I am taking a class taught by the professor who wrote this book about the economics of higher education: http://www.amazon.com/Tuition-Rising-College-Costs-preface/d...

One of the more interesting things he went over from the book was the discrepancy between males and females in PhD in the hard sciences. While many claimed discrimination, what it turned out to be a slew of different issues. Except from his lecture, taken without permission:

Why Are Female Faculty Members Underrepresented at Research
Universities Relative to Liberal Arts Colleges
1. Gender differences in preferences for teaching vs. research
2. Perceptions by female PhDs that research universities are not hospitable environments for them
3. Perceptions by female PhDs that there is more gender discrimination
against female faculty at research universities
4. Actual gender discrimination against female PhDs in the hiring process and against female faculty in salary, tenure, promotion and resource allocation decisions at research universities
5. The difficulty of combining family and career at research universities

The remainder of my discussion is going to focus on the last explanation and discuss some policies designed to reduce these difficulties that have been implemented at the University of California. However, before doing so, I want to stress that issues relating to the conflict between family and career that professional women face are not unique to academia. For example,
1. Why are female lawyers underrepresented among the partners of large law firms?
2. Why are female doctors underrepresented among neurosurgeons and orthopedic surgeons and overrepresented among family practice physicians and pediatricians?

National Research Council Committee (that I served on) Survey
In 2004-2005 the National Research Council Committee on Gender Differences in the Careers of Science, Engineering and Mathematics

Faculty surveyed departments in research universities in six disciplines
– biology, chemistry, civil engineering, electrical engineering, mathematics, and physics. A second survey surveyed over 1,800 faculty in these departments. Among its major findings were:
1. If a male and a female apply for the a position, the female is more likely to be invited for an interview
2. If a male and female are both interviewed for a position, the female is more likely to receive the job offer. Hence
3. Female under representation relative to their share of the new PhD pool is due primarily to their not applying for jobs at research universities as often as males do. Furthermore
4. Female assistant professors in these fields are more likely to leave their positions than their male colleagues prior to being considered for tenure. Given that they are considered for tenure, they are more likely to be promoted and receive tenure than their male colleagues, but their
average time until receiving tenure is longer than their male colleagues’ average time
5. There were no differences in the probabilities of being promoted to full professor or the time it took to receive this promotion.
Source: National Research Council, Gender Differences at Critical
Transitions in the Careers of Science, Engineering and Mathematics Faculty
(Washington DC: National Academy Press, 2009)

/end quote
Granted, this does not directly apply to top scientists but I think many of you would agree that the fact that the volume of highly trained (read: PhD.) women scientists is significantly lower than that of men probably leads, in part, to the discrepancy among top scientists.