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What your science teacher told you about sex chromosomes is wrong

The human sex chromosomes: X and Y. Y is the little one.
The human sex chromosomes: X and Y. Y is the little one.
UIG via Getty Images

Sarah Richardson is a historian and philosopher of science who focuses on the intersections between race and sex and the sciences. In her new book, Sex Itself: The Search for Male and Female in the Human Genome, she explores how cultural gender norms have influenced the study of sex in the genome, and vice versa. Earlier this week, I spoke to her on the phone about sex, gender, and two chromosomes called X and Y. This interview has been edited for length and clarity.

Susannah Locke: We've been calling the X and Y chromosomes "sex chromosomes" for about 100 years. What's wrong with that?

Sarah Richardson: When I first got into this subject, I just assumed that X and Y were always known as the sex chromosomes — meaning that they were naturally and obviously the markers of sex. But as I got into the history, I realized that the X and Y chromosomes were known as anything but the sex chromosomes for the first 20 to 30 years after they were discovered.

SL: My understanding from your book is that the only known role that the Y chromosome plays in sex is that it helps with the development of testes and sperm, and that's about it, right?

SR: That's about all we know — and we may know more in the future.

Some people are pushing claims that there are gene variants on the Y chromosome or at least differences in the dosage level of genes on the Y compared to the X that may in future be shown to correlate with sex differences in disease rates. I am highly skeptical of such claims. I really think that that seems to be a sex-chromosome-centric perspective that is quickly going to go out the window as research progresses.

SL: And yet I was taught that everything needed to make a male was on the Y chromosome. That's wrong?

SR: One might [incorrectly] presume that all of the typical traits of masculinity, including brain and behavior, would be coded for on the Y chromosome. I do think that this notion of the Y as the essence of masculinity has begun to pervade the culture — especially as we've moved into the more genomic age. I see it all over the place, percolating into the culture as a kind of metaphor.

In the book, I suggest that we might want to abandon the term "sex chromosomes." The reason is that [the term] has led us to focus our search for the biology of sex on these two chromosomes. But actually there are processes all across the genome that are critical to all the things we understand as sex — most importantly, the development of distinctive reproductive systems in males and females.

Secondly, [the term] has led us into the wormhole of thinking of the X as the "female" chromosome and the Y as the "male" chromosome. The X and Y have become little representatives of male and female at the genomic level, and that's perpetuated a really strongly binary way of thinking about maleness and femaleness. That is both empirically wrong and has misled scientists in a number of episodes in the 20th century.

SL: How has calling X and Y sex chromosomes misled scientists?

SR: One of the episodes that I talk about is the famous case of the XYY "supermale."

In the 1960s and 1970s, there was an intensive search for the behavioral qualities that were associated with crime and especially aggression and sexual aggression on the Y chromosome. And a large proportion of men with an extra Y chromosome were found in a few high-security facilities in Scotland.

On the basis of this, researchers hypothesized that the extra Y chromosome — the so-called XYY male — was a male with an extra dose of "maleness." So [the idea was that] this greater aggression had landed them in prison. And these conjectures were so compelling that between 1960 and 1970, 82 percent of all published studies on the Y chromosome focused on XYY men.

But in the end, it was found that there was no association between having an extra Y chromosome and heightened aggression. And the hypothesis has been fully debunked, except in the eyes of a few very marginal outliers. It represents one of the case studies of overstatement and hype and poor methodology in the history of behavioral genetics. And most scientists really hold it at arm's length as an example of old, bad science.

It was this assumption that researchers brought — that the Y chromosome must be the essence of masculinity — that led them to this intense conviction to overturn their own methodological principles and pursue this hypothesis. They were relying on their lay assumptions about what constituted masculinity.

SL: I was taught the XYY/aggression link as something that was true in psychology class. These myths can take a while to disappear.

SR: It was taught as a textbook example because it's such a simple, clean, clear, vivid, socially relevant example, that seems to intuitively make sense and is great for teaching middle-school and high-school students. For historians, textbooks are a really critical place to look because you see how knowledge gets crystallized, and once it gets there it can take decades to be removed.

SL: Let's talk about X, now. How did the X chromosome get this association with femininity when both men and women have them?

SR: Functionally speaking, both males and females have one active X chromosome in each cell.

