Male and Female Brains Are Different. Should It Matter?

Neuroscientist Larry Cahill on the great ‘neurosexism’ debate

Photo: Jonathan Kitchen/Getty Images

In a major policy change implemented in early 2016, the National Institutes of Health made clear its expectation that researchers seeking grants “consider sex as a biological variable in all stages of research.” The reason was simple — if overdue. Past studies often focused on males, yielding results that did not always apply to the other 50.8 percent of the population.

It’s not surprising that such a major decision would prove controversial in some quarters. What may be surprising, however, is that many of those taking issue with the idea are feminists.

That’s because among the various biological differences scientists must account for are those involving brain function. And the notion that sex hormones and sex chromosomes exert an influence on cognition and effectively make men’s and women’s brains “different” is vigorously disputed — in large part due to fears that the idea could lead to a kind of gender essentialism and feed into harmful stereotypes. There’s even a word for it: neurosexism.

“Can We Finally Stop Talking About ‘Male’ and ‘Female’ Brains?” went the headline of a 2018 New York Times op-ed by two prominent figures on the anti–sex difference side of the debate. The journal Nature, which less than a decade ago was routinely publishing articles that acknowledged the presence of genetic sex differences in the brain and other organs, ran an article in its February 27, 2019, issue entitled “Neurosexism: The Myth That Men and Women Have Different Brains.” The piece was a book review of The Gendered Brain: The New Neuroscience That Shatters the Myth of the Female Brain by Gina Rippon, a professor of cognitive neuroimaging in the U.K.

When neuroscientist Larry Cahill read the article, his first response was, “You’ve got to be kidding me.”

Cahill, a professor in the Department of Neurobiology and Behavior at the University of California at Irvine, is considered one of the world’s leading researchers on the influence of biological sex. He was accustomed to seeing popular books written by what he calls “ideologues.” But what stuck in his craw this time was that Nature had lent its imprimatur to a review that, aside from containing a multitude of what he considered misleading statements, was subtitled “the hunt for male and female distinctions inside the skull is a lesson in bad research practice.”

Cahill doesn’t do a lot of writing for lay publications. But on March 29, he published an article in Quillette entitled “Denying the Neuroscience of Sex Differences.” In it, he laid out some of the history of the field (it’s a short history, he says, since until about two decades ago, few people even thought to study the differences) and pointed out the inherent fallacy of equating such research with neurosexism. “By constantly denying and trivializing and even vilifying research into biologically-based sex influences on the brain,” he wrote, “[the anti–sex difference contingent is] in fact advocating for biomedical research to retain its male subject–dominated status quo so disproportionately harmful to women.”

I first met Cahill a year ago when I visited him in his UC Irvine office to talk about how his research related to theories about male brains being genetically predisposed to activities like computer coding. (His answer basically amounted to “it’s complicated.”) When I saw his piece, I was curious about what prompted him to write it and, moreover, why he did so in Quillette, a digital outlet that bills itself as “platform for free thought” and has been dubbed “the voice of the ‘Intellectual Dark Web.’”

I also wanted to talk with Cahill about what it’s like to study brain sex differences in an age when everything about gender is politicized and mainstream outlets often adhere to a safely progressive narrative. We spoke on the phone for about an hour last week. This is a condensed version of our conversation.

Meghan Daum: Why did you choose to publish in Quillette?

Larry Cahill: Because the editor asked me. The New York Times didn’t ask me. Nature didn’t ask me, and the Royal Society of London didn’t ask me. When I contacted Nature saying I felt they needed to do something to undo the damage they had done with that review, they did say I could submit a letter to the editor, but I chose not to do that. But when [Quillette editor Claire Lehmann] asked me to write a piece, I said sure. What prompted me to say yes was not Gina Rippon’s book itself. It’s yet another book by yet another ideologue that appeals to a certain wing of the non-neuroscientist population. I don’t care about that. The difference in this case was it was receiving the imprimatur of Nature. That one blew me away. To see that headline, honestly, knowing what I know about the dramatic change in neuroscience in the past 15 to 20 years, it’s not a lot different than seeing a headline like “The Myth That Evolution Applies to Humans.” That would be comparable.

You say in the article that, for decades, neuroscientists — yourself included — used only male research subjects because they assumed females were identical. And since females come with the complicating factor of the hormonal cycle, it’s just simpler to study males. Why was this the case, and what changed?

Like everyone else in neuroscience, I used to fully believe that there weren’t any sex differences or influences on brain function, with the exception of little-bitty brain parts explicitly associated with sex and sex hormones and sex parts. Those parts are different in males and females — okay, fine. But when you move beyond those little-bitty regions at the base of the brain, all of us, myself included, assumed that there were not fundamental sex differences, that they didn’t exist. This was the case no matter what we were studying: amygdala and emotion, the hippocampus and memory, the prefrontal cortex and working memory, vision—you name it. That’s precisely why we all studied males. The view was that if you want to understand the female, then, ironically, the best way to understand the female was to study the male. For most neuroscientists, the rationale was not a sexist rationale. It was the opposite. The rationale was there aren’t any differences between males and females, so you avoid the unnecessarily complicated feature of the female hormonal cycle and study the male.

