Some scientists have argued that human beings evolved to be monogamous, while others claim that we evolved to be nonmonogamous. So which view is correct? According to recent research on the subject, the answer isn’t one or the other—it’s a bit more complicated than that.

Rather than all of us having the same relationship orientation, research increasingly suggests that some people are predisposed to different styles of mating. One of the most provocative pieces of evidence supporting this idea comes from a neuroscience study recently published in the Archives of Sexual Behavior, which suggests that monogamous and nonmonogamous orientations just might be “wired” into our brains.

In this study, researchers put 20 heterosexual men (aged 34 on average) in fMRI scanners and looked at what went on in their brains while they viewed different kinds of images. These men were carefully selected to ensure that half were “highly monogamous,” whereas the other half were “highly nonmonogamous.”

To qualify as “highly monogamous,” guys had to say that they’d never been in an open relationship, had never cheated (and had no desire to do so), rarely fantasized about women other than their current partner, and reported fewer sex partners than the average person—specifically, fewer than five. For comparison purposes, data from the General Social Survey reveals that Americans today report an average of about 11 sex partners.

By contrast, to qualify as “highly nonmonogamous,” guys had to report experience with both open relationships and infidelity, express a preference for having multiple sexual partners at the same time, and be more sexually experienced than average. These men reported an average of 30 previous sex partners.

While inside the fMRI machine, participants were briefly shown blocks of images that varied in content. Some image blocks were sexual, featuring nude men and women engaged in vaginal intercourse. Others were romantic, featuring clothed men and women doing things like hugging or holding hands. And yet others were neutral. Some of the neutral images featured pairs of people doing everyday things that weren’t romantic or sexual in nature (like barbecuing), while others were nature scenes that didn’t include any people at all.

Researchers then compared the patterns of brain activation they observed. What they found was that men, regardless of whether they were monogamous or nonmonogamous, demonstrated significant activation of the brain’s reward pathways when viewing sexual images. In other words—and perhaps unsurprisingly—both groups of guys found the sexual images to be about equally pleasing.

Different groups of people might be predisposed to a certain style of mating based on the way our brains happen to be “wired.”

When it came to romantic images, though, the story was completely different. Monogamous men demonstrated significantly more activation of the brain’s reward system in response to romantic images compared to nonmonogamous men.

Further, the researchers found that the areas of the brain that were activated when monogamous men viewed romantic images were largely the same ones that were activated when they viewed sexual images. In other words, monogamous men seemed to be processing romantic and sexual images in a very similar—and highly rewarding—way.

By contrast, nonmonogamous men showed different patterns of brain activation when viewing romantic and sexual images. In fact, there were a few areas of the brain that were active when they were looking at romantic images that weren’t active when they viewed sexual photos.

According to the lead author of the study, Dr. Lisa Dawn Hamilton, “nonmonogamous men showed more evidence of higher-level processing of the romantic stimuli compared to the sexual stimuli, potentially meaning they are thinking about it more consciously than the sexual pictures.” In other words, nonmonogamous men may have been studying and thinking about the romantic images, rather than simply experiencing an automatic reward response.

So what does all of this mean? One possibility is that different groups of people might be predisposed to a certain style of mating based on the way our brains happen to be “wired.” Translation: Tendencies toward monogamy or non-monogamy may be rooted in our genetics and biology.

That’s not the only possible explanation, however. For example, as I’ve written about previously for [Playboy](http://www.playboy.com/articles/4-personality-traits-open-relationship), people in monogamous and nonmonogamous relationships differ in terms of their personality and attachment style. Maybe these psychological differences are the key to understanding the brain activation patterns we saw in this study.

It’s also possible that what we’re seeing here is evidence of a learning process. Maybe having more practice or experience with monogamy or nonmonogamy effectively “rewires” the brain. If true, this would mean that our brain activation patterns are a consequence of our relationship experiences, rather than the other way around.

Obviously, we need a lot more research to know which—if any—of these explanations are correct. We also need work that addresses the limitations of this study, including the fact that it only focused on heterosexual-identified men, which means we can’t say whether the findings would be the same for women and persons with same-sex attraction. Further, the “highly nonmonogamous” group didn’t include anyone who had experience with polyamory, where a person has several committed partners at the same time. It’s possible that polyamorous men’s brain results would look quite different compared to the nonmonogamous men in this study.

Though we still have much more to learn, the results of this fascinating study provide further evidence that blanket statements about how everyone is “meant” to have the same relationship orientation probably aren’t true. Different kinds of relationships seem to work for different people, and the reason just might be because our brains don’t all work the same way.


Justin Lehmiller, PhD is a sex educator and researcher at Ball State University, a Faculty Affiliate of The Kinsey Institute, and author of the blog Sex and Psychology. Follow him on Twitter @JustinLehmiller.