What Science Has to Say About Gender Identity

A few days ago, President Trump announced that transgender individuals would not be allowed to serve in the U.S. Military. This follows a previous Trump reversal of guidelines put in place by President Obama regarding how public schools should handle their transgender students’ use of bathrooms.

In discussions of these issues, a distinction is often made between “biological sex” and a psychological state of “gender identity” with the latter weighted as less important, less biological, or less real. Political commentator Ben Shapiro, for example, illustrates the attitude in his response to a transgender rights advocate questioning him at a recent lecture:

“I’m not going to modify basic biology because it threatens your subjective sense of what you are.” [Emphasis added.]

This sort of statement presupposes that Ben Shapiro understands the basic biology, which I’m sure he doesn’t given that biologists are still working it out.

What we do know from basic biology is that the classic model of sexual differentiation is probably wrong.

There is a classic understanding of the biology of sex: X and Y gene expression leads to the determination of female or male gonads (ovaries, testes), which in turn secrete hormones that lead to a wide range of sexual differentiation in females and males from external genitalia to body size and shape to behavior. Recent research, however, has demonstrated a more complex biology in which non-gonadal sex differences, including in the brain and the behaviors it controls, result from gene expression directly in these non-gonadal tissues. Much of the evidence for this new view has come from a range of animal studies demonstrating that manipulation of hormone levels does not fully account for non-gonadal sexual differentiation, even when it comes to behavior. To provide one example, castrated male zebra finches develop normal male song patterns and hormone-modified genetically female finches, who develop testes as a result of the hormone manipulation, nonetheless retain their normal female song pattern.

A zebra finch

If it is the case, as the existing science indicates, that biology operates along parallel pathways to determine and differentiate male and female phenotypes, then it is biologically feasible that genetic variation could lead to individuals with mixed sex differentiation, that is, with the gonads of one sex and a brain that leans the other way. One theory is that transgender individuals are the phenotypic realization of this biological state of affairs.

To put it into lay terms that policy makers and political commentators can understand, what this may mean is that your subjective sense of what you are is due to basic biology even if it disagrees with your gonads. And if this is true, the individual who Shapiro chastised might have responded, “I’m not going to modify basic biology because it threatens your subjective sense of what I am.”

To be clear, science does not yet have a definitive answer regarding the biology of gender identity. The underlying biology is complex and particularly difficult to study in humans. But at the same time, it is quite clear that if lawmakers, lawyers, and presidents are to engage in a debate that turns on biology, then state-of-the-art biological science must be part of the discussion.


Gregory Hickok is Professor of Cognitive Sciences at the University of California, Irvine where he is also a Fellow at the Center for the Neurobiology of Learning & Memory. He is author of The Myth of Mirror Neurons: The Real Neuroscience of Communication and Cognition. You can reach him on Twitter @GregoryHickok and follow his blog, Talking Brains


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9 thoughts on “What Science Has to Say About Gender Identity

  1. Thanks for the comments, everyone. Here’s a couple responses to the critiques.

    “the author has constructed a straw man of Shapiro’s basic biology belief” — The so called straw man has little to do with Shapiro; it was the standard model of the biology of sex in the field of biology until recently when experiments in a range of species suggested that is more complex than we thought.

    “It’s just so typical of the professorial class to attack and attempt to render mute conventional wisdom” — Actually, my point here is simply to look at what the actual science has to say rather than relying on conventional wisdom, which if left unchecked by science would still have us believing that the world is flat.

    “Go ahead, professor, call out Shapiro but give a pass to the free speech hating Leftists…” — Check my Twitter @gregoryhickok to see me call out a recent Berkeley cancellation of a supposedly controversial author.

    “The given examples demonstrate that birth biological sex differences remain even when sexual organs are manipulated. If anything this reinforces the biological determinism of binary sex differences even in the face of attempted interference.” — You get the biological determinism part correct but miss the point: the biology operates in parallel on more than one target organ. One target is the actual reproductive sex organs and the other, the brain, controls the behavior that is most cases will lead to the use of those sex organs by ensuring a sex drive, wiring in attraction to possible mates, and so on. If these are under different genetic control, this opens the door to possible mixed biological determination in sex organs versus the brain.

