Yes, women make less, but there are lots of factors.
I researched this extensively not relying, as Cecil [http://www.straightdope.com/columns/020823.html] did, so much on case law but interviewing various people, mostly female. I focused on three occupations: Law, medicine, and veterinary medicine, because those are three areas which used to be almost exclusively male (50 years ago) and which are now trending toward women.
In those three occupations, women still make less. Of course the overall average is skewed by the preponderance of men who are in their peak earning years vs. women who are still relative beginners.
But: Women who have been on the job for 15 years make less than men who have been on the job 15 years. And there are lots of reasons.
These are generalizations of course. (I myself adopted a strategy, in about 1979, of not taking a job unless the previous person in that job was a male, and this strategy served to keep my salary ahead of my husband’s for practically my entire marriage, so far.) But some of the reasons women don’t make as much are:
They aren’t (generally) as tough when it comes to negotiating. Or some of them—lots of them—don’t realize that everything is negotiable.
Those who do sometimes negotiate lower salaries in return for flex time. Or they trade down their benefits and get a child-care adjustment instead of company stock. Women are lots more likely to do this than men.
They are more likely than men to work part time or to job share.
The Mommy Track is real. I know (and interviewed for the article) three women and one man who worked either part-time or on a non-partnership track at law firms (at these law firms, “part time” was 40 hours a week) and made less money. (The man who did this made less money too, but his wife was on the partnership track at a different law firm; he rarely saw her. He did all the soccer/piano/ballet coordinating for their daughters and took them in on Take Your Daughters to Work Day.) This is a choice that more women than men make. Obviously, if the men who opted for this did it in great numbers it would bring their average salary down too.
Women are way, way more likely to look for another job if their bosses are assholes. Men are more likely to suck it up if the money’s good.
In all this, I am speaking of the three job categories mentioned above. In addition, female MDs got much less research grant money and female attorneys were significantly less likely to be rainmakers. Female veterinarians were somewhat less likely to specialize in larger animals. All these are choices (or maybe not choices; they could be dictated by temperament and physiology) that result in lower incomes.
Whew! I didn’t mean to write a damn term paper here. I’m blowing through several inches of research I did for a story on this in 1995, so this info is 8½ years outdated.
Here’s some depressing news, though. Men who worked in predominately female job categories, such as secretary, made 103% what women did in those jobs. This is balanced by the fact that women who go into he-man fields such as plumbing or construction make 105% to 108% of what men in those jobs earn. (I think these stats were from 1994 but could be ’93).