In other words, there were very few individuals whose brain regions were all malelike or femalelike. And there was no clear continuum between the two endpoints.
Instead, across both gray and white matter and in connectivity patterns, brains are so overlapping that calling a particular form male or female is meaningless, Joel and her colleagues wrote. [Men vs. Women: Our Key Physical Differences Explained]
"Our results demonstrate that even when analyses are restricted to a small number of brain regions (or connections) showing the largest sex/gender differences, internal consistency is rare and is much less common than substantial variability (i.e., being at the one end of the 'maleness-femaleness' continuum on some elements and at the other end on other elements)," they wrote.
"Anyone who is aware of current data on brain sex differences appreciates that there is no such thing as a monolithic 'male brain' or 'female brain,' in much the same way as there is no such thing as a male heart and a female heart," said Lise Eliot, a neuroscientist at Rosalind Franklin University in Chicago and author of "Pink Brain, Blue Brain: How Small Differences Grow Into Troublesome Gaps - And What We Can Do About It" (Mariner, 2010).
Eliot, who was not involved in the new study, said the research was an "innovative approach" to sifting through brain gender differences. While statistical differences between the genders exist, she told Live Science, the new study shows that the distribution of malelike and femalelike attributes is patchy, not uniform.
Indeed, previous studies have found large areas of overlap in the structure of male and female brain structures, even when population-level gender differences are found.
"Every individual could have part of both men and women in them," radiologist Ragini Verma told Live Science in 2013 after finding differences in connectivity between male and female brains.
The findings are consistent with an emerging line of research on the hormonal control of brain development, Joel and her colleagues wrote. Early on, neuroscientists had viewed sex-specific hormones as the key to sex difference in the brain, with testosterone "masculinizing" the brain and estrogen "feminizing" it.
Though hormonal influences are important, the real story is far more complex, according to a 2011 review in Nature Neuroscience.
A growing body of evidence suggests that development is a give-and-take among genetic, environmental and epigenetic (above the genome) factors, all of which are acting in parallel and influencing one another in complicated ways. Different brain regions react in different ways to sex-specific influences, which are not limited to estrogen and testosterone, that review found.
Meanwhile, environmental influences such as prenatal or early-life stress can feed back into this process, again altering how the brain develops.
All of these parallel processes could explain why simple stereotypes about male and female interests, abilities and intelligence so often fail on an individual level, Jordan-Young told Live Science.
"The idea of a unified 'masculine' or 'feminine' personality turns out not to describe real people," she said. "It describes stereotypes to which we constantly compare ourselves and each other, but more people are 'gender non-conforming' than we generally realize."
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