  2. The first two comments hit the nail on the head. The example given actually reinforces the fact that gender is extremely strongly tied to chromosomes, regardless of major environmental manipulations. The craziest part of the claim made by far left trans activists (who I don’t assume represent the majority of trans people) is that gender is completely independent of chromosomes and biology. This is a religious claim that defies all scientific evidence, and is in fact the same claim that fundamentalist christian conservatives made for decades. You can pray away the gay, conversion therapy can work, because there is no biological basis for homosexuality. It’s just a social and cultural creation.

    The other crazy part of this claim is that IF there is no biological link with gender, and gender is 100% a subjectivity steeped social cultural construct, then why take biological hormones and undergo surgery that were developed with objectivity based oppressive white male patriarchal medicine? Just be your own subjective self with your own subjective truth (in this instance “trans”).

    Personally, I have no problem with trans people doing whatever they want as long as it doesn’t infringe on other people.

  3. Transgenderism is more of a challenge to feminism than to biology.

    Feminism made the distinction between sex and gender, claiming the former was biological fact but the latter was a social construct. Biology determined the physical role of sexual differences in reproduction but any differences in behaviour were entirely down to socialisation.

    Men are aggressive and competitive and women are caring and nurturing because that is how they are brought up.

    Support for trans-rights has now led some feminists to concede that behaviour might be genetically determined after all, and that it is biological sex that is a social construct.

  4. Thank you for this logical perspective on the matter. This has been my position, albeit without much evidence beyond the obvious starting point.

    We know that we are physical, biological beings. We know that in large part people declare themselves to be (or accept or reject outside labelling as) male or female very early on in life. And this happens in places and at times where it could not possibly have been socially constructed to be so. So that tells me that there is no “gender”, but rather that sex in humans, although we are generally sexually dimorphic, is more complex than we’ve been led to believe (what a shock, I know) and that the biology behind creating your genitals is plainly not the same biology creating your brain, or else you could not end up with trans people. And it is pleasing to see this supported by evidence in other species as well. I look forward to more results in this area.

    Further, I believe that what we call “orientation” is really just another facet of “sex” like any of the others, and that all gay people are also trans (specifically, a kind of inter) in this regard.

    But always we must keep in mind that, in some sense, we are all our own species. Each individual should be treated with respect, and good grief if we have to police where people poop, that’s pretty disrespectful. The whole idea of segregated bathrooms itself assumes that men are evil, which is something I reject, and I don’t think is a message that our architecture ought to be sending.

  5. Swing and a miss…unless the true purpose of this article was to pick a fight w Shapiro, which is probably how publicity works in the era of Trump.

    But the article’s face-value “gender isn’t settled science” argument falls flat. I think Shapiro and other like-minded individuals would agree that male and female phenotype is highly variable (girly men, Tom boys, etc.). However, the author has constructed a straw man of Shapiro’s basic biology belief: chromosomes determine gonads determine hormones determine phenotype. The author then sites a publication demonstrating certain sex-specific phenotypes may develop independent of gonads, destroying the straw man. The author, however, misses the glaringly obvious conclusion that sex-specific traits ultimately aligned with chromosomal sex.

    Furthermore, the author’s example of phenotype (a bird’s song) is vastly ill-proportioned to the claim of Leftist vanguard HuffPo’s assertion that men have periods, too. It’s just so typical of the professorial class to attack and attempt to render mute conventional wisdom, giving activist extremists cover to affect real-world policy. Go ahead, professor, call out Shapiro but give a pass to the free speech hating Leftists who are infusing the educational system with notions that men may cyclically bleed from their vaginas as they shed their endometrium in preparation for ovulation and fertilization.

  6. This entire argument is a misdirection. There may be as many biologically based gender identities as finger prints, and it should make no difference to the conversation, because it is an argument over _words_, and words are part of a linguistic communication system, not some referent to ultimate reality. Thus words do not, and can not, have logical or hermetic definitions: they’ll always be fuzzy around the border. Even ostensibly simple words like life and death are like this.

    Male and female capture real statistical regularities in animals, including humans. Gender is distributed bimodally. That means that if we stop using the terms male and female, that humans beings will coin new terms that capture this bimodality. That’s the pragmatics of language. If someone doesn’t fit into a neat category, then it only means that we (whoever) were mistaken in thinking language has neat categories. That type of linguistics is 100 years out of date. And changing the language does nothing to change the statistical properties that language is referring to in the first place.

  7. Surely this argument is self defeating based on the evidence provided. The given examples demonstrate that birth biological sex differences remain even when sexual organs are manipulated. If anything this reinforces the biological determinism of binary sex differences even in the face of attempted interference. Nice try but defeated by logic

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