In Nature, Claire Ainsworth explains why it’s becoming more difficult to discuss sex as a binary:
Sex can be much more complicated than it at first seems. According to the simple scenario, the presence or absence of a Y chromosome is what counts: with it, you are male, and without it, you are female. But doctors have long known that some people straddle the boundary — their sex chromosomes say one thing, but their gonads (ovaries or testes) or sexual anatomy say another. Parents of children with these kinds of conditions — known as intersex conditions, or differences or disorders of sex development (DSDs) — often face difficult decisions about whether to bring up their child as a boy or a girl. Some researchers now say that as many as 1 person in 100 has some form of DSD.
When genetics is taken into consideration, the boundary between the sexes becomes even blurrier. Scientists have identified many of the genes involved in the main forms of DSD, and have uncovered variations in these genes that have subtle effects on a person’s anatomical or physiological sex. What’s more, new technologies in DNA sequencing and cell biology are revealing that almost everyone is, to varying degrees, a patchwork of genetically distinct cells, some with a sex that might not match that of the rest of their body. Some studies even suggest that the sex of each cell drives its behaviour, through a complicated network of molecular interactions. “I think there’s much greater diversity within male or female, and there is certainly an area of overlap where some people can’t easily define themselves within the binary structure,” says John Achermann, who studies sex development and endocrinology at University College London’s Institute of Child Health.
These discoveries do not sit well in a world in which sex is still defined in binary terms. 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.
“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,” says Arthur Arnold at the University of California, Los Angeles, who studies biological sex differences. “And that’s often a very difficult problem, because sex can be defined a number of ways.”