Co-authored by Dr Colin Wright (evolutionary biologist at Penn State, USA) and Dr Emma Hilton (developmental biologist at the University of Manchester, UK). The full article was published in the Wall Street Journal on 13th February 2020. The Dangerous Denial of Sex It's one thing to claim that a man can "identify" as a woman or vice versa. Increasingly we see a dangerous and anti-scientific trend toward the outright denial of biological sex."The idea of two sexes is simplistic," an article in the scientific journal Nature declared in 2015. "Biologists now think there is a wider spectrum than that." A 2018 Scientific American piece asserted that "biologists now think there is a larger spectrum than just binary female and male." And an October 2018 New York Times headline promised to explain "Why Sex Is Not Binary."The argument is that because some people are intersex — they have developmental conditions resulting in ambiguous sex characteristics — the categories male and female exist on a "spectrum," and are therefore no more than "social constructs." If male and female are merely arbitrary groupings, it follows that everyone, regardless of genetics or anatomy should be free to choose to identify as male or female, or to reject sex entirely in favour of a new bespoke "gender identity."In humans, as in most animals or plants, an organism's biological sex corresponds to one of two distinct types of reproductive anatomy that develop for the production of small or large sex cells — sperm and eggs, respectively — and associated biological functions in sexual reproduction.There is a difference, however, between the statements that there are only two sexes (true) and that everyone can be neatly categorised as either male or female (false). The existence of only two sexes does not mean sex is never ambiguous.Not everyone needs to be discretely assignable to one or the other sex in order for biological sex to be functionally binary. To assume otherwise — to confuse secondary sexual traits with biological sex itself — is a category error.The denial of biological sex also erases homosexuality, as same-sex attraction is meaningless without the distinction between the sexes. Many activists now define homosexuality as attraction to the "same gender identity" rather than the same sex. This view is at odds with the scientific understanding of human sexuality.Those most vulnerable to sex denialism are children. When they're taught that sex is grounded in identity instead of biology, sex categories can easily become conflated with regressive stereotypes of masculinity and femininity. Masculine girls and feminine boys may become confused about their own sex. The dramatic rise of "gender dysphoric" adolescents — especially young girls — in clinics likely reflects this new cultural confusion.The large majority of gender-dysphoric youths eventually outgrow their feelings of dysphoria during puberty, and many end up identifying as homosexual adults. "Affirmation" therapies, which insist a child's cross-sex identity should never be questioned, and puberty-blocking drugs, advertised as a way for children to "buy time" to sort out their identities, may only solidify feelings of dysphoria, setting them on a pathway to more invasive medical interventions and permanent infertility.