County-level IQ and fertility rates: A partial test of Differential-K theory

• Differential-K theory was proposed as a framework for explaining the origins of human IQ.
• Differential-K theory suggests IQ should correlate with other life history traits.
• We test portions of Differential-K theory at the county level.

Human intelligence (IQ) correlates with a number of important life outcomes ranging from mortality and morbidity to social rank and status. Noticeably absent from the literature, however, has been a unifying framework from which to examine why IQ should correspond to so many seemingly disparate outcomes. Rushton’s Differential-K theory represents a theoretical perspective capable of accounting for the role of IQ in understanding human flourishing. We tested predictions suggested by Differential-K theory by examining the association between IQ at the aggregate level and two life-history variables: parental investment and fertility rates. Our results provide tentative support for some of the predictions of Differential-K theory.