The traditional "Female Brain trapped in a Male Body" is more common than I thought - latest good data indicates at least 1 in 500 women have that problem, and rather less, but still substantial, evidence exists that 1 in 500 men have a similar one. It's also well correlated with other incomplete or abnormal brain differentiation syndromes such as dyslexia, ambidexterity, and Asperger's.
Fortunately you do get a 2 standard deviation IQ increase on average compared to the normal population. It also correlates well with aptitude for Computer Science, Teaching, creativity, military planning, and mechanical ability. Hence the number of Computer or Electrical Engineer academics (like Lynn Conway), electronic music pioneers (like Wendy Carlos), computer game designers (Danielle Berry, Catherine Mataga, Jennifer Reitz...) , model or actual railroad fans (Jessica Brisbane), Rocket Scientists (Zoe Brain)...
From Game Designers Just Wanna Be Girls :
Could the physical cause of transsexualism be directly related to high intelligence?
The physical causes are not well understood. The prevailing theory has neurological changes being triggered by prenatal hormonal events. Supposedly transsexual babies don't get masculinizing hormones and then develop "female brains." It is also believed that much of the neurological development in the later stages is subtractive. Maybe transsexuals grew better hardware to start with, but I suspect the nerd factor is more important.
When did you first notice that the percentages seemed higher than among, say, auto mechanics?
We have a support group here in Silicon Valley and one night several of us just showed up. It was something like three out of twelve people there. That's when I started keeping a list.
This higher percentage has been observed in other populations. For example: railroad fans. Supposedly many of the engineers that worked for NASA were also transgender. Of course, during those days, you were more likely to hide it.
Looking back on it, being a mildly dyslectic, ambidextrous gamer and model railway fan with mild Aspergers, who was into game design, and who taught Computer Science at the Australian Defence Force Academy and was involved in a space programme, should have sent a pretty strong hint about my condition - had I known any of this stuff before it hit me.