To find out whether these structural differences translated into cognitive differences, scientists examined detailed brain scans of more than 1,400 men and women. No matter which group of people they looked at, what type of scan was used or which part of the brain was examined, the researchers consistently failed to find patterns that set men and women apart.
“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,” the team wrote in a study published Monday in the Proceedings of the National Academy of Sciences. “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.”
To figure this out, the team — led by psychobiologist Daphna Joel of Tel Aviv University in Israel — went hunting for examples of brain “elements” that were either clearly male or clearly female. In other words, they looked for examples of measurements that appeared to cluster one way for men and another way for women, without much overlap in the middle. Then, after identifying these elements, the researchers looked to see whether women tended to have the “female” versions and men tended to have the “male” versions.
They started with a set of MRIs that measured the volume of gray matter in the brains of 112 men and 169 women ages 18 to 79. On these scans, they examined 116 separate regions and zeroed in on the 10 that showed the greatest difference between men and women. In each case, the 281 scans were divided into three categories — one-third considered “most male,” one-third considered “most female” and one-third in the middle.
Only 6% of the brains consistently ranked among the “most male” or “most female” in all 10 categories, the researchers found. On the other hand, 35% showed “substantial variability,” with male traits in some regions and female traits in others.
The study authors then repeated the analysis with different cutoffs for being “most male” and “most female.” Regardless of whether they used a threshold of 10%, 20% or 50%, the brains with a combination of male and female features far outnumbered the brains that were exclusively male or exclusively female.
Next, the researchers followed the same steps with other sets of brain scans that measured the thickness of gray matter in the outer layer of the cerebrum, the connections between different parts of the brain, and other features. As before, they found that consistently male or consistently female brains were rare, and brains with features related to both genders were common.
Finally, the scientists applied the same method to data from two large psychology studies of American teens. Using results from 570 participants in the Maryland Adolescent Development in Context Study, they found that only 1.8% of them scored consistently male or consistently female, compared with 59% who showed “substantial variability.” Among 4,860 participants in the National Longitudinal Study of Adolescent to Adult Health, the skew was even greater: 0.1% versus 70%.
Even in a data set of 263 college students who were asked about 10 “highly gender-stereotyped activities” like watching talk shows on TV or playing video games, the study authors still found that only 1.2% of the students could be classified as exclusively male or exclusively female, compared with 55% who had traits from both camps.
“This extensive overlap undermines any attempt to distinguish between a ‘male’ and a ‘female’ form for specific brain features,” Joel and her colleagues concluded. These findings have “important implications for social debates on long-standing issues such as the desirability of single-sex education and the meaning of sex/gender as a social category.”
As for genitalia, both kinds connect to the same part of the sensory cortex.