Women's luteal-phase sexual proceptivity and the functions of extended sexuality.

Women's sexuality, unlike that of most mammals, is not solely defined by sexual receptivity during the short window of fertility. Women demonstrate extended sexuality (in which they initiate and accept sexual advances outside of the fertile phase) more than any other mammalian female. In this light, surprisingly little research has addressed the functions of women's luteal-phase sexuality. On the basis of theory and comparative evidence, we propose that women's initiation of sex during nonfertile phases evolved in part to garner investment from male partners. If so, women should be particularly prone to initiate luteal-phase sex when the potential marginal gains are greatest. Results from a study of 50 heterosexual couples showed that women increasingly initiate sex in the luteal phase (but not the fertile phase) when they perceive their partners' investment to lag behind their own. These findings provide evidence for the distinct nature of women's extended sexuality and may contribute to a more comprehensive understanding of women's sexuality.