Estimating the Heritability of Intelligence with a Nuclear Twin Family Design

Intelligence, and the favorable life outcomes with which it is associated, are not shared equally between individuals or groups. In an endeavor to better understand the inequality, behavioral genetics (BG) have employed the classical twin design (CTD) to apportion the variability in the intelligence quotient (IQ) to genetic and environmental sources. Recent research has revealed that such parameter estimates are subject to bias: underestimating the influence of assortative mating, and nonadditive genetic effects. These biases are somewhat mitigated by the nuclear twin family design (NTFD). To date no published studies have applied the NTFD to IQ. This study addresses this limitation utilizing a meta-analysis of data comprising IQ measures from 660,365 twin, parent-child, and spousal pairs collected over more than 50 years from more than 15 countries. In a comparison of parameter estimates obtained under the CTD and NTFD, this study confirmed that the NTFD more accurately estimated the heritability of IQ. These findings will inform and guide future research, gene-mapping efforts, and public policy aimed at correcting the inequality in educational and life outcomes that are significantly associated with IQ.