Carrie Lukas has a fascinating article in the Wall Street Journal asserting that there is no systematic wage gap between genders. She argues that any overall differences can be explained by data showing that men, on average, work more and are willing to trade more quality of life for wages than women.
Even assuming that her data and analysis is accurate, bear in mind that her arguments only apply to the averages. Wage discrimination can and does still occur in some situations, against either gender. Remember also that the alleged wage gap, true or not, is perceived by many, and a smart employer will want to ensure that the perceptions of differences are avoided just as well as the employer avoids actual differences.