The recent conversations about gender expression have rightfully supplanted the ways that we once conflated psychological and sexual variance. As a society, we’ve grown become more cognizant and accepting of the ways in which one’s gender identity is not always tied to one’s body. Though we’ve grown more accustomed to thinking of gender as existing on a spectrum, it’s not often that we think the same of our biological sexes.

“The main problem with a strong dichotomy is that there are intermediate cases that push the limits and ask us to figure out exactly where the dividing line is between males and females,” Arthur Arnold, a biological sex research at UCLA explained to Nature magazine. “And that’s often a very difficult problem, because sex can be defined a number of ways.”

The basic biology that we’re all brought up to believe in states that men and women can be differentiated by the presence or absence of the Y chromosome. While that logic holds true in the most general of senses, a closer look at chromosomal variance reveals a much more interesting and not immediately obvious truth: in many cases physiological sex can shift on a genetic level.

Differences of sex development (DSD) manifests itself in a number of different forms depending on which different specific genes are functioning in a variety of different ways. The sort of DSD that we’re most familiar with are those in which a person’s body demonstrably belies their assumed sex such as genetically female women who discover that their gonadal tissue developed into small testes rather than ovaries. These cases, Claire Ainsworth explains for Nature, occur 1 in about every 4,500 births. If we were to take more minute sex variation into account, however, 1 in every 100 people could consider themselves intersex.

“These discoveries do not sit well in a world in which sex is still defined in binary terms.” writes Ainsworth. “Few legal systems allow for any ambiguity in biological sex, and a person’s legal rights and social status can be heavily influenced by whether their birth certificate says male or female.”

Recently Thailand, India, and Australia have made small inroads in allowing for a wider variety gender representation, but Ainsworth raises a valid point. The law, generally speaking, is no friend to those in the biological in-between.