This paper describes the reasoning and results of cross-cultural research on the conditions that may lead to the emergence of unilineal descent and some of the major variations thereof. We suggest that the presence of warfare in conjunction with a unilocal pattern of residence may be the impetus for the formation of unilineal descent groups, and that the kind of warfare which is present together with demographic factors may determine the particular kinds of unilineal descent groups which develop. The major variations of unilineal descent systems we deal with include contiguous versus dispersed descent groups, demonstrated descent groups (lineages) versus putative descent groups (clans, etc.), and dual (moiety) organization versus other kinds of putative descent systems. Finally, we present and discuss evidence suggesting that a system of putative descent groups (a "clan" system) generally develops before a system of demonstrated descent groups (a "lineage" system).