Back in April, Caitlyn Jenner came out as a trans woman during an intimate interview with Diane Sawyer. Celebrated for her bravery, the Kardashian star proudly exclaimed that she is—and always was—female. Even her brain.

“I am not stuck in anyone’s body. It’s just who I am as a human being,” Jenner explained to Sawyer. “My brain is much more female than it is male. It’s hard for people to understand that, but that’s what my soul is.”

But what does it mean to have a female brain?

The research

According to a new study shared by user Bossman1086 in Reddit’s science community and published in the Proceedings of the National Academy of Sciences, most human brains are not usually uniquely male or uniquely female. That doesn’t mean Jenner is completely off the mark when she says her brain is “more female,” but we’ll get to that later.

The idea of humans possessing either a “man’s brain” or a “woman’s brain” is not new. You’ve heard the cliché rhetoric before: Men’s brains are “superior” at reading maps. While women’s “larger” hippocampus—the part of the brain associated with memory and spatial relationships—renders them superior at learning languages (even if a previous study found that that men and women have about the same size hippocampus relative to the rest of their brains, so we can scratch that theory, too).

“The theory goes that once a fetus develops testicles, they secrete testosterone which masculinizes the entire brain. If that were true, there would be two types of brains: male brains in boys and female brains in girls,” the study’s author, Professor Daphna Joel, head of the psychology program at Tel Aviv University, tells Upvoted.

By analyzing the MRI scans of more than 1,400 people, Joel and her team found that you can’t simply label a brain as male or female. However, that doesn’t mean there aren’t any differences between a male and female brain.

So, what does this mean?

Most individuals have both male and female characteristics in their brain, Matthew Johnson, a neuroscientist who isn’t affiliated with the study told Upvoted.

“Most men have more ‘male’ brain characteristics than ‘female’ brain characteristics. Most women have more ‘female’ than ‘male’ brain characteristics, but very few people have all of their sex’s traits and none of the other’s,” explains Johnson, who is an assistant professor of psychology at the University of Nebraska-Lincoln.

Think of parts of the brain in the same way you would think about height. On average, men are taller than women. But this isn’t always the case. If someone is 5’7″ tall—and that’s all you know about them—you really can’t determine their sex purely based on that fact alone.

“There aren’t really that many features, outside of our chromosomes and what we’re packing in our bathing-suit areas, that are perfect reflections of our sex,” Johnson says.

Environment and the brain

Contrary to past theories, testosterone is not a main driver of masculinization or feminization of the brain. How one was raised can actually alter brain structure.

Take, for example, the brains of musicians. Scientists have observed brain structures changing after musical training. Studies have shown that musicians have increased gray matter in the auditory, motor, and visual spatial areas of the cerebral cortex, according to Oliver Sack’s Musicophilia.

There is no reason why this can’t be applied to gendered socialization: It’s not a stretch to infer that a woman—or man—being treated a certain way might cause shifts in the brain and cause them to be more prone to do “manly” or “womanly” things.

“Everything we experience changes our brain to some extent,” Johnson says.

He goes to to explain:

“Actions we perform can influence our sex hormones—for instance, aerobic exercise and having sex (which I guess can be the same thing if you’re doing it right) both boost testosterone levels temporarily. The long-term effects of socialization or behavior would be a little hard to assess specifically, though, because it would require a pretty meticulous longitudinal study that would be hard to execute.”

Transgender brains

Back to Jenner. If Jenner had said she was born with a female brain, that would have been scientifically inaccurate. Jenner, however, said her brain was “more female,” and there could be some truth in that statement.

“For trans folks, some studies have certainly suggested that their traits are shifted in favor of their perceived/desired gender,” Johnson explains.

For example, if someone was born with typical male anatomy, but perceives themselves as female, their brain will typically exhibit more female-like traits than a cisgender man.

“So, even if as this study claims there are few/no true ‘all-male’ or ‘all-female’ brains, trans [women] will typically exhibit more [female]-ness in their brain measurements than other folks with [XY] chromosomes, (i.e., cis men),” Johnson says.

The study’s author, Daphna Joel, emphasizes that it’s hard to know for certain.

“Our study suggests that there are many ways to be male, and many ways to be female, but we didn’t attempt to find the brain region/s subserving gender identity,” Joel tells Upvoted. “So maybe [trans women who say they have more female brain characteristics] are right. We did not test this.”

Fun fact: Johnson said there is some evidence to suggest that people who had male to female reassignment surgery experience fewer instances of penile “phantom pains”—aka feeling you still have a penis after amputation—than men who lost their penises unwillingly in accidents.

All in all, more research needs to be done on the brains of transgender people to draw any concrete, scientific conclusions. There is still much about the brain that we don’t yet understand. What this study can tell us, however, is that you can’t just place the brain into simple categories. Most people are not born with a “all-male” brain or “all-female” brain. Rather, our brain has a mix of traits from both genders.