I am from India and went to a high school where 60 percent of the girls wanted to go into medicine and the rest wanted to go into engineering (computer science/electrical engineering being the top choice). I did a B.Sc in Physics and my class had 60 percent females.
When I came to the U.S. for my Masters in Aerospace Engineering, I was completely stunned to find a university environment where most of the students in the engineering classes were male and a cultural environment where it was OK for women to say that shopping was a legitimate hobby, girls mostly wore pink, played with dolls, and worst of all, a complete lack of female role models.
Only one in five Physics degrees are awarded to girls. Why?
In fact, we're all classical physicists. We feel force, velocity, and acceleration at a gut level. In the science fiction novel Stranger in a Strange Land (1961), Robert Heinlein invented a word to express this kind of deeply intuitive, almost visceral understanding of a phenomenon: grok. 3 I grok force, velocity, and acceleration. I grok three-dimensional space. I grok time and the number 5 . The trajectories of a stone or a spear are grokable. But my built-in, standard-issue groker breaks down when I try to apply it to ten-dimensional space-time, or to the number 101,000, or even worse to the world of electrons and the Heisenberg Uncertainty Principle.
-Leonard Susskind 
Leonard Susskind (one of the fathers of string theory) may have a hard time "grokking" the Heisenberg Uncertainty Principle, but the majority of girls have a hard time "grokking" classical physics.
I think there are three main reasons: toys, experiences and role models.
Have you ever noticed how girls' toys are mostly static? Dolls, kitchen sets and stuffed animals don't demonstrate concepts of forces, momentum, cause and effect, control, friction, balance and inertia. Boys' toys on the other hand constitute trucks, balls, bats, gears, motors, remote controlled vehicles and sensors -- things that jump, roll, bounce, run, can be controlled etc. Lego sets, k'nex kits etc are more popular amongst boys, not girls. You just need to read customer reviews on Amazon to see how many more times grandparents will give lego sets to grandsons and not granddaughters.
To compound the above drawback in girls' toys, their play also serves as practice for the child care, kitchen work and home decoration they will likely take charge of as adults.
But adults prepare boys differently. Fathers in particular, encourage play that rarely involves taking care of someone else and is hardly ever direct practice for adult work. For instance, boys do not play at taking out the garbage or washing the car. If their play does involve someone's welfare like pretending to be a firefighter, the role is one for which adults are paid. However, most of boys' play remains play.
Although play is inherently enjoyable, adults shape children's preferences and proclivities for enjoyment. Adults encourage little girls to play at "pretend" housework and child care -- and most girls seem to find it fun. Boys would most likely feel the same, if encouraged in the same way. And, if boys were to spend as much time playing with dolls, they would probably develop the same nurturing skills; but boys spend very little time playing with dolls .
Most girls are usually entered into "girl-oriented" after school programs and summer camps such as ballet, theater, music, arts, dance or sports. Science, engineering, computer science experiences tend to be predominated by boys. And so the differences that first started as innocent and little, begin to widen each year.
Malcolm Gladwell's Outliers covers a similar point. It is all about practice. For example, it takes roughly 15,000 hours (or 7.5 years) to become a master programmer, 13,000 hours to become a master chef, 40,000 hours to become a neurosurgeon).
We all understand how music and sports require years of commitment. What we do not realize that it also takes years of practice to get familiar with "groking how the world works," solving problems, inventing solutions. The basic skills of asking the right questions, trying an idea, persisting through failure -- eventually lead to finding out how the world works. But, it takes years of practice and it starts with the toys you give a girl and the afterschool programs you sign her up for.
There are many amazing female role models, its just that the media doesn't give them much attention.
This is again one point at which the Indian culture gets it right when it comes to treating girls as intellectually equal to boys. There is a nationwide drive to encourage girls to take an interest in technical disciplines and role models such as Kalpana Chawla -- the first Indian woman in space -- exemplify these efforts. They are accorded much national attention and respect.
I believe mainstream media and popular Hollywood figures can accomplish much in this regard. That is why I was so thrilled to learn about Warner Bros' upcoming movie Gravity in which Sandra Bullock plays the role of a courageous and persistent engineer, problem solving her way through a malfunctioning space station. We created a Gravity Design Challenge to help teenagers get a taste of the problems Sandra Bullock was solving and to view science and engineering as something that is accessible to all -- even girls.
 Susskind, Leonard (2008-07-07). The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics (p. 5). Little, Brown and Company. Kindle Edition.
 Valian, V. (1998). Why so slow? The advancement of women. Cambridge, MA: M.I.T. Press.