This [the X as female] is an example of the way that we project the sex binary onto the natural world. It's as if we need the X to be the female chromosome if the Y is to the male chromosome. We need that symbolism somehow.

It was assumed until the 1950s that it was the double X in females that determined femaleness and lack of a second X in males that determined maleness. For half a century, that was official textbook knowledge. There's a way in which those older frames continue to persist even as that explanation is explicitly dismissed.

SL: One of the most interesting parts of your book was this scientific fight over whether the Y chromosome will disappear. It's a scientific fight and a cultural fight all at the same time.

SR: Our biological theories of sex are deeply intertwined with our cultural theories of sex and gender. The most recent prominent example is the way in which the prospect of the human Y chromosome "degenerating" has erupted into public debate.

The basics of Y chromosome evolution are not in dispute. It's been elegantly demonstrated that the Y evolved from an X chromosome and is a much smaller version of that. There's no question that it has lost a large number of its genes.

I think the term "degenerating," which is a technical term in this literature, has set off alarms. I think this is an interesting arena where cultural anxieties about masculinity in a feminist age are intersecting with a variety of scientific claims around the decline of maleness, male fertility, the Y chromosome.

And I look at the debate between two prominent sex chromosome scientists and try to understand how they are at once having a really serious scientific debate and at the same time having a debate about gender politics. One of whom [Jennifer Graves] argues that actually the Y chromosome could go extinct. The other researcher, David Page of MIT, has sought in every way possible to argue that the human Y chromosome is actually uniquely resistant to those pressures to further lose genes.

The upshot is we see how gender is always present in our science, but that doesn't mean that the science is bad or necessarily biased.

SL: You also have said that cultural ideas about gender and sex have helped science.

SR: There are some really striking examples where our broadening notions about gender and a more critical approach to gender have actually transformed science.

A great example of that is with theories of sex determination. In the 1980s, it was believed that scientists would find a single gene on the Y chromosome and that could be christened the "sex-determining gene."

And critiques of that male-centric model by all sorts of people — including openly feminist-identified scientists and gender analysts of science — really transformed the field. It caused scientists to ask tough questions about their model, and [now] there's a significant effort underway to look at genes all across the genome.

SL: The National Institutes of Health recently announced new rules that researchers need to include both male and female animals and cells in their experiments to the extent that they can find sex differences if they are there. What do you think of this?

SR: There has been a concerted effort by some activists to urge the study of sex differences at every level in every system in every tissue in every cell in every model organism. And their motto is "every cell has a sex."

But there's another kind of sex bias which is an overemphasis on differences. Documenting genomic sex differences [to this extent] could initiate a proliferation of uncritical findings of differences, many of which are likely to be false positives, to be very small differences that have no biological meaning.

I certainly agree that we do need to be studying sex differences and that the key is context and an appreciation for variation.

I see a dramatic increase in the number of sex difference studies in the genome in recent years, a huge increase in shabby sex-difference studies. And at the same time they're neglecting the variation within each sex and the variation between them.

One thing that's disturbing is the mandate to study sex differences and not a parallel mandate to study gender. For me, any claim of biological sex differences in humans has to involve either a ruling out or a documentation of interacting gender factors. For example, in brain research, the way we socialize males versus females produces what looks like a biological finding, but it might be a readout of cultural training.

SL: You wrote a similar book on race and the genome. How has that field played out differently than the one studying sex and genetics?

SR: I think that there's a much more robust critical cultural conversation as well as scholarly conversation about worries about studying racial differences in the genome as scientists responded to the potentially eugenic perils and the problems of enforcing facile ideas about human racial differences that contribute to racial discrimination.

Today you'll find dozens of conferences held a year where scientists are in conversation with social scientists, science studies scholars, historians, and philosophers, because they all realize while the scientific research is important, the social and ethical implications [of race and genetics] are also key.

Whereas in the case of sex differences in the genome, there is almost no attention. There's almost no engagement between researchers in that field and people who think deeply about and specialize in gender.

SL: Why is there a bigger cultural conversation about race and genetics than about sex and genetics?

SR: I think it is in part because of the history of racial science and eugenics, and the painful political history that that has had for many groups in our country.

I think that the contrasting direction that women's activism has gone in is to push for the inclusion of women in biomedical research. And having that be the goal has given sex differences research the gloss of women's health, a kind of pinkwash, if you will.