麻豆传媒

You may have read that having a male brain will . Or maybe that female brains are . But there is no such thing as a female or male brain, according to the first search for sex differences across the entire human brain. It reveals that most people have a mix of male and female brain features. And it also supports the idea that gender is non-binary, and that gender classifications in many situations are meaningless.

鈥淭his evidence that human brains cannot be categorised into two distinct classes is new, convincing, and somehow radical,鈥� says at the University of Bern, Switzerland.

The idea that people have either a 鈥渇emale鈥� or 鈥渕ale鈥� brain is an old one, says at Tel Aviv University in Israel. 鈥淭he theory goes that once a fetus develops testicles, they secrete testosterone which masculinises the brain,鈥� she says. 鈥淚f that were true, there would be two types of brain.鈥�

To test the theory, Joel and her colleagues looked for differences in brain scans taken from 1400 people aged between 13 and 85. The team looked for variations in the size of brain regions as well as the connections between them. In total, the group identified 29 brain regions that generally seem to be different sizes in self-identified males and females. These include the hippocampus, which is involved in memory, and the inferior frontal gyrus, which is thought to play a role in risk aversion.

鈥淭here are not two types of brain鈥�

When the group looked at each individual brain scan, however, they found that very few people had all of the brain features they might be expected to have, based on their sex. Across the sample, between 0 and 8 per cent of people had 鈥渁ll-male鈥� or 鈥渁ll-female鈥� brains, depending on the definition. 鈥淢ost people are in the middle,鈥� says Joel.

This means that, averaged across many people, sex differences in brain structure do exist, but an individual brain is likely to be just that: individual, with a mix of features. 鈥淭here are not two types of brain,鈥� says Joel.

Spatial awareness

Although the team only looked at brain structure, and not function, their findings suggest that we all lie along a continuum of what are traditionally viewed as male and female characteristics. 鈥淭he study is very helpful in providing biological support for something that we鈥檝e known for some time 鈥� that gender isn鈥檛 binary,鈥� says , a psychologist at the Open University in Milton Keynes, UK.

The findings will still come as a surprise to many, including scientists, says at the Rockefeller University in New York. 鈥淲e are beginning to realise the complexity of what we have traditionally understood to be 鈥榤ale鈥� and 鈥榝emale鈥�, and this study is the first step in that direction,鈥� he says. 鈥淚 think it will change peoples鈥� minds.鈥�

at Durham University, UK, isn鈥檛 surprised by the findings, however. He has been studying sex differences in cognition, such as whether men, as commonly believed, really do have than women.

鈥淎cross all kinds of spatial skills, we find very, very few that are sensitive to sex,鈥� says Hausmann. 鈥淲e have also identified spatial problems where women outperform men 鈥� the black-and-white idea of a male or female brain is clearly too simple.鈥�

Cultural expectations

Despite persisting stereotypes, girls are no worse than boys at science and maths subjects, either.

鈥淧eople get wedded to the idea that being male or female is highly predictive of having different aptitudes or career choices,鈥� says , who studies brain sex differences at the University of Maryland School of Medicine in Baltimore. 鈥淭his study fights against the idea that these outcomes are based on biological differences, as opposed to cultural expectations.鈥� Other body systems are also often wrongly considered to be either male or female, says Joel.

, head of the Gender Medicine Unit at the Medical University of Vienna in Austria, agrees that things aren鈥檛 so simple. 鈥淭here are differences between men and women when you look in large groups, and these are important for diagnosis and treatment,鈥� she says. 鈥淏ut there are always more differences within genders. We always need to look at culture, environment, education and a person鈥檚 role in society,鈥� she says.

If a neuroscientist was given someone鈥檚 brain without their body or any additional information, they would still probably be able to guess if it had belonged to a man or a woman. Men鈥檚 brains are larger, for example, and are likely to have a larger number of 鈥渕ale鈥� features overall. But the new findings suggest that it is impossible to predict what mix of brain features a person is likely to have based on their sex alone.

Genderless future

Joel envisions a future in which individuals are not so routinely classified based on gender alone. 鈥淲e separate girls and boys, men and women all the time,鈥� she says. 鈥淚t鈥檚 wrong, not just politically, but scientifically 鈥� everyone is different.鈥�

But other scientists contacted by 麻豆传媒 don鈥檛 think that will ever be possible 鈥� as a sexually reproductive species, identifying a person鈥檚 biological sex will always be of paramount importance to us, they say.

Even so, Joel鈥檚 findings can be used to help many people understand the non-binary nature of gender, says Barker. After all, some people don鈥檛 identify as either male or female, and others feel their gender identity shift over time. 鈥淚t鈥檚 a shame that people鈥檚 experience alone isn鈥檛 enough for us to recognise as a society that non-binary gender is legitimate.鈥�

鈥淲e need to start thinking a lot more carefully about how much weight we give to gender as a defining feature of human beings, and stop asking for it in situations where it simply isn鈥檛 relevant,鈥� says Barker.

Read more: 鈥淎 welcome blow to the myth of distinct male and female brains鈥