Toby was assigned female at birth. While living as a woman, he was attracted almost exclusively to women. But when he started taking testosterone at age 22, that began to change. “It was like a switch got flipped inside my brain,” he says. “I’d never really thought of men in a sexual way, and then all of a sudden, I’d look at some guys and have all these sexual fantasies about them.”

Toby’s story is far from unique.

Several studies suggest that changes in sexual orientation among trans people are quite common. Among 115 Dutch participants, for example, 33% of trans women and 22% of trans men reported experiencing changes in their sexual attractions. This was true of 49% of trans masculine and 64% of trans feminine individuals in a 2015 study of 452 participants from Massachusetts, with the majority of these changes occurring after social transition. In another 2013 study of 507 U.S. trans men who’ve started transitioning (including hormones and/or surgery), 40% reported some shift in sexual attractions. Almost identical results were found in a 2005 study of 232 U.S. trans women who had undergone surgical and hormonal transition, where 43% reported significant shifts in their sexual orientation (of 2+ points along the 7-point Kinsey scale).

Most of these changes include shifts from exclusive attraction to one gender pre-transition toward some level of bisexuality post-transition, as was the case with Toby, who declined to share his last name. But some people claim almost complete reversal of their sexual orientation. For example, 13% of the trans women in the 2005 study switched from exclusive or primary attraction to women to exclusive or primary attraction to men (there were no such drastic changes among those who were initially attracted to men). Of the trans men in the 2013 study who were initially attracted to either men or women, 6-7% experienced a complete reversal.

(All of these studies asked only about attractions toward men and women, so we don’t know about participants’ attractions to other trans or genderqueer people.)

Why these changes happen is less clear, and like many things in psychology, this may be a question with several correct answers.

Although we tend to think of sexual orientation as something fixed at birth (determined by our genes and prenatal hormone exposure) and usually unchangeable thereafter, it is undoubtedly fluid to some extent, so shifts in attraction could be no different from those that happen to many cisgender folks as well. Research with cisgender populations finds that almost 20% of young adult women and 5% of men report changes in their attractions over a 5-year period. But these general population numbers are much lower than those found in trans populations, suggesting there are likely other factors at play among trans folks.

Perhaps the most obvious candidate is the hormonal changes that trans people undergo as part of their transition. While the exact ways androgens and estrogens influence sexual orientation are not yet understood, most scholars agree that hormones — at least as far as prenatal exposure goes — play an important role. And many trans folks themselves directly link these changes in attractions to the hormones they are taking or suppressing.

Another possibility is that the changes in sexual attractions are precipitated by the physical changes that trans people experience after hormonal or surgical treatments. Dr. Seth Pardo, PhD, a behavioral health epidemiologist at the San Francisco Department of Public Health who has done research on transgender issues (and who happens to be a trans man who did not experience notable shifts in his sexual attractions over the course of his transition) explains how the lack of alignment between a person’s gender identity and their body can be an obstacle to sexual attractions, especially if those attractions carry more physical, social, and psychological risks.