To learn more about the motivations behind the trend toward smaller families in Western culture, UC researchers are studying the rural Kenyan Kipsigis. In that traditional society, where men buy multiple wives by paying bridewealth and where fertility rates remain high, researchers have found that despite strong correlations between wealth and reproductive success, it appears that materialistic motivations -- maximizing wealth by endowing sons with plentiful resources -- prompt a limiting reproductive behavior. The finding is not necessarily surprising, says Monique Borgerhoff Mulder, a UC Davis anthropology professor participating in the research, but it does provide new insights into how reproductive behavior changes and why voluntary fertility limitation occurs. In addition, the findings support scientific arguments that human psychology is designed to maximize the incomes of all descendants produced. The study makes it clear, Borgerhoff Mulder says, that "not only have materialist motivations been around for a long time," but that they can be prominent even where a strong correlation exists between wealth and fertility. She presented the paper earlier this month at the American Anthropological Association annual meeting.