Although there has been a recent increase in the mentioning of cross-gender behavior issues in the popular media, Gender Identity Disorder, as it is referred to in the American Psychological Association's Diagnostic and Statistical Manual IV, is not a new thing. Indeed, it may be as old as humankind itself. This consistent record across cultures and time adds to the notion that the disorder may be, at least in part, of biological origin. There is no clearly understood cause for Gender Identity Disorder. However, we have enough information about fetal and childhood development to implicate a complex interaction of events.
Nature starts out the human developmental process by using the female schema as a base.
For a male embryo to develop, something must be added. That something must be a Y chromosome inherited from the father. Embryos without the factor continue to develop undifferentiated as female. Nature takes male differentiation further by having the newly formed male testes flood the brain with male hormones at around the third month of pregnancy. Importantly, there is no corresponding feminizing hormonal surge sequence observed in females.