Evidence Against a Genetical Component to Performance on IQ Tests

Abstract

Eaves and Jinks¹ have failed to note the significance of Scarr-Salapatek's² data, which provides important evidence against the heritability of IQ performance. A straightforward overall evaluation of this study, together with a review of evidence^3–7 which Eaves and Jinks consider to be more secure in establishing a genetical component to IQ performance shows that: (1) the upper limit of IQ heritability in Scarr-Salapatek's study is 15%±6%. This is consistent with zero heritability and directly contradicts the higher figures claimed by other studies^3–7; (2) other studies^5–7 which use identical-fraternal comparisons to derive apparently higher upper limits to heritability do not take into account the more similar treatment frequently given to identical twins. When this is adjusted for, these studies are consistent with low or zero heritability; (3) the similarity between separated identical twins, used by Jinks and Fulker³ to derive an 80% heritability estimate, can be quantitatively accounted for by highly correlated placement (ref. 8 and L. J. Kamin, Invited Address, Eastern Psychological Association, Washington DC, March 1973) with little or no genetical component.