The tension between nature and nurture shapes Colapinto’s writing throughout As Nature Made Him. Dr. John Money’s theory is that people are “psychosexually malleable” when born, and so he believes that nurture creates gender identity. Dr. Milton Diamond, around the time that Money’s theory becomes standard understanding in the medical field, proposes that biology plays a much larger role in determining gender and sexuality than Money’s theory allows for. Throughout the second half of the 20th century, the two men battle over these competing ideas.

The debate between nature and nurture permeates Brenda/David’s childhood development story. As Brenda’s parents and doctors work to instill in her gendered behaviors, even applying extreme methods suggested by Dr. Money, Brenda feels herself resisting those behaviors, though she wants to please them. At puberty, Brenda’s biology seems to rebel against girlhood and femininity. Her experience feels biological: Once she knows the truth about her origins, Brenda decides to become David.

Importantly, Colapinto addresses the reality that David’s case is not uniform proof that biology, or nature, alone determines gender. He suggests only that “continuing to assert nurture’s primacy over nature” allows harmful sex assignment surgeries to be performed on children (278). His work helps tip the balance toward nature as a determining, but not single, factor in the development of gender and sexuality for a human.