Jon Chait points to research showing that the lack of women willing (or able) to work the night shift is one factor contributing to the wage gap.
The night shift story is part of the same basic pattern: women are less willing or able than men to put in hours at work that are incompatible with family life. Working 60 hours a week to get ahead is very difficult for mothers, and so is working the night shift. The nub of the issue is that we live in a society where men often feel comfortable, or at least justified, working those sorts of hours even if they're parents, while women don't.
Chait is spot-on in identifying the strong social pressures that push mothers to choose jobs with shorter or more convenient hours -- jobs that tend to pay less. But what about non-mothers? They face a wage gap, too. Women may want to work longer hours -- pick up an extra shift, stay late several nights a week to make partner -- but are still perceived as unwilling or unable to do so.
In other words, women workers aren't the only ones who are affected by social conventions about how career-oriented they should be. Their supervisors and co-workers are, too -- which can lead them to deny qualified women the promotions or extra shifts they seek. It's not all about women's choices. Gender discrimination still fits into this equation.
--Ann Friedman