I used to work at a science museum, and one of the pet projects I had was finding ways to get girls more involved with science and technology. What I figured out was, if you start very early on, girls are very, very interested in science/techy stuff, and are more likely to continue down that path on their own, regardless of peer pressure - parents are a whole different issue, of course. But if girls haven't been exposed to hands-on science by the end of elementary school (I'm Mexican, and our system runs different than in other countries), it's not likely they'll get involved.
I'm kind of an example of that. Growing up, I had some books about "how (insert everyday household item here) works??", and I liked reading them with my dad and brothers. I was allowed to play with things around the house, provided it wasn't something that could harm me, and any questions I had were answered - not always on the spot, but my parents were kind and insightful enough to provide me with an answer. That continued throughout junior high school and high school - which is how I ended up at the science museum. Now, my career of choice is not related to science (I'm studying a bachelor's degree in Translation & Interpretation), but I do want to get involved in science popularization, mainly translating texts so everyone can have access to the latest developments in science and technology. And I mean everyone - kids, adults, people with higher degrees of education and people who may not have finished elementary school.
On the other hand, I've female friends who know zero, nada, absolutely nothing about science and technology. Talking it out with them, their parents never allowed them to take apart an old radio, or help change a lightbulb, or watch a documentary on the Big Bang. Instead, they were provided with dolls and dresses and shielded from anything deemed as "man stuff".
Of course, I also have friends who were brought up like that, and are studying robotics… Difference is, somewhere along the way, they met someone who introduced them to that field. Or, like me, they got a book from Carl Sagan or Stephen Hawking that opened their eyes to the awesomeness of science and technology beyond gender.
It's a mix of a lot of things - upbringing, schooling, friendship, luck, and yes, even the will to get yourself into that field. I mean, I could've been a neurosurgeon, or researcher or something like that. I even studied psychology 2 years before switching majors. But I chose not to go that way, because my calling is different, and I feel I can do more good to science by helping spread what we already know than by researching new stuff.
That's just my take on things, though. But I do take some courage from the fact that, like WhiteyChan said, there's more girls and women stepping into the science/tech/math fields with each passing year.