Ignorance of biodata is costly in sociology. Gender theorists remain unaware that until the demographic transition, infants were suckled every 15 minutes for two years, less often another two. A nearly continuous cycle of pregnancy and lactation barred women from the activities that brought the most prestige and power until the advent of modern sanitation after 1880. Women entered the public arena in large number only after technology altered the social consequences of human physiology. Yet wives still spend twice as much time in housework and child care as husbands. Data about the effects of both biology and culture on social interaction would enhance studies of ethnocentrism within the household.