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.

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.