Director, screenwriter, and producer Lilly Wachowski (formerly Andy) announced Tuesday that she is a transgender woman. Wachowski’s statement comes about four years after her sister, Lana, announced she was living publicly as a transgender woman. The Wachowski siblings are famous for work including The Matrix trilogy, V for Vendetta, and the Netflix series Sense8.

Wachowski released her statement in the Windy City Times after being threatened by The Daily Mail that the “definitely not a tabloid” organization was going to profile her transition against her will. She wrote:

“To be transgender is something largely understood as existing within the dogmatic terminus of male or female…. But the reality, my reality, is that I’ve been transition and will continue to transition all of my life, through the infinite that exists between male and female as it does in the infinite between the binary of zero and one.”

Because Wachowski shares the same gender identity as her sister, public curiosity begs the question: Is it possible that there is some biological reason that siblings would both have a transgender identity?

The answer is, to put it mildly, complex.

“There are likely several pathways towards a transgender identity and there are indicators that there may be a biological basis for transgender identity, but it isn’t yet very clear,” biologist and neuroscientist Rachel Levin told Inverse. “I strongly suspect that there are biological underpinnings to many of the major roots to being trans — but that’s not to say there is only one root. The science needs to be cleaned up.”

Levin is the Chair of Neuroscience at Pomona College and a contributor to the academic volume Trans Bodies, Trans Selves. While she has doubts about any science being conclusive, she says some studies do suggest a possible role for biology and genetics in determining gender identity.

The most famous biological evidence comes from research conducted by psychologist Antonio Guillamon and neuropsychologist Carme Junque Plaja. In 2013, the pair used an MRI to examine the brains of 24 females who made the transition to males and 18 males who transitioned to females, before and after they went through hormonal treatments. They found that before these individuals went through treatment, their brains resembled the brains of their experienced gender. The cortical regions in the right hemisphere of the brains of male-to-female subjects tender to be thinner, which is a characteristic of the female brain. On the other side, the females who transitioned to males had relatively thin subcortical areas in their brain, which is typical of male brains.

Milton Diamond, director of the Pacific Center for Sex and Society at the University of Hawaii, tells Inverse that “there is definitely a genetic connection.” In a 2013 study, Diamond found that there is a statistically higher instance among twins that if one twin is a transgender individual, then the chance that the other twin will be a transgender individual goes up. In this study, it was also more likely for male twin siblings to both have a transgender identity than female twin siblings.

Many children who identify as the opposite gender begin to have a sense of this at a young age. For Levin, that’s another reason to believe there might be biological underpinnings. A 2015 study found that among 32 transgender children, each child had a strong, secure gender identity and didn’t express any signs of confusion.

But what may be the most likely biological driver of transgender identity, says Levin, are differences in hormone receptors.

“There’s long been the thought that since most of the differences we recognize between male and female are the result of prenatal hormone exposure,” says Levin. “A promising idea is that when particular areas of the brain develop, the receptor hormones may be flawed. For example, there may be parts of the brain that are unable to recognized testosterone in developing male bodies and therefore they are feminized.”

Levin firmly believes that the research conducted so far investigating the biological reasons behind transgender identity have been inconsistent in their results — yet those studies make sense on an intellectual level.

Nevertheless, she cautions that to definitively say, yes the cause of transgender identity can be biologically traced, is to discount the transgender experience of people who — when tested — may not reveal a biological connection.

“My fear,” she says, “is that if, in the end, we can claim that there is a genetic basis or hormonal basis — if one doesn’t have that gene or that hormone exposure but knows oneself to be trans, then that doesn’t mean you are any less trans than someone who does have it.

“I think this biological determinism is frightening. I do think there is a chance there is a biological component, but I have serious doubts to whether we’ll ever find it.”

Photos via Lilly Wachowski