The author is a professor of physiology and neurobiology at Dartmouth’s Geisel School of Medicine and a former fellow of the OpEd Project.

The countersuits being brought by the federal government and states such as North Carolina over the use of bathrooms by transgender individuals encompass many complex issues on federal versus states’ rights. But they also highlight a critical, common and incredibly damaging misperception: that gender and biology are two separate things. A letter writer to my local paper notes with respect to the Obama administration’s actions dictating bathroom policy, “Strangely enough, it seems to be predicated on the idea that one’s gender is a matter of choice rather than biology.”

The writer is right — it is biology, but not in the way he means. I have spent more than two decades studying sex-specific differences in the brain. I hazard a guess that I know more than most people on this issue, although I also know that we scientists have only a rudimentary understanding of how genetic, epigenetic, hormonal and environmental factors interact during brain development and into adulthood to form the incredibly diverse and complex spectrum that forms gender roles and gender identity.

But, I know for damn sure that biology is a big part of it, and it is not simply what sex gets put on your birth certificate — an assignment that is predominantly based on whether or not one has a penis. There are numerous biological conditions, such as androgen insensitivity syndrome, 5-alpha reductase deficiency or Rokitansky syndrome, in which individuals with either XY and XX chromosomes are born without a normal vagina or uterus but also without a penis, and are nearly always assigned to be “a girl.”

It has to do with the biology of your brain.

What most people don’t know is that our brain is both literally and figuratively our biggest sex organ. The parts of our brain that control behaviors that have to do with sex and things that differ by sex are exquisitely sensitive to hormones and chemicals that can mimic hormones. They also express genes in patterns that differ between males and females.

Tune in Tuesday at 11/10C for PBS’ new late-night series Point Taken to see OZY co-founder Carlos Watson moderate a spirited debate on gender.

What’s more, the factors that regulate these brain regions do so not only during the hormonal rages of puberty but also early on in our development. In humans and other animals, these brain regions are molded to be different from before the time we are born. Once established, many of these changes are permanent. And while we may not fully understand all of these early actions, they are key to sex-specific behaviors, sexual preferences and, just as likely, gender identity.

We don’t know how or if nonhuman animals have a sense of gender, and so can’t study that experimentally. But we do know that male versus female partner preferences in rodents can be changed by altering those brain regions that govern sexual behaviors. We also know that that there are significant differences between cis- and transgender individuals in brain structures and the connections between them; these are correlated with differences in behaviors, such as processing of positive affective and erotic imagery.

Brain differences are biological. We should know then that to be transgender is not a choice.

There is evidence to suggest that increased levels of gender dysphoria (i.e., the variance between gender identity and chromosomal sex) may result from developmental exposure to abnormal hormone environments, especially increased levels of androgens in XX fetuses that can occur in conditions such as congenital adrenal hyperplasia. Anecdotally, my neuroscience colleague Ben Barres has suggested that prenatal exposure to testosterone-like drugs may explain his own transgender identity.

In humans, care must always be taken relating correlation to causation. Differences in brain structures may reflect mutual interactions among innate brain processes, expressed gender roles and society’s response to those actions. But brain differences are biological.

We should know then that to be transgender is not a choice.

It is not a choice when meta-analysis of suicide rates indicates that lifetime prevalence of attempted suicide in transgender individuals is ~40 percent as opposed to ~4 percent in the overall population. It is not the same as deciding whether you will wear a red tie or a green one.

It is a choice for us to educate those who mistakenly believe that allowing transgender people to use bathrooms appropriate for their identity endangers women and children. Such people either do not know transgender people or, more likely, know them but do not know they are transgender. And those that believe that real sexual predators will be dissuaded by a sign on a bathroom door are truly lost.

It is a choice for those of us who study the complexities of biology and the human brain to inform those who are not neuroscientists so that they can understand why it is not “a choice” for transgender people; it is who they are.