By Kim Tingley

Men have a far greater appetite for sex and are more attracted to pornography than women are. This is the timeworn stereotype that science has long reinforced. Alfred Kinsey, America’s first prominent sexologist, published in the late 1940s and early 1950s his survey results confirming that men are aroused more easily and often by sexual imagery than women. It made sense, evolutionary psychologists theorized, that women’s erotic pleasure might be tempered by the potential burdens of pregnancy, birth and child rearing — that they would require a deeper emotional connection with a partner to feel turned on than men, whose primal urge is simply procreation. Modern statistics showing that men are still the dominant consumers of online porn seem to support this thinking, as does the fact that men are more prone to hypersexuality, whereas a lack of desire and anorgasmia are more prevalent in women. So it was somewhat surprising when a paper in the prestigious journal P.N.A.S. reported in July that what happens in the brains of female study subjects when they look at sexual imagery is pretty much the same as what happens in the brains of their male counterparts.

The researchers, led by Hamid Noori at the Max Planck Institute for Biological Cybernetics in Germany, weren’t initially interested in exploring sexual behavior. They were trying to find ways to standardize experiments that use functional magnetic resonance imaging (fM.R.I.) to observe how the brain responds to visual stimuli. In order to do that, they needed to compare past studies that used similar methods but returned diverse results. They happened to choose studies in which male and female volunteers looked at sexual imagery, both because doing so tends to generate strong signals in the brain, which would make findings easier to analyze, and because this sort of research has long produced “inconsistent and even contradictory” results, as they note in their paper. Identifying the reasons for such discrepancies might help researchers design better experiments.

A search turned up 61 studies that met Noori’s criteria for inclusion: Healthy adult men and women of different sexual orientations (including bisexual and transgender subjects) who had rated erotic images in terms of how arousing they were. Those participants had then been put in an fM.R.I. scanner — which detects changes in blood flow associated with neuronal activity — and been shown the most arousing images as well as neutral, nonsexualized ones. What Noori’s team found was that image type — whether it was a picture or a video — was the strongest predictor of differences in which parts of the brain became engaged. Unexpectedly, the weakest predictor was the subjects’ biological sex. In other words, when men and women viewed pornographic imagery, the way their brains responded, in the aggregate, was largely the same.

This latter, more provocative finding yielded the paper’s title, “Neural Substrates of Sexual Arousal Are Not Sex Dependent.” Headlines followed, along with controversy in the cognitive neurosciences. Researchers whose work has shown differences between men’s and women’s brains viewing sexual stimuli objected to such generalization. But the purpose of statistically analyzing many studies together, a process called meta-analysis, is precisely to be more conclusive: The goal is to reveal global patterns that smaller studies can’t.

The science of sex is inherently paradoxical. For centuries, social stigma, prejudice and misogyny have condemned as aberrant sexual pleasures we now know are healthy. Yet despite the growing realization of how much outside views shape even our most private behavior, we can still experience the mechanics of our own desire — never mind that of others — as a fundamental mystery. Noori’s team is trying to shed light on a big part of that lingering mystery: If men’s and women’s brains respond similarly to sexual stimuli, what accounts for the apparent differences in how they approach sexual practices?

Answering that question means connecting the dots from what triggers the firing of specific neurons to how those firings give rise to the myriad thoughts and feelings we have about sex to the actions we take in response to them. Knowing what all this should look like neurologically could give clinicians more ways to treat the 43 percent of women and 31 percent of men who, according to the Cleveland Clinic, report problems in their experience of sex. “Issues of sexual behavior and sexuality are highly associated with mental health, with life satisfaction, even with physiological health,” says Justin Garcia, director of the Kinsey Institute at Indiana University. That makes it crucial to find out what “the constituent parts” of sex are.

In fact, it is still extremely difficult to interpret what activity in a given region of the brain really means. When viewing erotica, women often (and far more often than men) experience a disconnect between their physiological arousal — measured by genital temperature, wetness and swelling — and what they describe feeling. This could mean that they do not realize or do not want to divulge that an image is turning them on, or that they believe an image is or should be arousing when it isn’t, physiologically. That dissonance raises a host of complications. To what extent do cultural attitudes toward pornography — historically, women have been shamed for consuming it — influence both our subconscious and conscious responses to sexual images? Because neuroimaging has been available for only the past 30 years — Noori analyzed studies from 2001 and later — there’s no way to compare similar scans from the ’50s, say, and see how shifting norms might have changed the results.

Complicating things further is the multifunctionality of brain networks. In 2017, Janniko Georgiadis and Gerben Ruesink, at the University Medical Center Groningen in the Netherlands, published a review that showed seemingly distinct patterns of brain activity for wanting sex, liking (or having) sex and the opposite, inhibiting sex. The broadness of these categories shows how opaque those concepts are. Melissa Farmer, a research assistant professor in the department of physiology at Northwestern University, points out that “desire might be anything from when you see someone up until you act on that and approach them. That’s a lot of steps.” Neuroimaging has the potential to delineate those steps more precisely and objectively than self-reporting possibly could.

But even “objective” brain activation can be ambiguous. In a previous study, Georgiadis found that in women, the same areas that tend to become active when viewing sexual imagery that neuroscientists have deemed pleasing also became active in response to photos of vomiting or feces. What scientists tend to regard as “arousal” on brain scans could also be its opposite, or perhaps some combination of each. Likewise, it’s conceivable that the “sameness” Noori found in male and female brain activity indicates that characteristics we’ve defined as opposites actually overlap. In 2015, researchers led by Daphna Joel at Tel Aviv University published an article that analyzed more than 1,400 M.R.I.s of male and female brains and concluded that in most brains, certain regions might be more “male” while others are more “female,” creating a unique gender “mosaic” that defies either-or classification.

Sexual behavior, in turn, is inextricable from other behaviors. Indeed, as Georgiadis and Ruesink point out, the same “sometimes quite generic” brain activity associated with erotic stimuli is also part of how we process “reward, memory, cognition, self-referential thinking and social behavior.” We use the same neural processes to determine if a sexual experience is valuable and worth repeating as we do for food and drugs. A more complete mapping of how men and women respond neurologically to pornography, and how that affects their behavior, might thus offer a model for explaining happiness or addiction.

“In the brain, sex is everywhere,” Georgiadis says, and recognizing its interactions with other mental processes might also argue for a different, less binary definition of it, both as a behavior and as a biological classification. As neuroimaging enables a more granular view of brain networks, we may find that new labels are needed. That could even argue for dispensing with categories like “desire” and “arousal” or “male” and “female” in favor of descriptors that better capture how those concepts intermingle and connect with others.

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