Behavior Genetics, VoL 21, No. 4, 1991 Increased Heritability for Lower IQ J. Michael Bailey 1,2 and William Revelle 1 Received 16 Oct. 1990--Final 13 Dec. 1990 Levels? Detterman et aL (1990) presented evidence based on twins that the her- itability of lQ may be higher in the lower part of the IQ range. We first offer an alternative test for differential heritability across the IQ range, based on the analysis of absolute intrapair differences of monozygotic versus dizygotic twins. We then review two previous studies, each con- taining more twins than the sample of Detterman et al., which examined the distribution of intrapair absolute differences. In contrast to the study of Detterman et al., both yielded results more compatible with higher heritability in the upper range of IQ. We discuss various interpretations of these findings and show how our proposed test might aid in distin- guishing among them. KEY WORDS: twins; heritability; environment;intelligence. INTRODUCTION In a recent paper Detterman et al. (1990) present evidence that the her- itability of IQ may be higher in the lower part of the IQ range. The evidence consisted of a series of complicated regression analyses, which yielded consistent, significant results despite the small sample [86 mon- ozygotic (MZ) and 54 dizygotic (DZ) same-sex twin pairs]. Though their study is, by itself, suggestive of higher heritability at lower IQ levels, Detterman et al. neglected two previous studies, both including much larger twin samples, which yielded results opposite those of Detterman et al. (1990). Below, we first explicate an alternative sta- tistical test for differential heritability across different ability groups, which is simpler and more intuitive than the approach taken by Detterman Department of Psychology,NorthwesternUniversity,Evanston, Illinois 60208. 2 To whom correspondence should be addressed. 397 0001-8244/91/0700-0397506.50/0 91991Plenum Publishing Corporation