There is no such thing as a “male brain” or a “female brain,” new research finds.
Instead, men and women’s brains are an unpredictable mishmash of malelike and femalelike features, the study concludes. Even in brain regions previously thought to show differences based on sex, variability is more common than consistency.
“Our study demonstrates that although there are sex/gender differences in brain structure, brains do not fall into two classes, one typical of males and the other typical of females, nor are they aligned along a ‘male brain–female brain’ continuum,” the study researchers wrote today (Nov. 30) in the journal Proceedings of the National Academy of Sciences.
“Rather, even when considering only the small group of brain features that show the largest sex/gender differences, 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.” [10 Surprising Facts About a Woman’s Brain]
The new research is the first to examine sex differences in the brain as a whole. If the brain is truly sexually dimorphic, coming in a male and a female form, it should be consistently different between the two sexes, Tel Aviv University psychobiologist Daphna Joel and her colleagues wrote.
Consider the peacock, with its sexually dimorphic tail: The difference in color and size is consistent between the sexes – there’s no subset of peahens brandishing iridescent purple feathers.
Clearly, brains don’t fit this pattern; there is far more variation in brains within sexes than between them, a fact that has been known for a long time, said Rebecca Jordan-Young, a professor of women’s gender & sexuality studies at Barnard College in New York and author of “Brain Storm: The flaws in the science of sex differences” (Harvard University Press, 2010).
Many neuroscientists had already concluded that brains are checkered with a mix of male- and femalelike structures, said Jordan-Young, who was not involved in the new study.
But despite this variation, there could still be a continuum of male-type and female-type brains, Joel and her colleagues reasoned, so long as the gender differences between structures are consistent between men and women. They decided to put the question to a test.
The researchers combed through more than 1,400 magnetic resonance images (MRI) from multiple studies of male and female brains, focusing on regions with the largest gender differences.
In the first analysis, using brain scans from 169 men and 112 women, the researchers defined “malelike” and “femalelike” as the 33 percent most extreme gender-difference scores on gray matter from 10 regions. Even with this generous designation of “male” and “female” scores, the researchers found little evidence of the consistency they would need to prove brain dimorphism.
Only 6 percent of brains were internally consistent as male or female, meaning all 10 regions were either femalelike or malelike, the researchers found. Another analysis of more than 600 brains from 18- to 26-year-olds found that only 2.4 percent were internally consistent as male or female, while substantial variability was the rule for more than half (52 percent).
— source discovery.com