Models of spouse similarity: applications to fluid ability measured in twins and their spouses

Genetic influences have consistently been reported to be the principal explanation for resemblance among relatives for intelligence, with shared environmental effects playing a much smaller role. However, crucial to understanding the nature of environmental influences are the mechanisms of assortative mating. Phenotypic assortment, albeit widely assumed or modeled in biometrical analyses, may be less important than other assortment processes, such as social homogamy. Consequently, effects of shared environment may play a greater role than prior studies have suggested. The goal of this study was to resolve environmental and genetic influences on fluid ability based on alternative models of assortment by examining the similarity of monozygotic (MZ) and dizygotic (DZ) twins and their spouses. Raven's Progressive Matrices scores were available from a population-based Swedish sample of 138 twin kinships. The effects of both social homogamy and phenotypic assortment were tested simultaneously in each of two alternate assortment models. A factor/delta path model represented social homogamy as a common factor and phenotypic assortment as a delta path, while a delta/delta path model represented both social homogamy and phenotypic assortment as delta paths. Overall, the factor/delta path model was found to be superior. Results suggested that social homogamy completely explained spouse similarity; phenotypic assortment was not significant. The results of these analyses suggest the presence of shared environmental effects among twins and their spouses, which would have been underestimated if only phenotypic modeled phenotypic assortment may have underestimated the effects of environment.