If you accept the premise that assertiveness (or lack thereof) is at least partly learned, it stands to reason that girls could learn to be more assertive negotiators; therefore, the standard solution for the wage gap has been to train women and girls to be more assertive. Teach them to ask for more, the theory goes, and they'll do it.
One problem with this theory is that it may not hold up in practice; women who behave assertively, according to a recent study, are regarded more negatively and are more likely to be perceived as incompetent. Men who behave assertively, meanwhile, tend to be generally admired and are often rewarded financially. The fault, then, resides at least partly in the person perceiving the woman, not the woman herself.
Another problem with this theory is revealed by another recent study, which found that "haggling" carries a social cost for women that is not borne by men.