I've talked to alot of different pre-meds, male and female, and the #1 reason given for not going MD/PhD for those thinking about it is the time involved. For most women, the time involved would prevent them from having children and starting a family. I don't want to make myself sound sexist, but I've never met a female MSTP wannabe cept RNAgirl, and the #1 reason cited is later family concerns given the strenuous and lengthy training.
Do women have to have kids? No. But, do most want to have kids and raise them well anyways? Yes. I don't want to hear this "The husband can do it" crap, cause women want to give their children a good start in life, and whether or not it's politically correct, they usually do bear the brunt of the child-rearing activities. My g/f is an anthropology major and a feminist, and we get into arguments in this almost weekly
I know many females who work in research labs in and around my University. In fact, I know more, and I think there are more female undergrads working in labs than males. But when I talk to them they're all considering MD OR PhD. Again, because of family concerns.
I really don't think there's some sort of underlying discrimination. Especially not with the numbers of oppertunities available around here for anyone of either gender. In fact, every once and awhile I'll get an e-mail from my Biology advisor giving research or pre-MD/PhD oppertunities that are minority or female only. Sometimes, when I was looking for a lab job, they would get me so mad I considered asking her not to send them to me anymore, since I'm a white male. To make a long story short, I just kept knocking on doors and found what I was looking for.
I think this comprises the largest reasons why you don't see many women in certain specialties also. Sure it's true that some women would only want (and reasonably so) to see pediatricians and OB/GYNs that are female. But, the sheer number of female pre-meds and med students that I've met that talk about their love for children and that as motivation for being a doctor makes me think that women really are looking to have kids and work with kids.
Please don't think I'm saying that men and women are unequal. To me, this is all COMPLETELY cultural. In America we put the baby doll in the little girl's hands and the race car in the little boy's hands and they grow up the way we've produced them. Then they do the same thing to their kids. It's not easy to fix. Women are equally capable of CHOOSING and PERFORMING in their chosen path. My new boss is a female MD/PhD, I'll have to ask her opinion on the subject.