Brain scans show that very few brains feature all male-associated regions or all female-associated regions, more than 90 percent feature a mix. Photo by Zohar Berman/Daphna Joel
TEL AVIV, Israel, Dec. 1 (UPI) -- Researchers at Tel Aviv University say there's no surefire way to systematically differentiate between male and female brains. Every human brain features a unique mix of "male" and "female" traits.
"Human brains do not belong to one of two distinct categories," researchers wrote in their new paper on the subject.
Neuroscientists arrived at their conclusion after analyzing the brain scans of some 1,400 people, ages 13 to 85. For each brain, scientists measured the size of 29 regions identified by previous research as being correlated with sex.
What they found was that very few brains featured all male-associated regions or all female-associated regions. The brains of self-identifying men and women boasted a mix of male and female regions. In other words, brains exist on a continuum, or sliding scale, of traits associated with sex.
"Most people are in the middle," Daphna Joel, a neuroscientists at Tel Aviv, told New Scientist.
Though the new study, published this week in the journal PNAS, looked only at brain structure and not function, the authors say it supports what science has made increasingly clear in recent years, that gender isn't binary.
The findings fight against the theory that the early introduction of testosterone, as the fetus develops testicles, effectively masculinizes the brain.
Researchers aren't saying there are no differences between male and female brains, only that there are no hard and fast distinctions.
"Each brain is a unique mosaic of features, some of which may be more common in females compared with males, others may be more common in males compared with females, and still others may be common in both females and males," scientists wrote in their study.
But even these small differences or correlations may tell us more about cultural norms than biology.
"The bottom line is that saying there are differences in male and female brains is just not true," Gina Rippon, a researcher at Aston University, in Birmingham, England, who wasn't involved in the study, told The Telegraph. "There is pretty compelling evidence that any differences are tiny and are the result of environment not biology."
Armed with this new information, some scientists say it's time for society to rethink its emphasis on gender.
"People get wedded to the idea that being male or female is highly predictive of having different aptitudes or career choices," explained researcher Margaret McCarthy, an expert in brain sex differences at the University of Maryland in Baltimore. "This study fights against the idea that these outcomes are based on biological differences, as opposed to cultural expectations."