To help explain the starting-salary gap, a reader points to this WaPo piece on the "social cost of haggling":
Another study quizzed graduating master's degree students who had received job offers about whether they had simply accepted the offered starting salary or had tried to negotiate for more. Four times as many men -- 51 percent of the men vs. 12.5 percent of the women -- said they had pushed for a better deal. Not surprisingly, those who negotiated tended to be rewarded -- they got 7.4 percent more, on average -- compared with those who did not negotiate.
However, another study:
found that men and women get very different responses when they initiate negotiations. Although it may well be true that women often hurt themselves by not trying to negotiate, this study found that women's reluctance was based on an entirely reasonable and accurate view of how they were likely to be treated if they did. Both men and women were more likely to subtly penalize women who asked for more -- the perception was that women who asked for more were "less nice".