A (light) form of
Gender Identity Disorder.
Transvestites feel the compulsion to sometimes dress and act as a person of the opposite sex.
Most transvestites are men (probably because women don't get labelled as such when wearing male clothing). Estimates are that about 1 to 5 percent of the male population experiences some form of transvestism. More than 85% of transvestites are heterosexual. Most transvestites feel no urge to actually become women, although some may find out through their cross-dressing that they are really transgenderist or transsexual. This does not mean that transvestites become transsexuals! Most transvestites are content with dressing up once in a while and are perfectly happy with their lives as a man.
Not much is known about the cause of transvestism. There is a theory that Gender Identity Disorder is caused by a disturbance in the timing or amount of the administration of hormones to the foetus during pregnancy. This would result in a child that has a body of one particular sex, and a brain of the other sex, completely or partly. The theory that it is caused by some flaw in the upbringing of the transvestite has been abandoned by most psychologists. Most transvestites state that there is a feminine side to their personality that needs to be expressed through cross-dressing. Some transvestites are actually fetishists, and are turned on by feminine clothing. Most are not, however.
The main problem for transvestites is often shame and acceptance. Many transvestites have thrown away all their female clothes and attempted to stop cross-dressing at least once, because they feel it is a wrong thing to do or because they fear being caught. Most of them end up either very depressed or with a complete new wardrobe, because stopping is very hard. Partners of transvestites often have a hard time accepting the transvestism as well, because they fear their partner is gay or transsexual, because they're afraid of the opinion of family and friends or simply because they cannot handle the idea of a (their) man dressing up in female clothing. Many transvestites stay in the closet because of that.
In some cultures transvestites and transsexuals are accepted as some kind of 'third sex'. A good example are the berdache among Native Americans.