If you plot women's share of doctorates in different fields, it's pretty telling that CS is the only field in which the fraction of women dropped significantly between 1920 - 1960. In 1920, 20% of mathematics and CS PhD degrees went to women, by 1960 it was down to 5%. It took until around the turn of the century for it to get back up to 20%. (See http://www.nsf.gov/statistics/nsf06319/)