Gut Check is a periodic look at health and science claims made by studies and by newsmakers. We ask: Should you believe this?

The Claim: The human brain is a mosaic of sex differences: Most women have brains with numerous “male” features, and most men have brains with many “female” features. That undercuts the popular idea that there are two distinct kinds of brains, male and female.

The Backstory: The search for sex-based differences in human brains has not been science’s proudest moment. Many claims are based on small numbers of brains and do not replicate in different or larger samples, as psychologist Cordelia Fine of Melbourne University noted in a 2014 review: “Male” traits found in one group of brains aren’t necessarily seen in the male brains in a different study. Other brain-sex studies are based on problematic neuroimaging techniques.

Pop psychology books and the media swallow even dubious claims, citing them to argue that differences in how men and women think, feel, and behave are hard-wired into the cortex — the “men are from Mars, women are from Venus” myth. For instance, studies reporting that women’s brains have stronger cross-hemispheric connections while men’s are more “lateralized” turn into claims that women are innately better multitaskers and holistic thinkers, while men are laser-focused thinkers. Studies reporting that women’s brains have more active “empathy” regions when they process facial expressions, while men’s barely pay attention, are trotted out to justify the stereotype of sensitive women and emotionally clueless men.

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First Take: A new study, published in Proceedings of the National Academy of Sciences, examined MRIs of 1,403 brains. Scientists led by psychologist Daphna Joel of Tel Aviv University focused on 16 regions for which previous studies had reported the greatest sex differences, such as the size of a structure or the density of connections between two regions. They tested something that seems like a tautology: whether male brains have mostly male traits and female brains almost all female traits. If so, Joel said, then brains are like genitalia: if you have one piece of female (or male) equipment you likely have all the rest, but nothing male (or female).

But brains aren’t genitals, Joel and her colleagues report. The vast majority of brains have some female bits and some male.

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In one group of brains, for instance, the left and right hippocampus (a structure involved in memory) were, on average, larger in females than males. If brains were like genitals, then people with a large left hippocampus would also have a large right hippocampus, while those with a small version of one would have a small version of the other. “But it turned out that these types of brains are rare,” Joel told STAT. “Most brains are composed of combinations of large, intermediate, and small” versions of a left and right hippocampus, with “combinations of features, some more common in females compared to males, some more common in males compared to females.”

A mere 6 percent of the brains had only “male” characteristics, only “female” ones, or only in-between ones, the study found. “You can be highly toward the ‘male-end’ at one feature but highly toward the ‘female-end’ on another,” Joel said. “Females will have more brain characteristics with the ‘female’ form than with the ‘male’ form and vice versa for males,” but “we cannot predict the particular array of ‘male/female’ brain characteristics of an individual” from his or her sex.

Second Take: These results add to a complex picture that you’d never guess from media accounts and pop psych books. After decades of research, “we’re still debating the question of whether there are sex differences in the [human] brain and whether they make any difference,” said neuroscientist Bruce McEwen of Rockefeller University, a longtime leader in the field.

Contrary to claims about men’s brains being “wired for” math and focused thinking, or women’s being wired for empathy and intuition and holistic thinking, whatever differences exist “don’t add up in a consistent way to create ‘female brains’ and ‘male brains,’” said Fine. The new finding that brains are mosaics of male and female is also “problematic for the idea of ‘male natures’ and ‘female natures.’”

How can these findings be reconciled with the thousands of lab-animal studies showing how sex hormones shape the developing brains to have male or female traits? For one thing, those effects aren’t as powerful as once thought. But perhaps the key fact is this, McEwen said: Brains belong to people who have lived for decades in societies that treat men and women differently and gave them different experiences and opportunities. Experiences change the structure and function of the brain, as the many discoveries in the field of neuroplasticity show. “The male-female mosaic might reflect how these brains lived,” said McEwen.

The Takeaway: The idea that brains are distinctly male or female is too simplistic. At the very least, everyone’s brain is probably a mosaic of “male” and “female” characteristics created by a complex interplay of genes, hormones, and the lives we lead.