On and after January 1, 1978, no employer having employees of both sexes shall discriminate, within any establishment in which such employees are employed, between employees on the basis of sex by paying wages to employees in such establishment at a rate less than the rate of wages paid to employees of the opposite sex in such establishment for equal work on jobs, the performance of which requires equal skill, effort and responsibility, and which are performed under similar working conditions, except where such payment is made pursuant to: (a) A seniority system; (b) a merit system; (c) a system which measures earnings by quantity or quality of production; or (d) a differential based on a factor other than sex. An employer who is paying wage rate differential in violation of this section shall not be required, in order to comply with the provisions of this section, to reduce the wage rate of any employee.