A sweeping analysis of research on the human brain found very few sex-based differences in structure and function, noting that once brain size is accounted for, any differences that remained were small and rarely consistent from one study to the next.
The researchers examined three decades of data from MRI scans and postmortem brain tissue studies, and their study was published Feb. 20 in Neuroscience & Biobehavioral Reviews.
“Like many neuroscientists, I was under the impression that if we can just get up to bigger sample sizes, we will get rid of the noise; we’ll be able to see these reliable differences,” said Lise Eliot, a professor of neuroscience at the Chicago Medical School of Rosalind Franklin University and the first author of the study. “And the sample sizes are getting larger and larger — well into the thousands — but we’re not finding these reliable differences.”
While working on her 2009 book Pink Brain, Blue Brain: How Small Differences Grow Into Troublesome Gaps — And What We Can Do About It, Eliot realized that there were few comprehensive overviews of the research on differences between male and female brains. What summaries did exist often focused on papers that included terms such as “sex difference” or “gender difference,” which means they likely missed many studies that compared men's and women's brains but failed to find a difference.
Additionally, many of the studies Eliot and her colleagues looked at didn’t distinguish between sex and gender. In their analysis, the team adopted the term "sex/gender" because researchers don't know when looking at the brain the degree to which male/female differences are the product of sex or gendered life experience, Eliot says.
Adult males generally have brains that are 11% larger than those of females. Larger brains have relatively more white matter — the “highways” of tissue that connect messages from one distant part of the brain to another.
“The bigger the brain, the longer the distance,” Eliot said. “But it doesn’t necessarily have functional implications, given that a woman with a large brain will have a higher ratio of white matter than a woman with a small brain, and the same thing within males.”
Much of the research that she and her colleagues examined presented conflicting results about structures alleged to be larger in one sex than the other.
“One large study will find a difference in one direction; another large study will find a difference in another direction,” Eliot said. “There’s not a consistent finding across all large studies that we can reliably, slam-dunk know for a fact that, say, the hippocampus is larger in one sex, or the orbitofrontal cortex is larger in another,” Eliot said, referring to brain structures involved in memory and decision-making, respectively.
After Eliot and her team took brain size into account, they concluded that the amygdala — another structure that plays a role in fear and other emotions — was about 1% larger in males, but this difference wasn’t statistically significant. Additionally, although there are algorithms that can discriminate between male and female brains from MRI scans with 80% to 90% accuracy, once brain size is controlled for, the predictions become only about 60% accurate. That's barely above chance, or not much more accurate than a coin toss.
Taken together, the findings indicate that — unlike with ovaries and testes, or the presence of horns on a ram and their absence on a ewe — there aren’t species-wide sex differences when it comes to brain structure, Eliot says.
She and her team also examined differences in lateralization, which refers to the amount a person uses one hemisphere of the brain at a time during a task, rather than both. Despite past claims that males are more lateralized, Eliot and her colleagues concluded that any actual differences were extremely subtle and cannot account for any behavioral sex differences.
The researchers also examined studies that used functional MRI to determine which parts of the brain became active during tasks that demanded verbal, spatial or emotional skills, such as recognizing the expression on another person’s face.
“The findings are different across nearly every study,” Eliot said.
Although overall differences between men and women have been recorded in psychology and the occurrence of conditions such as ADHD and anxiety, they don’t seem to correlate with any difference in brain structure or function that researchers have been able to reliably detect.
“The present synthesis indicates that such ‘real’ or universal sex-related difference do not exist,” Eliot and her colleagues wrote in the study. “Or at best, they are so small as to be buried under other sources of individual variance arising from countless genetic, epigenetic and experiential factors.”
The team did not address potential biochemical or hormonal differences that researchers have speculated might contribute to behavioral sex differences. Still, from what they could conclude, “It’s really not appropriate to think of the brain as coming in male-type and female-type, just like we don’t really think of the kidney or the lungs or the heart as coming in male- or female-type,” Eliot says. “We can’t transplant brains yet, but we do transplant all these other organs between males and females.”
The article, “Dump the ‘dimorphism’: Comprehensive synthesis of human brain studies reveals few male-female differences beyond size,” was published Feb. 20 in Neuroscience & Biobehavioral Reviews. The authors of the study were Lise Eliot, Adnan Ahmed, Hiba Khan and Julie Patel, Rosalind Franklin University of Medicine and Science.