Half the men in the control group were heterosexual and half were homosexual. When the researchers examined the BSTc of the transsexuals, they found a marked difference. The transsexuals' BSTc was more like the women's than the men's. In fact, the transsexuals' BSTc was, on average, slightly smaller than the women's. The researchers seemed to have found at least one biological motive for the transsexuals' desire to change sex, although it may not be the only one.
Several explanations are possible.
One is rooted in the process by which embyros take on sex differences. All human embryos develop in the very earliest stages of gestation along more or less feminine lines. Those destined to become males differentiate from the master template after a complex series of hormonal secretions starts to masculinize the embryo. Miscues in this process could result in crossed signals in the portions of the brain that are responsible for gender identity. Not everyone is convinced, however.