In his study of 43 Cases Jan Wålinder reports on a case study of a 29-year-old transsexual patient who after the taking anticonvulsant medication for only 21 days, watched his proclivities towards a gender transition evaporate, only to return after the medication was stopped. (See Case Study #15 below)
The Wålinder study was the first such study that demonstrated that a transsexual's proclivities toward a gender transition can be effected by the introduction of antiepileptic drugs.
Further, of the fourty three case study there is an very high incidence of patients that have a male relative (e.g., a father, an uncle or a brother) who was known to have struggled with alcohol abuse. Alcohol abuse is indicative of a presence of addiction. Addiction, including substance addiction and behavioral addiction, is a mental disorder that is passed from the male parent to the offspring via the male parent's chromosomes. The gene that carries the code for addiction within the chromosomes is three times more likely to be passed from the male parent to male offspring than passed from the male parent to female offspring. This proportion explains why the male-to-female ratio of transsexualism is 3:1. 
The high correlation between the incidence of substance abuse being reported in male relatives of the transsexual patients corroborates the hypothesis that transsexualism is a type of behavioral addiction, like an sex addiction, Internet addiction, or pathological gambline addiction.
 Elklund, P.L, Gooren, L.J., Bezemer, P.D.: (1988) Prevalence of Transsexualism in the Netherlands
read the case studies: Jan Wålinder's research paper, Transsexualism, A Study of Forty-Three Cases, Review