Men and women do not always make the same moral choices. In particular, men are more likely to agree that killing someone can be morally acceptable provided that this death saves the lives of several other persons (Fumagalli et al. 2010; Lotto, Manfrinati, and Sarlo 2014; Youssef et al. 2012). In other words, men seem to be more utilitarian than women, more accepting of inflicting harm for the purpose of a greater good.

To the best of our knowledge, this gender difference has only been observed in cases in which utilitarianism implies to harm, and its causes are yet to be discovered. In this article, we suggest that this greater male utilitarianism is only one specific consequence of broader differences in male and female intrasexual competition. The main strength of our account is to make new predictions about the moral dilemmas for which men will become less utilitarian than women to the point of becoming anti-utilitarian.

Our sexual selection account is based on three claims which we develop and reference below: (a) men have better reproductive prospects when the operational sex ratio (OSR) in the population is biased toward females; (b) men have a greater propensity than women to inflict violence upon their sexual rivals; and (c) the aversion to utilitarian killings derive in part from the aversion to inflict violence.

Both sexes need to attract high quality mates and to outcompete rivals, and their difficulty in doing so is affected by the OSR in their population. When the OSR is female biased (more females than males), men have enhanced mating prospects, and the reverse is true for populations with a male-biased OSR (Emlen and Oring 1977; Kruger and Schlemmer 2009; Pollet and Nettle 2008). Accordingly, men have an evolutionary interest in augmenting the ratio of fertile women to men.

The advantages of a male-biased sex ratio for women are not as clear, given the potential dangers caused by having too many men in a population (Schacht et al. 2014). The two sexes, in any case, do not possess the same adaptations for outcompeting rivals. Because the parental investment of women (e.g., pregnancy, childbirth, and child care) is largely greater than that of men (Trivers 1972), their death would result in a larger fitness cost (Sear and Mace 2008). In part, because of this greater potential fitness cost, women tend to compete primarily with physically low-risk strategies, such as derogating their rivals (Campbell 2013; Campbell and Cross 2012; Vaillancourt 2013). In contrast, direct physical competition appears to have been a primary mechanism of sexual selection in men (Archer 2009; Hill et al. 2013; McDonald, Navarrete, and Van Vugt 2012; Puts 2010). Accordingly, sexual selection may have prepared men for physical conflict, both through physical traits such as greater upper body strength and through psychological traits such as a lower aversion to inflicting harm (Sell, Hone, and Pound 2012).

Let us now apply this framework to utilitarian dilemmas starting with a bare bone dilemma which does not specify the gender of the potential victims: “Would you kill one person if it saved the lives of three other persons?” Given that one’s aversion to inflicting physical harm is known to predict one’s moral aversion to utilitarian killings (Wiech et al. 2013), we would expect men (the more physically aggressive sex) to give more utilitarian responses, and this is indeed what has been generally observed in the literature, in which the gender of the victims has not been systematically manipulated.Footnote 1

The true strength of the sexual selection account, though, is to make novel predictions about gender differences in moral thinking for novel versions of this classic moral judgment vignettes. In particular, the sexual selection account predicts that men can be less utilitarian than women, or even anti-utilitarian (i.e., willing to kill several in order to save just one life), when specific conditions are met. More precisely, consider this genderized version of a sacrifice moral dilemma:

The genderized sacrifice dilemma Given the choice, would you decide to cause the death of three members of your own sex, or to cause the death of one member of the opposite sex?

For men, the dilemma translates as: Would you rather cause the death of three men (saving one woman), or cause the death of one woman (saving three men)? For women, the dilemma translates as: Would you rather cause the death of three women (saving one man), or cause the death of one man (saving three women)?Footnote 2 The sexual selection account assumes that men are more prepared than women to eliminate sexual rivals by the infliction of physical harm, especially in situations such as that described in the dilemma, which pose no physical risk to themselves. Accordingly, we predict that men will make the anti-utilitarian decision to cause the death of three men (same sex rivals) and let one woman live in the genderized dilemma, whereas women will make the utilitarian choice of saving the three instead of the one.

In the rest of this article, we offer repeated evidence for this phenomenon, in online and offline studies, and we identify moderators and boundary conditions which are consistent with its sexual selection account. We reason that if male anti-utilitarianism in the genderized dilemma derives from sexual selection via intrasexual competition, then it should be especially strong in contexts where sexual resources are scarce (and intrasexual competition is fiercer Arnocky et al. 2014). Furthermore, and for the same reason, we expect anti-utilitarianism to be lower for homosexual males and to disappear when the dilemma features a woman in their 50s, who no longer has reproductive value.