Evolutionary psychology has made attempts to understand why humans hate cheaters so much, but no one has come up with an ironclad explanation.
At a superficial level, sexual jealousy is arguably one of the most counterproductive emotions we have evolved to experience. Think of all the hours we humans waste pining over stolen partners or unrequited love, not to mention the devastation caused by crimes of passion. Plus, from an evolutionary perspective, surely more people having sex would mean more people, period. So why do we get so upset over what someone else does with their naughty bits?
The answer to the question is complex and still being investigated, though many theories have been posited around how sexual jealousy may have provided an advantage in our ancestral environment. What we do know is that while females and males are equally jealous creatures, the trigger points for jealousy differ greatly between the sexes.
According to decades-old research by psychologist David M. Buss, men are hardwired to feel jealous over a partner's sexual infidelity, while women are more likely to feel jealous when a partner is emotionally unfaithful. If we look back at our ancestors this kind of makes sense, and has led to what is known as the parental-investment model. From an evolutionary perspective, men needed to make sure their sexual partners were faithful so they didn't waste time and resources raising children that weren't theirs. Women didn't need to worry about that, but they would have had to depend on their male partners for resources while they raised children. A woman would therefore feel more threatened by emotional infidelity, as it could result in a partner giving his resources to another woman and her children instead.
This dynamic obviously isn't in place in relationships where pregnancy and childrearing isn't a factor, but our emotions remain the same when it comes to online relationships, for instance. Research published in the Journal of Applied Social Psychology in 2010 found that men are more likely to experience jealousy at the prospect of their female partners having cybersex than the thought of them forming an online emotional attachment. Women, on the other hand, found the idea of their male partner's forming emotional relationships online more threatening.
While the sex differences in jealousy have been replicated in multiple studies, the theory is not without its critics. Many of these studies involve asking subjects whether hypothetical sexual or emotional infidelity would distress them more. It is widely known that humans are bad at predicting their future emotional reactions, which throws the validity of these results into question. But, even studies that have observed actual jealous behavior (such as a 2011 study by Barry X. Kuhle that involved coders analyzing episodes of the reality program Cheaters) have reported findings that mirror this theory.
However, gender doesn't absolutely explain everything. In 2010, psychological scientists Kenneth Levy and Kristen Kelly focused on the types of attachment people display in relationships in a study and found that rather than being hardwired in us, attachment style is shaped in our formative years by parents and caregivers, and in later life by friendships and intimate relationships. Levy and Kelly found that people with "dismissing" attachment (those who value autonomy in relationships over commitment) are more likely to be distressed by sexual infidelity, and that this attachment style is more common in men. By contrast, men and women who have either "secure" or "anxious" attachment styles (the latter of which is slightly more common in women), find emotional infidelity more upsetting.
Like most of the other research in this field the work of Levy and Kelly has not been immune to critics. These include the authors of a 2015 study published in the Human Ethology Bulletin who showed that, in a sample of 88 men and 170 women from Chile, sex differences in jealousy were not explained by attachment style. In fact, gender was again the only predictor of the type of jealousy that was most distressing. (Perhaps the conclusion there is that Chilean relationships don't hinge on such predictable attachment styles?)
Like everything else in evolutionary psychology there's a number of factors at play, with varying levels of influence from our genes, our upbringing, and our environment. Still, there is evidence that men tend to get angry about sex, while women tend to get upset about emotional intimacy. And none of this is likely to change anytime soon.
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