A variance decomposition analysis using maximum likelihood methods was employed to examine the genetic architecture of sexual dimorphism in anthropometric traits in a large pedigreed sample of Mexican American individuals from San Antonio, Texas. For this analysis the magnitude of sexual dimorphism was viewed as arising from a special case of genotype by environment interaction (G × E), that of genotype by sex (G × S). Evidence indicates a marked G × S interaction for 9 of the 12 traits examined and 1 of the 4 indices, findings which are interpreted as indicators of a strong genetic component to the degree of sexual dimorphism expressed in these traits. Such results have important implications for the use and interpretation of these traits in an epidemiological as well as an evolutionary context. © 1993 Wiley-Liss, Inc.