variables were collected during the follow-up periods. (More information on these longitudinal studies and the data sets themselves are found here.)

Marks found that intelligence, as assessed by the AFQT, was a much stronger influence on educational attainment, occupational status, income earned, and wealth accumulated for the study populations than the SES of the homes in which individuals were raised. He also found that intelligence predicted income earned even after adjusting for the SES of the homes in which individuals were raised and for their educational attainment. This led Marks to conclude that ”contrary to dominant narratives, cognitive ability is important to a range of social stratification outcomes, and its effects cannot be attributed to socioeconomic background or educational attainment.” Referring to intelligence or cognitive ability, he stated, “it is a simple matter to substantially reduce the estimates for ability by controlling for variables that are themselves strongly predicted by ability.”

Marks’s research showed that intelligence was the most important factor affecting the SES that individuals achieved as adults. According to FCT, stratification in SES in a population leads to disparities in health outcomes. (At least two other studies have found that an individual’s intelligence level is more important in predicting the SES reached as an adult than the SES of the home of origin; see here and here.) FCT states that people use flexible resources including knowledge to achieve better health outcomes, but it does not explain how someone is to acquire and utilize a flexible resource like knowledge to achieve better health outcomes without making use of intelligence.

The proponents of FCT list racism as a cause of health disparities: “We conclude that racial inequalities in health endure primarily because racism is a fundamental cause of racial differences in SES and because SES is a fundamental cause of health inequalities.” However, because FCT denies any role for intelligence, the theory overlooks the significant and well-documented racial and ethnic group differences in intelligence in the US. Harvard economist Roland Fryer analyzed the NLSY79 and NLSY97 datasets and concluded that, after taking AFQT scores into account, the sizable differences in income earned between blacks and whites and between Hispanics and whites essentially disappeared. This led Fryer (who is black) to conclude that discrimination in the US is decreasing.

Besides ignoring the role played by differences in intelligence in causing health disparities, FCT also ignores the possible role of genetic, cultural, and biological factors that may be involved. FCT may be an example of the “sociologist’s fallacy,” described as “the common scenario in sociology in which SES is prejudged as the operative cause of a variety of human outcomes, while other possible determinants like GCA [general cognitive ability or intelligence] are neglected.”

How does intelligence lead to better health outcomes? Linda Gottfredson has provided this working definition of intelligence:

Intelligence is a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience. It is not merely book learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings—“catching on,” “making sense” of things, or “figuring out” what to do.

Gottfredson and Deary have stated that “intelligence enhances individuals’ care of their own health because it represents learning, reasoning, and problem-solving skills useful in preventing chronic disease and accidental injury and in adhering to complex treatment regimens.” The authors went on to state that The socioeconomic measures that best predict health inequality also correlate most with intelligence (education best, then occupation, then income). This means that instead of IQ being a proxy for SES in health matters, SES measures might be operating primarily as rough proxies for social-class differences in mental rather than material resources.”

One can think of maintaining one’s health as a job, one of the most important in a person’s life. Extensive research has found that intelligence is a key ingredient for good job performance. The more complex the job, the greater the role of intelligence. As healthcare options become more abundant and complex, intelligence increases in importance in helping us stay healthy.

Sociologists have been puzzled as to why European countries with generous welfare benefits and wide access to healthcare for all continue to experience health disparities among their populations and even widening health disparities over time. As one report observed: “There is good evidence that welfare policies have contributed to a reduction of inequalities in income, housing quality, health care access and other social and economic outcomes, but they have apparently been insufficient to eliminate health inequalities.” One interpretation of these findings is that government programs to equalize living conditions and access to healthcare do nothing to equalize differences in levels of intelligence in the population because intelligence is highly heritable, and there is no proven way to increase it.

At the individual level, intelligence is only one of many factors that affect our health. Many diseases have a genetic basis that puts us at increased risk of a bad outcome. We are not responsible for our genes because we don’t get to select our parents and some diseases are more preventable than others. Although it is not possible to increase intelligence, it is possible to make health services more accessible and understandable to those with low levels of health literacy. It has been recommended that health messages be made understandable to those with a low level of literacy as well as to those whose native language is not English.

David Lubinski has pointed to psychological factors that can affect health outcomes, stating “compliance has at least two psychological components: a ‘can do’ competency component (ability) and a ‘will do’ motivational component (conscientiousness).” However, the available evidence points to an important role for intelligence in helping us to navigate our way through life in the healthiest way possible. If health disparities are to be mitigated effectively, it is important that we acknowledge the causes, even when the implications are politically incorrect.

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