I went digging around in the science too, to find out exactly what was going on, and I found some of what Professor Jenny Graves has written about the development of sex in human beings. I want to quote something from The Conversation site—so she's put it into language that ordinary, everyday people can understand. She says, "Mismatch between biological sex and gender identity, culminating in its severest form as gender dysphoria, has been ascribed to mental disease, family dysfunction and childhood trauma. But accumulating evidence now implies biological factors in establishing gender identity, and a role for particular genes. Variants—subtly different versions—of genes linked with gender identity might simply be part of a spectrum of gender and sexuality maintained through … history." Her research shows that there is no simple XX and XY but that development of gender identity in human beings seems to be part of a complex web of about 30 genes that affect the development, and so on it goes.