It seems to me that there are many factors working here. One is the push
to have more students take classes like physics and calculus. Many more
opportunities are open to women, so I think it is natural that there
numbers would increase. I,too, find many female students are more
aggressive in doing lab work and group work, although others are content
to remain in the background. Generally speaking, my high achievers are
the leaders in labs, whether they be male or female. My lower achievers
tend to sit around and watch air molecules collide, whether they be male
or female.
Part of the problem is that certain high school age groups, such as
african-american and hispanic, tend to downplay academic achievement
when it comes to males. However, it seems ok for females. I wonder if
this accounts somewhat for more females than males. I will say that my
own experience over the last 10 years shows this changing as more
minorities move into the middle class and beyond.
Great Britain apparently sees the female/ male ratio as a major problem
and is working on bringing more males into science and math classes.