Although stratified societies have typically been characterized by intensive polygyny, socially imposed monogamy has developed in the stratified societies of Western Europe. Following a critical review of other theories of socially imposed monogamy, a multivariate, nondeterministic theory is developed. Within this theory, a variety of internal political processes can result in socially imposed monogamy, but this phenomenon—while consistent with evolutionary theory—is underdetermined with respect to (1) evolutionary theory, (2) human nature/nurture (i.e., the characteristics of humans), and (3) external ecological variables. Data on the origins and maintenance of socially imposed monogamy in Western Europe are reviewed, indicating that post-antiquity socially imposed monogamy originated in the late Middle Ages and has been maintained since that period by a variety of social controls and ideologies, including political activities of the Christian Church and, in later periods, of women and lower- and middle-status males. As a result of institutionalized controls on reproduction, non-monogamous Western sexuality has been directed at obtaining psychological rewards deriving from evolved motivational systems (e.g., sexual pleasure, excitement, feelings of dominance, status, or intimacy), but this non-monogamous sexuality has not typically been a major source of increased reproductive success.