June 14, 2022 - 2:15pm

This week, headline-grabbing research has found that roughly one in 500 men do not have the standard XY sex chromosomes, instead having an extra copy of either the X (XXY, also called Klinefelter syndrome) or Y (XYY) chromosome. These differences are caused by mutations in the egg or sperm before conception, and both are associated with health issues including a risk of infertility or of minor learning difficulties.

Much of the reporting on this either implies or directly states that these conditions have been revealed to be much more common than previously thought. But actually, this latest prevalence estimate is slightly lower than the figures currently given by the NHS and the US-based National Organisation for Rare Disorders.

What this new research provides is a reliable estimate for both how common and how serious these conditions are, using a large sample from UK Biobank, a fantastically useful database containing genetic data as well as demographic and lifestyle information from half a million people. This means they aren’t relying on individuals happening to come to the attention of doctors — instead, they check more than 200,000 men in one fell swoop.

Their finding was that a large majority of men with either of these mutations, XXY or XYY, do not realise that they have them — implying that on average, symptoms may be much less serious than you’d assume if you’re only looking at the subsection who receive medical attention for them. Similarly, in 2019 the same team discovered that many women who carry a sex chromosome mutation in some of their cells (having just a single X or three X chromosomes instead of the usual two) are not aware of it. In both studies, many affected individuals were able to have children naturally.

Conditions like these are often referred to as “intersex” and cited as evidence that sex exists as a spectrum from female to male. It’s frequently claimed that being “intersex” — biologically neither male nor female — is as common as having red hair. The argument goes like this: if sex is a cluster of characteristics, including genitals, gonads (ovaries or testes), and chromosomes, and someone’s sexed characteristics depart from the norm, then they are not fully male or female, and instead are somewhere in between.

However, when we consider people who have atypical sex chromosomes, this argument leads us to the strange position of trying to claim that someone is not “really” male or female despite looking and functioning like other members of their sex to the extent that they live their whole life not realising anything is out of the ordinary. Sex is about which reproductive role your body is developed towards; and though XX and XY are by far the most common, there are multiple chromosome combinations that can prompt a person to develop down either route. So how can we say that someone who produces sperm and fathers a child is anything other than male?

In fact, advocates dislike the term “intersex” as it falsely and hurtfully implies people with these conditions are not “real” men or women, preferring to refer to “differences of sexual development” or DSDs.

This new research does not show that it’s common for people to be “intersex” and that sex is a continuum. Instead, it illustrates that when people cite these conditions as an example of “intersex”, they are referring much of the time to physiologically normal males or females.

Ellen Pasternack is a PhD student in evolutionary biology at Oxford University.