In Sex/Gender: Biology in a Social World, scientist Anne Fausto-Sterling argues that gender researchers are shortsighted when they structure the debate about gender identity as nature vs. nurture or biology vs. psychology. Why does she suggest this is the case?
Anne Fausto-Sterling demonstrates all throughout chapter 5, titled "Am I a Boy or a Girl?--The Emergence of Gender Identity," in her book Sex/Gender: Biology in a Social World, that the scientific investigatory framework of nature vs. nurture, otherwise phrased as biology vs. psychology, is simply too narrow. The main reason why it's too narrow is because evidence for both nature and nurture can be found.
As Fausto-Sterling explains, John Money and his colleague Anke Ehrhardt were the first to propose the idea that gender identity did not become fixed until the age of 2, and up to that point, gender identity was completely malleable. Their theory led the way to many gender corrective surgeries, but their theory was also shot down when the "famous Joan/John case" became known, "filled with personal tragedy and high drama" (p. 44). In contrast, Milton Diamond proposed the theory that a fetus's gonadal hormones, hormones responsible for stimulating reproductive organs, activate the fetus's brain to "produce 'brain sex'" (p. 43). What's more, the fetus's hormones "preconditioned the brain" so that, normally, gender identity accordingly develops alongside other sexual development (pp. 43-44).
However, even Diamond's theory is called into question when we take into account persons with gender identity variance, also called transsexuals or transgenders. These individuals are biologically born as one gender but feel "trapped in the 'wrong' body," so they seek corrective surgery to make their bodies conform with their identities (p. 58).
Hence, all in all, Fausto-Sterling is showing that it's simply not enough to understand gender based solely on nature vs. nurture theories. Instead, the reality seems to be that gender is a very complex web of both nature and environmental conditioning. She proposes that while it's true biology can influence gender identity, it's also true that psychology can influence gender identity.