Multivariate models of mixed assortment: Phenotypic assortment and social homogamy for education and fluid ability

Phenotypic assortment is assumed to be the principal mechanism of spouse si milarity in most biometrical studies. Other assortment mechanisms, such as social homogamy, may be plausible. Two models are presented that consider p henotypic assortment and social homogamy simultaneously (i.e., mixed assort ment), where selective associations between social background factors (Mode l I) versus selective associations between total environments (Model II) di stinguish the models. A series of illustrative analyses was undertaken for education and fluid ability available on a sample of 116 Swedish twin pairs and their spouses. On the basis of several fit criteria Model I was prefer red over Model II. Both social homogamy and phenotypic assortment may contr ibute to spouse similarity for educational attainment and fluid ability. Fu rthermore, spouse similarity for fluid ability may arise indirectly from so cial homogamy and phenotypic assortment for educational attainment. Power a nalyses indicated greater observed power for Model I than Model II. Additio nal power analyses indicated that considerably more twin-spouse sets would be needed for Model II than Model I, to resolve social homogamy and phenoty pic assortment. Effects of mis specification of mechanisms of spouse simila rity are also briefly discussed.