For those who agree that the wage gap is real (it is; so is the glass ceiling), there are all kinds of reasons why it exists. One prevailing theory is that women, by and large, are socialized not to negotiate to their greatest advantage. New research has some thoughts on that — namely, that it's bunk.
LeHigh University political scientists have just released a study on how gender affects the way people interact during negotiations between positions of higher and lower status. According to co-author Holona Ochs, "Individual behavior in hierarchical negotiation settings, like between a boss and employee, is more likely affected by the context than by gender difference." In other words, women can hold their own just fine (although negotiating, particularly when it comes to salary, isn't innate and everyone can learn or improve on the skill).
Another way of framing these results is that the wage gap is not the fault of women. Structurally, women are swimming against the current when it comes to things like investing, career advancement, freelancing, networking, and even who gets to be a genius. Even negotiating a salary can reinforce inequality rather than level it. But companies have an opportunity to give everyone a fair shake. There are loads of general steps institutions can implement, as well as more specific policies, such as for hiring referrals.
Individual women are doing great. They're doing the work. It's up to the system to change, and it's up to anyone with the power to do so.