People often question why so much of morality is focused on sex. Journalist Richard Schiffman, for example, asks “is sexuality the big deal (morally speaking) that the religious hierarchy makes it out to be?” In support of his view that the answer is no, he quotes ethicist Margaret Farley: “In Western culture, at least since its Christian formation, there has been a perduring tendency to give too much importance to the morality of sex… ‘morality’ is almost reduced to ‘sexual morality’” [1]. This emphasis on sex probably seems especially odd if you believe, like sociologist Catherine Hakim, that in reality “sex is no more a moral issue than eating a good meal.”

Schiffman and Farley imply that Western morality is peculiar in its focus on sex, but it’s important to not exaggerate this claim. Rules about sexual behavior are very common cross-culturally; sex is something that human beings are strongly inclined to try and regulate (although there is considerable variation across cultures about what constitutes ‘appropriate’ sexual behavior). Further, compared to the rest of the world, most Western countries are not especially conservative sexually, and are generally much more permissive than, for example, most Islamic and Asian countries.

Nevertheless, the question of why so much morality is focused on sex is a compelling one. It’s also a complex question from an evolutionary perspective, with different answers depending on what specific moral rule you’re talking about: rules about polygamy, homosexuality, and incest, for example, probably all have quite different biological and cultural evolutionary origins. However, in this post I’ll focus on rules about one particular kind of sexual behavior: promiscuity. In many cultures, much of the moralizing that goes on about sex probably stems from efforts to discourage promiscuity, and thereby increase paternity certainty and encourage male parental investment.

From an evolutionary perspective, there is a clear and direct link between a man’s certainty of paternity and his willingness to ‘invest’ in—that is, to pay the costs of caring for and supporting—his putative children. (For an earlier post I wrote about this topic, click here). Maternity was always certain in ancestral environments, whereas paternity never was. In order to reproduce successfully, men needed to ensure that their support of their mate and her offspring was benefitting their own genes, rather than those of another man. Evidence for a positive relationship between a man’s paternity confidence and his willingness to invest has been accumulating in behavioral biology for decades, and is reviewed in the excellent book Fatherhood [2], written by my fellow PT bloggers Peter Gray and Kermyt Anderson. One of the most interesting early studies on this topic [3], for example, shows that in cultures where promiscuity is low and paternity certainty is high, men tend to transfer resources to their mate’s sons, whereas when promiscuity is high and paternity certainty low, men tend to transfer resources to their sister’s sons. (When promiscuity is high, even if men don’t know if their mate’s children are carrying their genes, they at least know that their sister’s children are carrying their genes).

That’s not to say that a man won’t engage in any parental investment unless he has 100% paternity confidence, only that that the extent of his investment will vary positively with his confidence. And in some environments, women may benefit from the ‘paternity confusion’ caused by having more than one male partner, if it allows them to acquire investment from multiple men [4,5]. But in most cultures, the norm is for women to seek investment from one male mate, and these cultures tend to develop mechanisms for promoting and reinforcing paternity certainty. These mechanisms may not condemn just promiscuity specifically, but also demonize anything seen as related to or promoting promiscuity (lust, pornography, non-reproductive sex, premarital and extramarital sex, abortion, dancing, alcohol, etc.).

Why have so many cultural mechanisms evolved to reinforce paternity certainty, in, for example, Western and Muslim societies? Probably because fatherly investment has traditionally been important to children in these societies, and has therefore also been important to mothers, fathers, and other relatives of these children. In a culture where a child is relatively dependent on fatherly investment, everyone concerned with the child’s welfare will be relatively interested in ensuring that the putative father will have enough paternity certainty to be motivated to invest in the child. Further, fatherless children in these societies have historically tended not just to be disadvantaged, but to impose costs on society as well (for example, by increasing the need for publicly-funded orphanages).

If anti-promiscuity moral rules are calibrated to past environments in which male parental investment was essential, then these rules may become more relaxed as male parental investment becomes less essential in some of these environments (due to, for example, women becoming more powerful economically). You may see this outcome as either good or bad, depending on your own interests and cultural affiliations. From a scientific perspective, however, it's morally neutral, and just a good example of how our biological human nature can generate cultural change.


1. Farley M. (2006). Just love: A framework for Christian sexual ethics. Continuum International Publishing Group.

2. Gray P. B., Anderson K. G. (2010). Fatherhood: Evolution and human paternal behavior. Harvard University Press.

3. Hartung J. (1985). Matrilineal inheritance: New theory and analysis. Behavioral and Brain Sciences 8: 661-688.

4.Hrdy S. B. (1981). The woman that never evolved. Harvard University Press.

5. Walker R. S., Flinn M. V., Hill K. R. (2010). Evolutionary history of partible paternity in lowland South America. Proceedings of the National Academy of Sciences 107: 19195-19200.

Copyright Michael E. Price 2013. All rights reserved.

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