Imagine your response to picking up a copy of the leading scientific journal Nature and reading the headline: “The myth that evolution applies to humans.” Anyone even vaguely familiar with the advances in neuroscience over the past 15–20 years regarding sex influences on brain function might have a similar response to a recent headline in Nature: “Neurosexism: the myth that men and women have different brains” subtitled “the hunt for male and female distinctions inside the skull is a lesson in bad research practice.”
Turns out that yet another book, this one with a fawning review in Nature, claims to “shatter” myths about sex differences in the brain while in fact perpetuating the largest one. Editors at Naturedecided to give this book their imprimatur. Ironically, within a couple of days of the Nature review being published came a news alert from the American Association for the Advancement of Science titled, “Researchers discover clues to brain differences between males and females,” and a new editorial in Lancet Neurology titled “A spotlight on sex differences in neurological disorders,” both of which contradict the book's core thesis. So what in the name of good science is going on here?
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For decades neuroscience, like most research areas, overwhelmingly studied only males, assuming that everything fundamental to know about females would be learned by studying males. I know — I did this myself early in my career. Most neuroscientists assumed that differences between males and females, if they exist at all, are not fundamental, that is, not essential for understanding brain structure or function. Instead, we assumed that sex differences result from undulating sex hormones (typically viewed as a sort of pesky feature of the female), and/or from different life experiences (“culture”). In either case, they were dismissable in our search for the fundamental. In truth, it was always a strange assumption, but so it was.
Gradually however, and inexorably, we neuroscientists are seeing just how profoundly wrong — and in fact disproportionately harmful to women — that assumption was, especially in the context of understanding and treating brain disorders. Any reader wishing to confirm what I am writing can easily start by perusing online the January/February 2017 issue of the Journal of Neuroscience Research, the first ever of any neuroscience journal devoted to the topic of sex differences in its entirety. All 70 papers, spanning the neuroscience spectrum, are open access to the public.
In statistical terms, something called effect size measures the size of the influence of one variable on another. Although some believe that sex differences in the brain are small, in fact, the average effect size found in sex differences research is no different from the average effect size found in any other large domain of neuroscience. So here is a fact: It is now abundantly clear to anyone honestly looking, that the variable of biological sex influences all levels of mammalian brain function, down to the cellular/genetic substrate, which of course includes the human mammalian brain.
The mammalian brain is clearly a highly sex-influenced organ. Both its function and dysfunction must therefore be sex influenced to an important degree. How exactly all of these myriad sex influences play out is often hard, or even impossible to pinpoint at present (as it is for almost every issue in neuroscience). But that they must play out in many ways, both large and small, having all manner of implications for women and men that we need to responsibly understand, is now beyond debate — at least among non-ideologues.
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