Twins' IQs: a reply to Schwartz and Schwartz.

It is argued that the criticisms by Schwartz and Schwartz of Jensen's analysis of IQ data on monozygotic twins reared apart fall down on two central points, methodological and theoretical: (a) Valid inferences concerning the broad heritability of intelligence can be drawn from analysis of the combined data from several independent studies of MZ twins reared apart, even though the groups differ significantly in mean IQ; (b) Mean population differences within a given racial category can involve genetic as well as environmental factors.