Intelligence and fertility: Do smarter people have fewer children? The strongest evidence suggests a smaller, more complicated link
People with higher intelligence scores tend to have slightly smaller families in American and British surveys. But when researchers wait until people finish having children and compare brothers from the same families, the link shrinks or even reverses.
Intelligent individuals tend to have fewer children, a finding most apparent among women in American and British studies. However, the strongest studies, which only consider children after a person has finished having them and compare siblings from the same families, suggest that the link is quite weak. This relationship is strongly influenced by when people have children and their family background, with a notable difference in Scandinavia for men.
Researchers pursue this topic as it poses a challenge for evolutionary biology. Intelligence test scores are linked to higher education, income, and status, but in many modern societies, status and resources correlate with fewer surviving children. Multiple hypotheses have been proposed, including better contraception usage, the time required for higher education, and changing family size values.
This area of research has a controversial past, with Eugenics being a movement aimed at improving human populations by encouraging desired traits in offspring and discouraging others, often through forced sterilization. Early studies on intelligence and fertility emerged from eugenics concerns about less intelligent individuals outbreeding more intelligent ones.
Some researchers use subtle statistical trends to argue that modern populations may experience a genetic decline in intelligence, while others dismiss these assumptions. The quality of intelligence measurement varies across studies, with military conscription tests and comprehensive school tests providing more reliable results, while brief vocabulary quizzes depend heavily on someone's education level.
The correlation between intelligence scores and fertility is typically small, with a 2017 study in Intelligence reporting 0.18 for Taiwanese adults, compared to 0.65 for education levels. Researchers must account for the timing of parenthood, as higher test scores often lead to delayed childbearing. To address this, scientists analyze completed fertility, counting children only after age 45 or 50.
Population-level trends can be misleading due to diverse backgrounds, confounding factors, and the need for controlling variables through statistical techniques. Sibling comparisons help remove some family background influences but cannot completely prove a direct cause. Multiple studies have analyzed the same datasets, potentially creating an illusion of more independent confirmation than exists.
Some research draws from the National Longitudinal Survey of Youth, General Social Survey, Health and Retirement Study, National Child Development Study, and UK Biobank. While findings have been replicated by independent teams, controlling for timing and family background remains a complex challenge in this field of study.
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