Nutt begins this chapter by explaining how humans have long thought they could influence a newborn’s sex through various rituals. She then describes some of the biological processes that shape sex and gender. Humans have twenty-three pairs of chromosomes, the last of which make a person a genetic female (XX) or a genetic male (XY), but at least fifty genes play a role in the development of sexual identity. Plus, sexual anatomy and gender identity come from two different processes that happen at different times, along different neural pathways. Genes and hormones are involved in both of these processes, and there are numerous biological events that can cause a person to have a gender identity that differs from his or her sexual anatomy.

Hormones play a large role in determining a person’s sexual anatomy. Sex organs begin to form when an embryo is about six weeks old, but the brain-based processes that affect sex and gender are at least somewhat distinct. This includes the determination of gender identity. Part of the brain called the “straight gyrus” seems to affect how feminine a person is, no matter if this person is a male or a female, genetically speaking. The larger the straight gyrus, the more feminine the person tends to be.