This started as a response to smandal
's question of what my hypothesis is as to why physics has a particularly low population of women, compared to the other sciences/engineering/math. It got long, so I am posting it separately.
First let me give two quick links to previous things I posted, an article about the success of women in physics
and a long memo
summarizing a lot of studies. You may have seen these before but I'm putting the links up anyways as a refresher.
Obviously there are lots of factors which contribute to keeping women out of science/engineering/math in general, like straight-up gender bias in terms of giving grants and accepting papers, the publish-or-perish mentality which usually rewards lots of papers over a few high-impact papers (women usually publish fewer, more highly-cited papers, probably because they are so underrepresented and feel more pressure to do well), the time commitment combined with the heavy load of child-rearing which is still mostly shouldered by women, etc. etc. But you asked about physics in particular versus chemistry or biology, which have done a lot better. My ideas on factors that specifically impact that:
1. (This is the most important one) Women, regardless of what field they go into and whether or not they have children, are taught from a very young age that they need to be nurturing. They have to care for others, help others, be supportive. Look at the predominance of baby dolls and miniature strollers, and what gender of child has them (and is encouraged to have them, and sees other kids of the same gender on tv and in ads with them). There is so much pressure to be nurturing. And to me this is a lot of why you find a correlation between how directly a scientific field can "help people" and how many women are in it. Men, on the other hand, are encouraged from an early age to play with objects, build things, take things apart: various skills that speak more to the less helping-people bits of the physical sciences. So I have known a lot of women who liked science but chose biology or chemistry (or med school) to help people, or who went for physics or EE degrees but then turned in their research to the interfaces with biology. Yes, most science aims to help people--technology developed from my research, for example, could lead to new electronics or solar cells or machines which would improve lives--but the more biological sciences do it in a more evident and tangible way. This combines with the following effect in a very interesting way.
(Making my point for me: while I was writing this, a female physics graduate student who had been planning to work in our lab came in to tell us that she wanted something with more biology and is joining a different lab.)
2. Now, physics is kind of a weird field, because it can be one of two very different things. It can be experimental, applied, in some ways very close to engineering in terms of thinking of new inventions or apparati and then building them, or studying some interesting and important system which has never been created before. Or it can be intensely theoretical, mostly math and simulations, not hands on at all; this is much closer to math than engineering. But both things are physics. And within this, I would posit that there are two main goals you can have in physics, those goals being the development of new technology which improves the human condition, or knowledge for its own sake. Yes, they can feed into each other. I think that in general, the theoretical stuff maps more onto "knowledge for its own sake" and the experimental stuff maps more onto "improving society", on average, but I emphasize that there are exceptions.
Given that, I have found there to be a lot of elitism in physics about the 'purity' of the field. Some of the people in my year at grad school consistently made disparaging comments about various subfields of physics (like condensed matter, my field), saying that it was "just engineering", whereas they were in particle physics which they saw to be "real physics" because it was so abstract, at the heart of things, but also not useful. I saw a lot of disdain for the more experimental, more helping-people parts of physics, from physicists. Physicists are also often snide about chemistry and biology, saying they are just "applied physics" because in an ideal world you could derive all of those fields from physics, and also because chemists and biologists often mess things up due to a poor grasp of physics. (We won't get into how the physicists greatly underestimate the complexity of biological systems and screw things up in an equally stupid way.) Women often have lower self-esteem than men, and less confidence in their own abilities, for reasons that in my opinion amount to unfair societal pressures. And, they value biological sciences more highly to begin with because of the pressure on them to be nurturing. The result is that women, more so than men, are turned off by this elitism, both in terms of confidence in themselves and in terms of their respect for biology.
You could make here the counter-argument that math is even more abstract and pure, but has more women than physics does. I think that math is so disconnected from reality that it doesn't feel like the source of everything, the way some theoretical physics can; it feels to me more like learning a new language or figuring out the rules in a bizarre logical alternate universe. I never felt the math elitism because of that disconnect. This isn't to say that math isn't at the root of everything; it is, and it is more "pure" so to speak than physics. But it doesn't feel that way, at least not to me. I am not in a math department though, so I could be off here.
3. Speaking of math. Physics is the math-iest of the sciences and engineering fields, most definitely. If you think about it, most physics problem sets are very difficult math problems within a set of constraints, where the constraints are the physical laws. You learn harder math just to get a degree in physics than you do in any other science or engineering field, unless you take the initiative and learn more math than necessary in those other fields. And women are told, over and over, that they are innately worse than men are at math. If you tell a group of girls that studying hard improves math test scores, they do just as well as boys, and if you tell a group of girls that women are innately worse, they do worse. Also, the math test score gap seems to be very cultural and vary a lot in different countries and in different ethnicities. To me, this screams "sociological effect", and I think that women on average are probably just as good at math as men. But that isn't what they're told, and so they are more intimidated by a math-heavy field like physics than by biology.
4. Something that really just amplifies (2) and (3) is the fact that the classes you take up until your second or third year in physics graduate school are mostly theoretical, which is to say mostly math and mostly not experiments. There aren't many lab classes, and in some schools there may only be one, so unless the students get some laboratory research experience they may not be exposed to that side of physics at all. I think in biology, chem, and engineering, there are a lot more labs. And as I said above, because they are told they are bad at math and because of the unappealing elitism of theoretical physics, I think more women than men are likely to drop out of physics before getting to the part where you can choose to be an experimentalist. I know that I would have, if not for my research experiences.
There might be factors that I'm forgetting, and of course there are a lot of other effects which serve to drive women out of the sciences in greater numbers than men. But those are, in my experience, the most important ones.