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Analysis of reproductive data from 4829 mothers in the Ten-State Nutrition Survey of 1968-70 shows that associations between five measures of physique and fertility are explained by socioeconomic influences on each, here indicated by per-capita income. This socially-determined differential fertility of physique categories appears to be linear rather than modal in character, while income and fertility interact in a curvilinear manner. These data also suggest the existence of a secular trend for certain physical characteristics, particularly fatness, which are subject to social transmission between generations. The demographic implications of such a trend may include an increase in total population fatness.