1 Larmuseau M.H.D.

et al. Cuckolded fathers rare in human populations.

2 Trussell J. Contraceptive efficacy.

3 Harris D.J. Does contraceptive use lead to increased affairs? A reply to Larmuseau et al. 2016.

3 Harris D.J. Does contraceptive use lead to increased affairs? A reply to Larmuseau et al. 2016.

3 Harris D.J. Does contraceptive use lead to increased affairs? A reply to Larmuseau et al. 2016.

In a recent article we showed that human extra-pair paternity (EPP) rates did not substantially decline in contemporary relative to historical Western societies over the past few hundred years []. In both cases, ∼1–2% of all children appeared to be the product of cuckoldry. This came as a great surprise, given the known steady increase in the adoption rate and reliability of contraception over the past century []. We proposed several hypotheses to explain this paradox. One, which Harris [] focused on, was that women may have become more sexually liberated, and now engage in relatively more extramarital affairs. Harris [] counters our hypothesis by arguing that the low historic EPP rate of ∼1% would in fact be consistent with very high rates of extra-pair copulation (EPC) that exceed the estimates in contemporary societies, and that the available data therefore do not support the idea that woman may have become less faithful. We show here that the implicit assumptions by which EPC rates were estimated by Harris [] are erroneous. We then go on to review two explanations for the observed patterns: one being based on increased infidelity, and the other based on the idea that EPC could have rare adaptive benefits for woman in specific circumstances, such as when their husband is infertile.