The federal Equal Pay Act prohibits wage discrimination between men and women in the same establishment who perform jobs that require substantially equal skill, effort, and responsibility under similar working conditions. The jobs need not be identical, but they must be substantially equal. The law, in short, requires employers to pay women equal pay to men for substantially equal work.
The EPA has four main exceptions, that is, situations in which an employer might pay a woman less than a man without violating the law. These exceptions are when the difference in pay actually results from a seniority system; a merit system; an incentive system, which measures earnings by quantity or quality of production; or any other factor other than sex.
It is not enough that a higher-paid male in the same job happens to be more senior than you, or have higher performance ratings than you, or differ from you in some way unrelated to sex. To escape the EPA’s equal pay requirement, your employer must be able to show that its decision to pay the male employee more than you was in fact the result of a factor other than sex.