Human heteroparity

“Semen displacement theory” is the most intriguing part of Gallup’s story. We may prefer to regard our species as being blissfully monogamous, but the truth is that, historically, at least some degree of fooling around has been our modus operandi for at least as long we’ve been on two legs. Since sperm cells can survive in a woman’s cervical mucus for up to several days, this means that if she has more than one male sexual partner over this period of time, say within 48 hours, then the sperm of these two men are competing for reproductive access to her ovum. According to Gallup and Burch, “examples include, group sex, gang rape, promiscuity, prostitution, and resident male insistence on sex in response to suspected infidelity.” The authors also cite the well-documented cases of human heteroparity, where “fraternal twins” are in fact sired by two different fathers who had sex with the mother within close succession to each other, as evidence of such sexual inclinations.

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