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Genetic and environmental sources of variation in body mass, as measured by Quetelet’s index, have been assessed within a total community of families in Tecumseh, Michigan. Correlations of body mass are calculated for sets of related and unrelated individuals living together or living apart. For those living together, correlations are calculated among biological siblings and between these siblings and their parents, as well as between spouses. For those living apart, correlations are calculated among older siblings and between first cousins, as well as between unrelated individuals who marry at a future time (i.e., to-be-weds). The latter correlation provides a measure of assortative mating. These correlations are analysed using a biological model which partitions the total phenotypic variance into components associated with shared genes and components associated with shared household environments both within and across generations. Separate models are used for both direct and indirect assortative mating. Heritabilities are estimated to be 0.312 when indirect assortative mating is assumed and 0.372 when direct assortative mating is assumed after controling for age, sex and socioeconomic status. Regardless of the mating system used, the model confirms the hypothesis that the determinants of body mass consist of both significant genetic (p < 0.005) and environmental (p < 0.005) components. Shared household environments across generations are found to be significant (p < 0.01), but for shared household environments within