One explanation for the wage gap between men and women is that men are negotiating higher salaries and resources than women. This line of inquiry has spurred a large body of research into whether negotiations are a potential cause of the gender wage gap. A meta-analysis conducted in 2015 compiled the results of 51 studies about the gender gap and showed that men do achieve better economic outcomes than women on average, but this finding had several important caveats. First, the outcome gap is reduced when women negotiate on behalf of other people. This means that there is no gender difference among professional negotiators or people negotiating for their families. Second, negotiation experience also reduces the outcome gap. This is evidence of the importance of negotiation training and practice, particularly for women. Finally, men are more likely to initiate negotiations and push harder when economic outcomes are ambiguous. So when the salary range for a job is unclear, men tend to push for higher salaries than women. This is consistent with the premise of the book "Women Don't Ask" by Linda Babcock.
To this last point, many people feel that pushing hard will cause resentment, and research suggests this does happen sometimes. It is a case of winning the battle only to lose the war. On the other hand, a wage gap when first starting a job can accumulate to hundreds of thousands of lost dollars if a person were to hold that job for their entire career and rely only on percentage raises. So the choice is between causing potential resentment or a large financial loss. In cases like these, it is not easy to decide when to negotiate and how hard to push.
So when should you negotiate for a higher salary? Books like "Getting Past No" by Robert Ury or negotiation seminars like the ones offered by 98ÌÃ's Thurmond Negotiation Lab can help you make this decision.