The issue with such studies is the social and cultural baggage about sex and desire that women likely bring into a controlled laboratory environment. It’s tough to get an unfiltered biological response to a sexual stimulus, especially when most studies rely on women’s own reporting about their responses—which is often shaped by the burden of sociocultural influences.
With the goal of stripping away that baggage and focusing only on a biological response, a research group has collected and processed brain-imaging findings from 61 relevant studies, producing a result likely to generate some controversy. It turns out, from the group’s analysis, that men and women don’t differ much at all in the brain pathways that respond to sexually arousing visual stimuli. These conclusions were published on July 15 in the Proceedings of the National Academy of Sciences USA.
Other studies have often used subjective reports of sexual arousal with a narrow focus on specific brain regions, says senior study author Hamid Noori, head of the Neuronal Convergence research group at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany. His team sought to look more holistically at brain networks that respond to visual sexual stimuli while excluding influences such as touch. In this way, he says, he and his colleagues focused on how sexual imagery alone triggers “fast, uncensored biological responses, looking beyond the curtain of social assumptions.”
With that curtain out of the way, Noori says, the team found that being male or female contributes very little to which neural networks fire up in response to visual erotica. To reach this new take on existing evidence, Noori and his co-workers examined studies that used functional magnetic resonance imaging (fMRI) to compare brains of biologically male and female participants of different sexual orientations: homosexual, bisexual and heterosexual (these analyses do not typically include nonbinary or intersex participants). The fMRIs reflected neural blood-flow changes when the brain kicked into gear while participants viewed arousing images. Noori and his colleagues looked at regions that became active together during arousal, using statistics to control for connections that cropped up by chance. What they found was scarcely any sex-based differences between men and women when brain networks responded to the visual stimuli. When the researchers dug deeper and looked at sexual orientation, however, they found separate arousal circuits were activated, depending on whether a participant was homosexual, bisexual or heterosexual.
These findings counter earlier results suggesting sex differences, Noori and his co-authors say, including the oft-cited work of Kim Wallen and his colleagues, who reported sex-based arousal variations in a brain region called the amygdala. In response to the new study, Wallen, a professor of psychology and behavioral neuroendocrinology at Emory University, questions some of the statistical conclusions. He points out that 90 percent of the participants in the 61 studies were heterosexual. For sexual orientation to really have had a big impact on the results, the other 10 percent of the population would have had to have a disproportionately big effect.
“In other words, 10 percent of the sample makes the difference,” says Wallen, who would like to have seen an analysis using only the 90 percent of subjects who were heterosexual. If men and women within the heterosexual group were still similar in how their brains responded to arousal, even as their heterosexuality affected the arousal response, “then I would be convinced,” he says.
Wallen points to a study his group published in 2014 that replicated its 2004 findings of differences in amygdala activation. That study included 13 biologically male participants, 13 biologically female participants and 13 female participants who carried a Y chromosome but developed as “female-typical.” Wallen and his colleagues found that the women differed from the men in their amygdala response to visual sexual stimuli and that the two groups of women were similar to each other in their brain activation. Noori and his co-workers included the 2004 study in their meta-analysis, Wallen notes, but not the 2014 replication of those results.
Noori says that he and his colleagues ran tests to see if adding in missing or overlooked data would have changed their results. “As we report in the paper, the findings are very robust,” he says. Because the 2014 replication study Wallen cites involved only 13 people per group, he says, its inclusion would be unlikely to move the needle much on these latest results.
Another co-author of the 2004 and 2014 studies showing sex differences in the amydala says that their conclusions were quite similar to what Noori and his team report: “For the majority of the brain, the neural response was actually similar between men and women,” says Stephan Hamann, a psychology professor and neuroimaging expert at Emory University, regarding his group’s studies. His team found a distinction only in a “few small structures,” such as the amygdala. Hamann also points out that the smallest resolution Noori and his colleagues used in their brain measures would cover up to 20 percent of the amygdala. Lurking within this one fifth, he says, could be finer-grained brain differences that went undetected.
Noori says that with the large number of people included in his team’s analysis, it should have been able to detect most differences that exist. “This does not mean that we claim there are absolutely zero differences, no matter what,” he says, and future studies with finer resolution might show other results.
Pursuing such fine-grained resolution to achieve the final word on differences—or lack thereof—between the sexes appears to be daunting. “Even if we resolve all individual subregions at some point,” Noori says, “every voxel still contains hundreds of thousands of [brain cells (neurons and glia)] forming complex networks that can be sex-specific.” In the meantime, he adds, “all that we can say is that based on the measurements so far that we collected consistently, the meta-analysis doesn’t indicate differences with respect to biological sex.”