Also, note that modern psychology and anthropology has suggested that men negotiate more (and/or better) than women. It has been suggested, theorized, and seems reasonable that this is actually the cause of the gender pay gap... men are more willing to ask (and better at asking) for more.
http://onlinelibrary.wiley.com/doi/1...00008/abstract
http://onlinelibrary.wiley.com/doi/1...175.x/abstract
https://books.google.com/books?hl=en...iation&f=false
http://www.tandfonline.com/doi/abs/1...87150903553790