Those who have it, according to the association’s Diagnostic and Statistical Manual of Mental Disorders, experience “a persistent and intense distress about assigned sex, together with a desire to be (or insistence that one is) of the other sex. There is a persistent preoccupation with the dress and activities of the opposite sex and repudiation of the individual’s own sex.”
And, it adds, “mere tomboyishness in girls or girlish behavior in boys is not sufficient” to warrant the diagnosis. It requires “a profound disturbance of the normal gender identity.”