AssassinatedBeauty I can't speak for trans people in general, but there is a big overlap between intersex and transgender in people who were exposed to DES. I, along with most of DES "sons" I've talked to, have symptoms of hypogonadism (chronic below normal male testosterone production), and "eunuchoid habitus" (a type of body structure that's usually associated with intersex conditions, and makes you look a bit like a cross between a man and a woman).
Also, most of the people I've talked to with a history of DES exposure have genital anomalies of one kind or another associated with incomplete male development, including things like hypospadias, undescended testicles, and micropenis. In my case, although I developed normal male genitals and my testicles did fully descend, I was born with a hydrocele, which is an abnormality associated with testicular descent - it occurs when the channel the testicle descends through fails to close up afterwards. I have the eunuchoid body structure as well, and when I was younger, was very androgynous looking. No one ever said anything about it to me though, I think everyone was just seeing a feminine looking man with female body language, and immediately assuming I must be gay rather than thinking of intersex conditions.
Genital development is largely complete by the end of the 12th week after conception, and the way DES and other miscarriage preventatives tend to be prescribed means that most or all of the exposure occurs later in the pregnancy, during the second and third trimesters (by which time it's too late to have much effect on genital development, but it can still affect brain development, since brain development continues right up until birth). I think that's why you often end up with people who look male, but identify as women.
The David Reimer case on its own doesn't prove anything, however there have been plenty of other cases where male babies were surgically reassigned to female, with similarly disastrous outcomes, e.g.
www.ncbi.nlm.nih.gov/pmc/articles/PMC1421517/
So there's now plenty of clinical evidence showing that your gender identity is hardwired into your brain, and not learned. Zucker's theory is wrong, and you can't beat the trans out of kids (or use electroshock, hormones, reparative therapy or anything else). The only way to change someone's gender identity would be to go back in time and expose their developing brain to the correct hormones. Since there's no way of doing that, unfortunately we're stuck with the results.
One thing I will say though, is that it is an intersex condition. It's entirely possible to end up in a situation where some of your brain development has occurred as male and some as female, so you don't fit in very well as either sex. The system doesn't currently cater at all well for those of us in that situation, it seems to be geared towards pushing everyone into a nice neat male or female box.