This very interesting NY Times opinion piece (and the studies is cites) is a must read for human resources personnel and members of management involved in making promotion decisions.

A short overview of the argument:

"Women help more but benefit less from it. In keeping with deeply held gender stereotypes, we expect men to be ambitious and results-oriented, and women to be nurturing and communal. When a man offers to help, we shower him with praise and rewards. But when a woman helps, we feel less indebted. She’s communal, right? She wants to be a team player. The reverse is also true. When a woman declines to help a colleague, people like her less and her career suffers. But when a man says no, he faces no backlash. A man who doesn’t help is 'busy'; a woman is 'selfish.'" (link)