Physics Today says the culture of physics itself is to blame for the gender pay gap.  

Staff writer Toni Feder cobbled together data from a 2010 National Academies of Sciences, Engineering, and Medicine report “Gender Differences at Critical Transitions in the Careers of Science, Engineering, and Mathematics Faculty” and unpublished information - grey literature - from the magazine publisher (American Institute of Physics) to suggest physicists are suppressing salaries of women. Or they are suppressing themselves. The unpublished data claim that a woman in academic physics identical in every way to a man will make, on average, 6 percent less.

This is a puzzle. The disparity does not exist in the private sector so academics are discriminating against women? No, the article instead suggests that women are less aggressive in their salary negotiations and also less likely to ask for a raise. The article also claims there is implicit bias in university departments because there are more men. The combination explains the difference. Ordinarily, academics resent that implication because it would aksi explain why there are so few handicapped people or political conservatives among tenured faculty, and that smacks to the public of regular old discrimination. 

Credit: Katrina Cole, CC BY-NC

That's exactly what it is, according to one anonymous claim used by the writer: "Boys in the department give money to boys in the department.” 

The writer recommends that more women and other under-represented groups serve on hiring committees, though it will be odd if a female insists a female in someone else's lab get more money than they requested...and that money is coming out of someone else's budget.

Hopefully there will be an analysis to find out if men in psychology are paid worse than women. Gender disparity in the social sciences is even more pronounced than in the physical sciences.