Nature or Nurture.

David Reimer was a boy whose penis was destroyed in a botched circumcision (a topic for another blog post), the doctor did gender reassignment surgery and Reimer was raised as a girl.

http://en.wikipedia.org/wiki/David_Reimer

Prof. John Money oversaw the case, and had the theory that gender is a social construct, not a biological one.
The end result was tragic, as David grew up, rejected the gender reassignment, and commited suicide after depression. One of Money's comments is telling.

" He claimed his detractors believed "masculinity and femininity are built into the genes so women should get back to the mattress and the kitchen."

It seems that denying nature versus nurture has become an all or nothing proposition. Either you believe that gender is a social construct, and human beings are totally blank slates, waiting to be programmed, or you're a right wing misogynist.
Which of course, is a straw man attack. Just because masculinity or femininity are in the genes, doesn't mean that women have to be baby slaves. It means that biology should be considered just as much as social influences.

The excellent Norwegian documentary Hjernegask (Brainwash) looked into the roles of nature, nurture, and the current feminist dominated thought about the subject in norwegian countries.

http://www.youtube.com/watch?v=TgDGqi1MbOc

During this part of the documentary, Prof. Kristian Tambs, when studying the educational levels of twins, found that 60% of their results were genetic, 10% influenced by family, and 30% influenced by peers.

It's interesting that Prof. Tambs was reluctant to be interviewed, as his conclusions were unpopular. That is a subject for yet another blog post.

So how much is nature and how much is nurture? One thing is for sure, nature is not 0%, and we're never going to find out as long as people cling to their ideologies instead of honestly investigating the matter.