It is a condition where the person is born with the genitals of one sex but the gender of another sex. N.B. Sex is of the body, Gender is of the mind.
Nobody knows for certain, however, there are several theories.
It is known that every foetus in a womb starts life as female and remains that way until about the sixth week of pregnancy. Then the mothers body produces a rush of hormones, either Testosterone, the male hormone or Oestrogen, the female hormone. It could be said that every male child has undergone a natural sex-change before birth. There is no need for transsexuals to suffer in the late twentieth century as treatments are available. The first thing a transsexual would need to do on discovering there is something wrong is to see their own doctor and ask for a referral to a psychiatrist who has the knowledge of gender issues.