Women in Science (xkcd)

This cartoon is hotlinked from cartoonist xkcd's web site. It struck a chord with me, because it made me think (again) about the abundance (or the lack thereof) of female scientists. I would like to share just a few facts from my own scientific career.
First of all, we must define a starting point for my becoming a scientist. I suggest we pick the time when I entered the final two years in secondary school (Gymnasium), because that was the first time I was allowed to choose a slant in my class schedule toward science and mathematics.
Specifically I picked Physics and Chemistry as two topics in which I took the written Abitur exam. There was only one female student in the Physics class, but I would say that in Chemistry it was almost even. In fact, there was another Chemistry class at other times of the week, so as to ensure all students get their favorite combinations of classes, and thus we should look at both Chemistry classes combined. I don't remember everyone after 15 years, especially since the second class took place at our partner school, but my feeling is that the percentage of female students in the combined classes ranked between 30 and 50%.
Then I went on to do my civilian service, a military service substitute, which I did in a kindergarten. There, the staff consisted of four female full-time employees, a female trainee, and myself. The principal was also female, but since she was in charge of multiple institutions, she had her office somewhere else, and she showed up only once a week.
College was next, which I started as an Electrical Engineering major. Slightly more than 50 people started with me, of which only 3 were women. One actually quit after the first term, but another one, who is from an Arabic country, later went all the way to her PhD, and in the process she kicked everyone's behind whenever it came to do math.
In fact, I also quit doing EE, to pursue a Chemistry degree instead. Again, the gender distribution was about balanced, actually with a slight excess of women (due to that fraction of students who specialized in Food Chemistry). Eventually I graduated, after a year-long final research project, and when I left for Berlin, the three graduate students who were in that group at that time for their own doctorates were all female.
I don't have comprehensive information on the gender composition of the CP department of the FHI Berlin, because not only was the group too big to assess something like this in retrospect only - there was also a significant fluctuation of staff, so this variation makes it impossible for me to correctly remember everyone. Yet I can offer a survey on the people who worked directly with me: four females and two males.
In Chicago I was in charge of providing training to the students who were assigned to work on the same equipment: three females and one male (even though the male student overlapped with me only for a few weeks, after which he went to Berlin, where he worked on "my" old experiment, and he actually taught me more about the Chicago chamber than I taught him about science in general).
To sum up, there is much female talent in science, and the fact that that there are only few female professors is most likely due to the "glass ceiling" effect. - Other explanations are welcome, especially from female scientists!