But then there's also relatively more women in Mathematics and Astronomy. There's definitely not a simple relationship between how "hard" the science is and how many women are in the field.
Maybe my point is more that when people think of biology/medicine/psych they will often put it in a context of people,animals,plants which is more stereotypical "women stuff". Whereas physics/chemistry/CS is more often thought of in the context of "things" like buildings,bombs,cars,computers etc which is stereotypical "guy stuff".
Mathematics is difficult to generalise because it has such a wide range of applications.