What kind of brain differences might these behavioral differences reflect? Would the brains of nonmonogamous people process romance-related information differently from the brains of monogamous people? A new fMRI studysuggests so.

Researchers at the University of Texas at Austin brought 20 sexually active heterosexual men (aged 22-50, average = 34) into the lab, laid them down in an fMRI scanner and showed them different images, including sexual (e.g., naked people having vaginal intercourse), romantic (e.g. fully clothed people hugging or hand holding) or neutral (e.g., landscapes or people barbecuing).

A pre-study interview classified 10 of these men as highly monogamous: They had fewer than five sexual partners in their lifetime, never dated more than one person at a time, had never cheated on a partner and reported fantasizing about women other than their current partner less than once per month. In contrast, the other 10 men were similar in age but highly nonmonogamous: They had relationships with multiple partners (including both cheating and consensual nonmonogamy), preferred having more than one partner at a time and reported a median of 30 lifetime sexual partners (ranging from 15 to 200).

Results showed no differences in brain activation between the monogamous and nonmonogamous men when they were watching sexual images: Both groups found these images equally arousing. Given that sexual behavior is inherently pleasurable for humans (no continuation of the species without it, really!), this was hardly surprising.

However, the brains of the two groups differed quite a bit when it came to romantic stimuli. Monogamous men had increased activation in limbic and reward-related areas of the brain (for the neuroscience nerds out there, the right hemisphere thalamus, nucleus accumbens, caudate, pallidum, putamen, insula and prefrontal cortex) compared to nonmonogamous men. These are the same brain areas that past studies find light up when people who are madly in love look at photos of their loved ones. And the same brain areas in which animal research finds different distribution and density of oxytocin, vasopressin and dopamine receptors (neurotransmitters responsible for pair-bonding and reward processes) between the socially monogamous prairie voles and the nonmonogamous montane voles. There were no brain areas where the nonmonogamous men’s brains lit up more to romantic images than monogamous men’s brains.

Furthermore, among monogamous men, areas that were active when viewing romantic pictures were still active while viewing sexual pictures, indicating that sexual and romantic stimuli are closely related for them. By contrast, nonmonogamous men’s brain showed additional activity during romantic stimuli in several regions of the cortex that were not active during the sexual photos (including bilateral frontal and orbitofrontal cortex, RH pre- and postcentral gyri, bilateral superior temporal cortex and LH angular gyrus). This suggests there is a greater difference between romance and sex for nonmonogamous men. As Lisa Dawn Hamilton, the lead researcher in the study explains, “It seems nonmonogamous men are processing these images at a higher cognitive level instead of responding mostly with the ‘reptile brain’ (as Dan Savage would say). It’s as though they are studying a piece of art rather than something that is intrinsically rewarding to them.

Past studies have found that people in polyamorous relationships have higher testosterone than people in monogamous relationships, and that the sexually adventurous (like those more likely to have casual sex, to have cheated on a partner or to have interest in group sex) are more likely to have a certain type of dopamine-receptor gene. This new study adds yet another piece of evidence for the biological underpinnings of people’s tendency for monogamy versus nonmonogamy.

Of course, finding differences in brain activation doesn’t necessarily mean that these men have a biological predisposition toward (non)monogamy driving their behavior. It could be that men who are monogamous have had more rewarding relationships in the past and thus conditioned to associate romance with pleasure. Or it could be a combination of the two: Some brain differences were already present at birth, and later relationship experiences only amplified those initial brain differences.

To determine which came first, we’d have to scan the brains of the same people many times over the course of their lives, starting from early childhood before they had the chance to have any relationship-related experience that could shape their brains one way or the other. Given the current political climate when it comes to science funding, especially sex science, I doubt such research will happen any time soon.

There are a few study limitations worth mentioning: The sample included no women, trans folks, or people in openly nonmonogamous relationships, and the researchers didn’t control for a few factors that could potentially affect the findings, such as sexual desire or attitudes, attachment style or susceptibility to social norms.

Yet, this study offers a rare glimpse into the brains of people often socially stigmatized and vilified, and I truly hope we see more research of this kind.

Forbes

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