There are many reasons for the lack of women in physics – all correctable. I’ll highlight some of the arguments below, some coming specifically out of physics, some from science in general, and others coming out of social-psychology studies. While almost all of it is focused on the United States, similar cultures would share these effects. I want to point out that many of these effects also apply to underrepresented racial minorities, whose numbers in physics are far lower than for women .
We are not too far from an era where female graduate students were asked to double as babysitters or when female physicists had a hard time getting a paid job at CERN. We aren’t far enough for all experimental buildings to have women’s bathrooms. While there are still a few giant jerks left, most of the problems are from people who wouldn’t think they are doing anything to disadvantage women in the field, including women themselves.
We’re scientists, so what can actually be measured? Studies can be done on how decisions are made and what the field actually looks like. Studies have been done (not specific to physics) showing that white names favored over African-American names in otherwise identical CVs for interview callback (3:2), “Brian” was preferred over “Karen” (2:1) using identical resumes, and that women had to be 2.5 times more productive to rate equally in scientific competence as the average male for a postdoc fellowship . The people making these decisions may not be consciously racist or sexist, but have picked up unconscious bias from societal messages .
Perhaps you want to believe that these studies – done in labs or outside of physics – don’t actually relate to what we experience. A study was done on members of D0 (an accelerator experiment at Fermilab) to look at male and female post-docs. There was evidence that the female post-docs did more “service work” (40% more) than their male peers and had to work on twice the number of internal physics analyses papers to go to the same number of conferences. In physics, recommendations and supervisors’ support are necessary to advance a career, and women are again disadvantaged. According to Prof. Urry, a physicist who has written about these issues,  (and I have certainly witnessed it for years), “Young men are talked about as superstars, while young women are described as “very good”.” There are many ways in which recommendation letters for female students aren’t as impressive as those for males, including shorter length, more gender-stereotypical adjectives, lack of use of title, and more negative language or irrelevant comments that could raise doubts. 
Our society socializes men and women to like different things and behave in different ways. This can disadvantage female physicists (or women who could have been physicists otherwise) in many different ways, including
- Trained to not have personality traits beneficial to being a physicist
- Not encouraged to participate in activities that would foster an interest in the physical sciences
- Not encourages to pursue technical careers
I think the first point is often ignored, and perhaps one of the most important. Many physicists (perhaps this is just in the US, but I doubt it) are egotistical, hard-headed jerks. In some groups, meetings seem to be a contest to either talk the most or interrupt each other the most. These are habits that society discourages far more in girls than boys. Urry writes that “To succeed as a physicist, one must not only do good work but also aggressively promote one’s ideas and accomplishments.” and
Men who are highly aggressive, intrusive, peremptory, and obnoxious still enjoy the confidence and respect of their colleagues. Women with even a fraction of the same toughness are characterized as “difficult” (there are other words for this) and demoted or told to play nice.
Obviously there are some ‘nice’ men who have succeeded in physics and numerous women that have. But the majority of the women I have met who entered physics in the 70′s and 80′s are absolutely tough as nails. They didn’t succeed by playing nice.
There is much that could be said about exposure to and encouragement in technical fields, but I don’t think it is a dominant problem. Freshman physics is hard and boring, but you would still expect both male and female students to quit. About 50% of high school physics students are female, while only 22% of bachelor’s degrees are . Much of this can be attributed to the students entering physics culture – with all of it’s problems – and being discouraged by the issues I am discussing.
If socialization and familiarity with technical fields and the physical sciences was simple, we’d expect to see similar gender breakdowns in other sciences, math, and engineering. Not only does physics have fewer female PhD’s than other technical field, but the improvement over time is also slower . One argument is that physics has an ancient history of being tied to the clergy , and that particle physics still has much of the same mentality. While it isn’t the whole story, it does provide one reason why particle physics has the smallest percentage of female PhD’s .
Stereotype Threat and Solo Status
When a department or field lacks women or minorities, the problem is much deeper than young people not having role models. While it is important for people to have mentors who can give helpful advice, there are extensive psychological challenges for both the minority individual and those belonging to the majority. Some of this relates to bias: “As the ratio of women to men, or racial minorities to Whites, increases, women and racial minorities receive lower evaluations and are less likely to be promoted than White males.”
The idea of “stereotype threat” and “solo status” is that self-perception of being in a disadvantaged group decreases ones own performance. In research discussed in , a math test is given to subjects in two different circumstances. If told it measures their ability and given the test, men outperform women. If told beforehand that it is a “special” type of test that has no gender bias, women perform the same as men. This is the same test- the difference is only stereotype threat! Additionally, if a person thinks they are the only member of a marginalized group in an evaluation, the person performs worse than if they are in a group with the same race or gender. Research has shown that women have lower expectations of their performance if they are in a solo situations, and these expectations lower their actual performance .
Research into these issues is extensive. Believing that you are representative of your group has been shown to hurt performance in a multitude of ways.
Low-status groups engage in different communication styles when interacting with high-status groups… Women tend to use more tentative language (e.g. weakening the strength of a statement by using phrases such as “sort of” or “maybe”) when interacting with men. Solos … may say only what is neccessary, without elaborating on answers, in order to provide less room for error.
With the culture of physics being what it is, these effects can be debilitating to someone facing stereotype threat and solo status. Urry says that “To succeed as a physicist, one must not only do good work but also aggressively promote one’s ideas and accomplishments.” This is in great conflict with tentative communication and lowered expectations of one’s performance.
Often the “leaky pipeline” – women leaving academia after each stage – is discussed with regards to the lack of women in physics. Pointing to family or child-raising issues is sometimes just a cop-out – many issues apply. This can’t be solved by “fixing” the women. The environment and mentality must be changed. Perhaps you have never experienced these issues in physics – perhaps that explains why you have made it as far as you have. Yes, there are some amazing women in physics- but there are plenty of mediocre men in physics! If some of the women who would be good (but not amazing) physicists could make it, they would still be improving the field.
For an excellent summary, see this excellent APS article written by Prof. Urry. For some interesting stories on the bias female professors face, I recommend reading through the archives at FemaleScienceProfessor
 Report On Women In Scientific Careers At Cern Gaillard, M. K. 1980
 “Are Photons Gendered: Women in Physics and Astronomy” Urry, C. M. In Gendered Innovations in Science and Engineering.2008
 Title: A Case Study of Gender Bias at the Postdoctoral Level in Physics, and its Resulting Impact on the Academic Career Advancement of Females Towers, S. 2008
 “Nepotism and Sexism in Peer-Review” Wenneras,C. and Wold,A. Nature 337, pp 341. 1997
 “When Being Different is Detrimental: Solo Status and the Performance of Women and Racial Minorities” Thompson, M and Sekaquaptewa, D. Analyses of Social Issues and Public Policy Vol 2 No 1 pp 183-203, 2002
 Diversity in Science Association 2007 Report (Nelson Diversity Survey)
 Implicit Association Test
“Exploring the Color of Glass: Letters of Recommendation for Female and Male Medical Faculty “, Trix, F, Psenka, C, Discourse & Society, Vol. 14, No. 2, 191-220. 2003
 “Pythagoras’ Trousers: God, Physics, and the Gender Wars” Wertheim, M. 1995