Given the primacy of reproduction, same-sex sexual behavior poses an evolutionary puzzle. Why would selection fashion motivational mechanisms to engage in sexual behaviors with members of the same sex? We propose the alloparenting hypothesis, which posits that sexual fluidity in women is a contingent adaptation that increased ancestral women's ability to form pair bonds with female alloparents who helped them rear children to reproductive age. Ancestral women recurrently faced the adaptive problems of securing resources and care for their offspring, but were frequently confronted with either a dearth of paternal resources due to their mates' death, an absence of paternal investment due to rape, or a divestment of paternal resources due to their mates' extra-pair mating efforts. A fluid sexuality would have helped ancestral women secure resources and care for their offspring by promoting the acquisition of allomothering investment from unrelated women. Under this view, most heterosexual women are born with the capacity to form romantic bonds with both sexes. Sexual fluidity is a conditional reproductive strategy with pursuit of men as the default strategy and same-sex sexual responsiveness triggered when inadequate paternal investment occurs or when women with alloparenting capabilities are encountered. Discussion focuses on (a) evidence for alloparenting and sexual fluidity in humans and other primates; (b) alternative explanations for sexual fluidity in women; and (c) fourteen circumstances predicted to promote same-sex sexual behavior in women.

The daughter-guarding hypothesis posits that "parents possess adaptations with design features that function to defend their daughter's sexual reputation, preserve her mate value, and protect her from sexual victimization" (Perilloux, Fleischman, & Buss, 2008, p. 219). One way that parents may attempt to guard their daughters' sexualities is by conveying to them certain messages about sex. To explore this possibility we administered an online questionnaire that tested 8 sex-linked predictions derived from the daughter-guarding hypothesis about the content of parent-child communications about sex. Participants were undergraduates from a Northeastern U.S. Jesuit Catholic university (n ϭ 226) and young adults recruited through Facebook (n ϭ 391). As predicted, daughters were more likely than sons to recall receiving messages from their parents that (a) emphasized being discriminating in allocating sexual access; (b) emphasized abstinence; (c) encouraged them to deter, inhibit, and defend against their partners' sexual advances; (d) encouraged them to not emulate depictions of sexual activity; (e) stipulated when they were old enough to date; and (f) curtailed contact with the opposite sex. Results supported several hypothesized design features of the daughter-guarding hypothesis. Parents may be socializing children in ways that fostered ancestral reproductive success through sex-linked birds-and-the-bees talks and messages.

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