A spirited dispute in Harvard's 1960s Social Relations/Anthropology doctoral program centered on whether a society's language, beliefs, values, traditions, and technology—its culture—derive from accidental and historical factors, or from adaptation to particular conditions. The fact is, traditional societies limit reproduction in ways that may be bitter: women and babies are often victims. Men are luckier. Some religious vocations demand temporary or lifelong celibacy, The mirror image of polygyny is polyandry. Practiced in Tibet, polyandry depresses fertility because many women cannot find a husband when it is common for several brothers to share a wife. The human biological potential for childbearing is probably about ten or eleven births per woman. But a far lower fertility rate typically occurs, whether in historical, contemporary, modern, or premodern societies. Correlation does not prove causality. Variation in the rate at which fertility is declining within regions suggests that land hunger does, indeed, drive the more cautious approach to childbearing.