Transsexualism is today defined as psychical disorder, but there are reasons to believe that also fysiological factors play a role, since transsexuals have memory of belonging to the opposite sex as early as they can remember.
A strong reason to believe that mental gender identity is decided already at birth is research on intersexuals (people born with the genitalia of both sexes) who have been given a gender by surgery soon after birth.
This does not preclude that transsexualism can appear as a psychosocial disorder. Both situations can in their most serious form cause much anxiety. The only possible treatment is regarded as surgery combined with hormone therapy.