What kinds of discoveries were made over the past 20 years?

In the past, we just assumed there were no differences, so we didn’t bother looking. Now it’s the complete opposite. More and more dominoes are falling. More and more neuroscientists are saying, “Wow, if I’m studying the hippocampal synapse—the center of the field of brain memory—in rats, I’m seeing that it’s operating somewhat differently in the male than in the female.” Once you discover this, you can’t pretend it’s not true. You can’t go, “Oh, it’s a bunch of bullshit, and I hear it’s neurosexist.”

But what would a discovery like that mean in practical terms?

As I point out in the Quillette article, it can indeed get tricky — like all of medicine and neuroscience. What exactly does a sex difference in the hippocampal synapse mean to you and me walking around? It can be very hard to say, which is a testament to the complexity of neuroscience. That’s why the anti–sex difference crowd always jumps on that. They’re saying, “Sure, there are sex differences, but we don’t know what it means.” On one level, that’s correct; we don’t know what it means. But we know exactly what it means on a fundamental level, which is that a foundational assumption of how we did neuroscience is wrong and has to change. A foundational assumption of how I spent the first 20 years of my career doing science was wrong.

What exactly are you trying to change?

I’ve spent the past 20 years of my career basically saying to my neuroscience colleagues and all of medicine — that means immunologists and cardiologists and everybody — “Hey, guys, you may not believe it, but there are sex influences all over the damn place. And you simply cannot be treating women equally by ignoring them.” This has been my mantra for 20 years now. This is something that I think people outside of science have trouble grasping.

One of the arguments by those who are made nervous about this kind of research is that the information could be twisted or misused for harmful purposes, sort of like how evolutionary psychology can get extrapolated into reductive and sexist conclusions about mating rituals and gender roles. What do you say to that?

Yes, that is one of their arguments. I say by that logic we shouldn’t study genetics. We’re not going back to the Dark Ages. [The German philosopher] Arthur Schopenhauer said there are three phrases to the acceptance of any truth: First, the truth is ridiculed. Second, it’s opposed. Third, it’s accepted as self-evident. I spent much of the past 20 years in the ridicule phase: “Oh, Larry Cahill, you used to do such good work. Now he’s studying the sex-differences bullshit!”

But it’s reached a critical mass now where both within neuroscience and outside of it, it’s getting scary for people who believe there can’t be sex differences in brain. So, now we’re in phase two, which is fighting it. I tell people to stay away from the ideologues on both sides of the issue. Stay away from the ones who tell you, “There’s male and there’s female and never the two shall meet.” But also stay away from the ideologues on the other side, who unfortunately are given a voice by editors at places like the New York Times, who know nothing about the issue except that they’re afraid of appearing to be on the wrong side of it.

Is there a battle between actual scientists and science journalists who may be in thrall to a progressive or woke ideology?

No, because most scientists are blissfully unaware. The world you operate so much in, the world of Twitter and media, is one that most of them don’t care about. They’re really more worried about how they’re going to deal with the issue in their next grant. Scientists don’t care about the latest book being talked about in the New York Times. That’s where I’ve lived most of my life. It’s just that I’ve been drawn into this now by having to deal with this small but ridiculous vocal group of ideologues, and I feel an obligation to not let them get away with utterly misrepresenting or even vilifying my science.

What happens on your own campus? Are you considered controversial?

You might think they’d be protesting outside my door. They are not. I teach a class here called Sex Differences in the Brain. It has a max of 60 people, and it’s enrolled about to the max. My students come out of the class going, “Wow, I had no idea about this stuff!” I point out to them that there are people on this campus in the Gender Studies Department who use books by one of these ideologues and literally have classes basically saying that what I’m saying is wrong. But there are no protests. I’m on a very liberal campus. Everything here is all about diversity and inclusion, and I’m doing fine. I should be a lightning rod, but I am not.

What’s an example of the kind of evidence for sex differences that your students find surprising?

One of the things that blows students’ minds is to realize that women disproportionately, about twice as much, suffer from all anxiety and depression disorders relative to men, and almost all our models for studying anxiety disorders are based on male animals. Even in those cases where the results in the male animal don’t necessarily generalize to the female animal, we still use them as models for human depression and anxiety disorders.

You write in the Quillette piece that we “should not run wild with implications of sex difference findings for brain function and behavior.” You also “reject the illogical conclusion that sex influences on the brain will mean that women are superior, or that men are superior,” and you say your colleagues agree. Why do you think some people remain so resistant to what really now appears to be the scientific consensus about sex differences in the brain?

The ancient Romans had an expression: Quod volumus, facile credimus. “We readily believe what we want to believe.” I wish we’d grow out of that as a species.