Sex is one of those things that nearly everyone has an opinion about. Politics in the U.S. often revolves around topics of sexuality. Should gay people have the right to marry? When should a woman be able to seek an abortion? Do employers have to cover birth control in their health insurance plans? Why do we care so much about the sex other people are having that we make it a cornerstone of many domestic policy issues?

Promiscuity is a topic that comes in for plenty of this kind of moralistic attention. When my colleagues and I published a paper on promiscuity in Archives of Sexual Behavior [1] last summer, our research was covered by media outlets like Slate, the Atlantic, and New York Magazine. And when I blogged about our paper for Psychology Today, it became my most popular post in over three years of blogging, so far attracting about 170,000 page views.

As noted in that blog post, our Archives paper suggested that anti-promiscuity morality originated to maximize the likelihood that fathers would support their children. But in the reader comments about this post that appeared on Psychology Today’s main site and Facebook page, a commonly-suggested alternative explanation was that anti-promiscuity morality serves primarily to prevent sexually transmitted diseases (STDs) among the general population. I’m skeptical about this alternative hypothesis, but I’ll use it here to illustrate a larger point: evolutionary theory can illuminate the obscure and non-conscious sources of our moral beliefs, including our beliefs about the sex lives of others.

Compared to most mammalian males, men (on average) spend a lot of time, energy, and effort taking care of their offspring. In most human cultures, fathers are very involved in their children’s lives, and only one man is recognized as a child’s father (however some cultures represent important exceptions to these patterns [2,3]). In order to support his child, this man must be able to recognize it as his own. Because promiscuity undermines a man’s confidence in his paternity, it makes it harder for him to identify, and thus to invest in, his children. Therefore, in environments in which investment from the father is especially crucial for families —for example, cultures in which women tend to depend heavily on one male mate for economic resources—you would predict that people would express greater moral opposition to promiscuity.

In our study, this is precisely what we found: opposition to promiscuity was stronger among people who knew more women who depended economically on a male mate, and it was stronger in US states in which female economic dependence on men was higher. This relationship between female economic dependence and anti-promiscuity morality remained significant even after we controlled for other predictors of anti-promiscuity morality like religiosity and conservatism. Other evidence also suggests that anti-promiscuity morality is related to promoting paternal investment. All over the world, in countries where women’s incomes are lower than men’s, people are less interested in short-term mating [4]. Anti-promiscuity morality is also linked to religious and political conservatism, which are themselves ideologies associated with environments of high female economic dependence.

So there’s some compelling evidence suggesting that anti-promiscuity morality was designed, by biological and/or cultural evolution, to solve problems related to fatherly investment. What’s the case for the suggested alternative explanation, that people oppose promiscuity because it spreads diseases among the general public? On the surface, at least, this might seem like a solid argument. After all, promiscuity really can contribute to the spread of STDs in society, and STDs really are bad, so opposition to STDs seems like a practical and down-to-earth reason to oppose promiscuity. But I don’t think this argument really adds up when you think seriously about its implications. For one, it implies that people who are relatively opposed to promiscuity will also be relatively concerned about public health in general. This prediction strikes me as intuitively implausible, simply because people I’ve encountered who seem most anti-promiscuity don’t seem more concerned than anyone else about public health. But intuition and personal experience can be unreliable guides to reality, so I wondered if I could somehow test this prediction, using a methodology similar to that of our Archives paper.

One of the strongest effects we reported in that paper was that in states with lower median female income, people are on average more opposed to promiscuity (and this effect is specifically related to female as opposed to male income). I wondered if opposition to promiscuity would also be higher in states that invested proportionality more in public health programs. If so, it would imply that public health in general was a higher funding priority in more anti-promiscuity states. To test this prediction, I analyzed the relationship between states’ per capita 2012-13 public health budgets [5] and average within-state levels of anti-promiscuity morality (the same variable used in our Archives study). I controlled for the effects of within-state per capita income [6] and median age [7] on public health budgets, since these budgets may be higher in states with older populations and wealthier tax bases. And as in our Archives analyses, I ensured that states with larger sample sizes were weighted more heavily in the analysis. Results indicated an extremely weak negative relationship between the two variables (that is, public health spending was very slightly lower in states that were more opposed to promiscuity, the opposite direction predicted by the “concern for public health” hypothesis). However, this relationship was nowhere close to being statistically significant [8].

The test presented above could not produce evidence that moral opposition to promiscuity is associated with greater concern for public health. I won’t pretend, however, that a single test presented in non-peer-reviewed article could settle the issue of whether anti-promiscuity morality is related to such concern. And even if anti-promiscuity morality did have little to do with concern for public health, it could still be related to the individual’s private desire to avoid catching (or transmitting) an STD. Indeed, it has been cogently argued that individuals are less likely to be promiscuous if they think they’re more vulnerable to disease [9]. Anti-promiscuity morality could very well be related to STD aversion on some level, and it’s an issue that merits much more investigation. Still, it’s also plausible that anti-promiscuity morality has less to do with STD-related concerns of any kind, and more to do with evolutionary principles of paternity certainty and parental investment, as outlined above and in our Archives paper.

If anti-promiscuity morality does turn out to be more related to fatherly investment than to STD aversion, why would the STD explanation seem so much more intuitive to so many people? The reality is that we have no reason to expect that people will be aware of the true origins of their moral beliefs. People often experience a moral belief initially as an emotional reaction, and then attempt to rationalize it in a post-hoc way. This rationalization may seem unrelated to the actual motivations behind the belief [10], and have little to do with the belief’s evolutionary function; a belief may be held with passionate intensity by someone who is completely ignorant of its biological and cultural evolutionary origins. It’s therefore entirely possible for people to believe strongly that promiscuity is wrong, because this belief emerged in past environments to promote fatherly investment, but to have no understanding that this is where their belief came from. They may rationalize their belief in whatever terms seem most reasonable, accessible and defensible to them—such as concerns related to public health or to religious doctrine—regardless of whether their rationale has anything to do with their belief’s actual origins. In cases such as these, evolutionary theory can be an indispensable tool for helping us discover the true sources of our moral beliefs.

This article was co-authored with Jade Gibson Price. Copyright Michael E. Price and Jade G. Price 2014. All rights reserved.

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