Human evolution is far from a relic of the past, as a new study in Behavior Genetics demonstrates. By analyzing data from three generations of Americans, researchers found that natural selection is favoring certain health-related traits while selecting against traits linked to higher educational attainment. These findings offer new insights into how societal and economic factors continue to influence the course of human evolution.
Natural selection is the process by which certain traits become more or less common in a population over generations, based on their impact on an individual’s ability to survive and reproduce. Traits that increase reproductive success—such as those that enhance health, adaptability, or fertility—tend to become more prevalent over time, while traits that reduce reproductive success may diminish.
In humans, natural selection is influenced by a complex interplay of genetic, environmental, and social factors. Unlike in other species, where survival may depend largely on physical characteristics, in humans, cultural, economic, and societal factors significantly shape reproductive patterns and, by extension, genetic evolution.
The new study is grounded in an economic theory of fertility, first proposed by economist Gary Becker, which explains how economic considerations influence reproductive behavior. The theory has two main components.
The first is the “substitution effect,” which posits that individuals face a trade-off between working and raising children. For individuals with higher education or income, their time is more valuable in the labor market, meaning that the opportunity cost of having children is higher. As a result, these individuals are more likely to have fewer children, prioritizing their careers and economic productivity over reproduction.
The second component of the theory explains how this trade-off plays out differently across socioeconomic groups. According to the theory, the substitution effect is more pronounced for people with lower education, lower income, or those who are unmarried. In these groups, marginal utility—the value of each additional dollar of income—is higher, meaning they are more sensitive to the economic trade-offs between work and family. In contrast, higher-income or highly educated individuals may experience a balancing effect known as the “income effect,” where increased wealth makes raising children more affordable, partially offsetting the substitution effect.
To conduct their analysis, the researchers used data from the Health and Retirement Survey, focusing on a sample of 8,827 genotyped white participants born before 1965 for men and before 1970 for women. This ensured that most participants had completed their fertility by the time of the study. The researchers also examined data from 2,319 genotyped Black participants for comparative purposes. Polygenic scores—genetic predictors for traits like educational attainment, self-rated health, and age at first birth—were calculated using pre-existing genetic data repositories.
The key variable in the study was “relative lifetime reproductive success,” which was defined as the number of children a person had, divided by the average number of children for people born in the same year. By examining how polygenic scores correlated with reproductive success, the researchers could estimate the direction and magnitude of natural selection on various traits. The study also extended its analysis across three generations, looking at the participants’ siblings and grandchildren to capture how selection effects accumulate over time.
The researchers found that polygenic scores associated with educational attainment tend to correlate negatively with reproductive success, confirming that these scores are being selected against. This means individuals with higher genetic predispositions for educational attainment are, on average, having fewer children. This pattern held consistently across the three generations studied, suggesting that the effects of selection on educational traits have persisted over time. The researchers noted that this aligns with the first prediction of the economic theory of fertility, which posits that individuals with higher human capital face a trade-off between investing time in work and raising children.
However, the results for the second prediction of the theory were less consistent. This prediction states that selection pressures should be stronger among socioeconomically disadvantaged groups, such as those with lower income, less education, or unmarried parents. While the researchers found stronger selection coefficients for low-income and unmarried parents, they did not observe significant differences based on education level or age at first birth. These mixed results suggest that while economic factors may explain some aspects of natural selection, they do not fully account for the observed patterns.
In addition to educational traits, the study also examined selection on health-related traits. Interestingly, some health traits, such as self-rated health and age at first birth, showed selection pressures comparable to those for educational attainment. This indicates that natural selection is not solely acting on traits related to socioeconomic outcomes but also on traits that influence overall health and reproductive timing.
For example, the polygenic score for self-rated health was positively associated with reproductive success, suggesting that individuals with better genetic predispositions for health are having more children. Conversely, polygenic scores for traits like smoking behavior were being selected against.
The researchers also estimated the magnitude of genetic change across generations, finding that natural selection is causing measurable, though modest, shifts in polygenic scores. For instance, the genetic predisposition for cognitive performance is estimated to decrease by approximately 0.055 standard deviations per generation. While this translates to a small decline in average IQ, the researchers caution that these estimates are influenced by various factors, including environmental changes and genetic measurement errors.
The study’s limitations are important to consider. One major limitation is the focus on white participants, which means the findings may not generalize to other racial or ethnic groups. Although the researchers included Black participants in some analyses, the smaller sample size for this group limited the precision of the results. Additionally, the study relies on polygenic scores, which are imperfect predictors of complex traits. These scores capture only a portion of the genetic variance associated with traits, and their predictive power may vary across populations.
The study, “Natural Selection Across Three Generations of Americans,” was authored by David Hugh‑Jones and Tobias Edwards.
