Some people dream about being the other gender. The fact that transgender people do so, are increasingly becoming accepted, but a recent Israeli study indicates that more than one third of non-transgender people have dreamed about belonging to "the other side" too.
It becomes harder to separate transgender people from non-transgender (cisgender) people if cisgender people are crossdreaming, but if we think of gender as a complex continuum rather than a strict binary, it starts to make sense. The Israeli studies tell us that the boundary between cis and trans is very fuzzy, indeed.
One in three people have had erotic crossdreaming fantasies
In my recent presentation of Justin J. Lehmiller's comprehensive study of sexual desire in the book Tell Me What You Want, I noted that one third of his respondents report that they have had erotic crossdreamer fantasies. In other words: They have imagined themselves as having the body of the opposite sex in their sexual fantasies.
Given that the great majority of his respondents identify with their assigned gender, this could mean that at least one third of Americans have been crossdreaming. Since there are many who have non-erotic crossdreamer fantasies, the percentage may be higher.
For instance, about one-quarter of men and women had fantasised about cross-dressing, and nearly a third had fantasised about trading bodies with someone of the other sex. In addition, about one in four men and one in six women had fantasised about having sex with a cross-dresser, and even more (about one in three men and one in four women) had fantasised about sex with a transsexual partner. [My emphasis]