There is strong evidence that most transgender people are born with a predisposition to being transgender that was formed prenatally.
Generally identity seems to be fully formed before the age of 6 and then becomes very difficult to change or influence later.
There are thought to be three major factors in development: chemical/hormonal, genetic and environmental.
We know from brain studies that men and woman have small specific and identifiable structures in the brain that are different in size and proportion. When the brains of transgender person are analysed they almost always consistently match the brain structure of their adopted gender not their birth sex. Male to female transsexuals have a female brain structures. These changes are thought to be caused due to hormonal or chemical imbalances that cause the wrong hormones to be expressed by the mother prenatally. Sample sizes are small in these studies due to the low number of transgender persons’ brains available for medical study after death, but research is on going. There is a consistent trend even with the small and growing sample size.
CREDITS AND SPECIAL THANKS:
STEVE ROBERTS, BIRGITTA HOSEA, NILLY BROOK,
LOUISE KELLY INFORMATION MANAGER AT STONEWALL