Mo. Rev. Stat. § 290.410

Notwithstanding any other provisions of the law, no employer shall pay any female in his employ at wage rates less than the wage rates paid to male employees in the same establishment for the same quantity and quality of the same classification of work, provided that nothing herein shall prohibit a variation of rates of pay for male and female employees engaged in the same classification of work based upon a difference in seniority, length of service, ability, skill, difference in duties or services performed, difference in the shift or time of day worked, hours of work, or restrictions or prohibitions on lifting or moving objects in excess of specified weight, or other reasonable differentiation, or factors other than sex, when exercised in good faith.