AbstractProsimian lemurs differ fundamentally from anthropoid primates in many traits related to social structure. By exploring the demography of Milne‐Edwards' sifakas (Propithecus diadema edwardsi), and comparing it to other well‐studied primates, we explore the effect of demographic and life‐history factors on social structure. Specifically, we compare lemur survivorship and fertility patterns to two published composite models: one created for New World and another created for Old World monkeys. Using longitudinal data collected on individual Propithecus diadema edwardsi from four study groups from 1986–2000 in Ranomafana National Park, Madagascar, we quantify 1) group composition, 2) birth seasonality, 3) interbirth interval, 4) life‐table values, and 5) population growth estimates. The mortality, survivorship, and life‐expectancy schedules indicate high infant and juvenile mortality. Fertility remains high until death. The intrinsic rate of increase and net reproductive rate indicate a shrinking population. We suggest that high mortality rather than low fertility causes the observed population decline. While sifaka survivorship closely resembles New World patterns, fertility resembles Old World patterns, i.e., like New World monkeys, few sifakas survive to reproductive age, and those that do, reproduce at a slow rate resembling the Old World pattern. This necessarily impacts social structure. An adult sifaka at the end of her lifespan will have one only daughter who survives to reproductive age, compared to 3.4 for New World or 2.7 for Old World monkeys. Demography limits the formation of large kin‐based groups for sifakas, and survivorship and fertility patterns do not easily permit sifakas to form large same‐sex family groups. Am J Phys Anthropol, 2004. © 2004 Wiley‐Liss, Inc.