From CBS News :
A new study in the journal Organization Science  finds that when managers have to explain their pay-raise decisions to employees, they give more money to men than they do to women -- even if the workers' performance is equal.
In the study, originally done for Emory University, 184 managers were given a set amount of money that they needed to distribute among employees with identical skills and responsibilities. Half of the managers were told they would need to justify their decisions to their employees, and half were told there would be no discussion afterwards.
Unfortunately, women can't overcome an initially low raise by negotiating because the corporate budget has already been spent. In many companies, each manager receives a budget for raises that is then divided among employees. All workers are notified at the same time (or over a very short time period) of their increases. Because every penny has already been allotted, there is no money left to give to someone who questions a small raise. Managers won't typically go to an employee with a higher raise and say, "Oops! Jane needs a few more bucks, so we're taking a percent off your raise and giving it to her!"
The only way a raise can go through at this point is for an exception to be granted. And that requires a lot of hard work on the part of the manager and (most likely) the manager's manager. HR and senior management must be convinced that this additional raise, outside of the spent budget, is worth the money. And managers, who are cognizant of their own reputation, will try to do this without stating that they made a mistake in allocating raise money.