Nutt examines the physiological evidence for being transgender. One source may lie in the bed nucleus of the stria terminalis (or BNST), an area of the brain that is responsible for sex and anxiety responses, which is twice the size in men as in women. Scientists find that the BNST in transgender women looks exactly like those of cisgender (biologically female) women and is not affected after sex reassignment surgery or hormonal treatment. For cisgender (biologically male) men treated with estrogen for testicular cancer, the scientists found no shrinkage of their BNST.
Nutt returns to another scientific chapter in order to provide yet more information: in this case, what the biological source of being transgender might be. The fact that the BNST is the same in both transgender women and cisgender women, but not cisgender men treated with hormones, provides a strong case for the neurological basis of one’s sense of gender.
In 2008, Australian researchers discovered a genetic variation in transgender women: their receptor gene for testosterone was less efficient at signaling the uptake of male hormones in utero, resulting in a more “feminized” brain.
Nutt then addresses how two identical twins like Nicole and Jonas could share DNA but have a different gender identity. The answer, she posits, can be found in epigenetics, which looks at the environmental triggers that turn certain genes “on” or “off.” Those triggers often depend on the environment inside the womb, which differs for each twin since he or she each floats in his or her own amniotic sac and has his or her own umbilical cord. This is why identical twins also have unique fingerprints.
Nutt provides even further information on a confusion borne of Nicole and Jonas’s situation specifically: she enlightens her readers as to the fact that twins may share the same DNA but do not have all the same physical features or genetic expressions. She reminds readers of a common knowledge example of this idea in order to make it more readily understandable.
Epigenetics causes researchers to question the Darwinian principle of sexual selection: the idea that there are two genders because it helps ensure the survival of the species. Under these rules, traits like homosexuality are outliers, decreasing survival because they do not aid in reproduction. Nature contradicts these Darwinian theories, however. Sex change is a normal process in many fish species. There are intersex deer and male kangaroos with pouches. In 2015 researchers discovered a species of Australian lizard that changes sex when the temperature rises. These examples complicate the idea that only two genders enhance survival.
Nutt makes a concession to some of the theories that hold that being gay or transgender isn’t supported by sexual selection, but then counters that these variations are found innately in nature. Her examples support the point that multiple sexes and genders exist beyond the binary in nature. This is an important fact, because people who exist outside of this binary often face discrimination (as Nutt elaborates on in the next chapter) and are widely considered outsiders.
Some human societies embrace the reality of multiple genders. In Papua New Guinea, a third gender is recognized: individuals who appear mostly female at birth but masculinize at puberty. This same group is recognized in the Dominican Republic. In India, Pakistan, Nepal, and Bangladesh, millions belong to a transgender group known as the hijra, who, according to ancient myths, could confer luck and fertility. In Indonesia, the Bugis people believe there are five genders. Gender might be necessary, but not necessarily binary.
Nutt admits that having two genders is biologically necessary due to how humans have evolved, but that does not mean that only two genders must exist, nor that biological differences aren’t already found. She also cites other societies that have accepted these variations throughout history in order to contrast those societies with the U.S., which has often been intolerant and unaccepting by comparison.