Even where the work is substantially equal, a pay difference is lawful if it results from a seniority system, a merit system, a system measuring earnings by quantity or quality of production, or a differential based on any factor other than sex. The burden of proving one sits with the employer, and several states require the factor to be job-related and to explain the entire difference.
The first three
Each has to be an actual system, applied consistently and known to the people it affects. An employer cannot describe an ad-hoc pattern of decisions as a merit system after the fact. Documentation and consistency are what turn a practice into a defense.
The fourth, where the cases live
“Any factor other than sex” sounds unlimited and is not. It has to be a genuine explanation for the difference, not a label. Common candidates:
Education or certification the role genuinely uses. Shift or location differentials. Market conditions at the time of hire, which is accepted in some jurisdictions and treated sceptically in others. Prior salary, which is where the law has moved most.
Prior salary is increasingly excluded
Because a pay difference explained by earlier pay simply carries forward whatever produced that earlier pay. A number of states and the courts in some circuits have held that it cannot justify a difference, either at all or on its own.
What states have tightened
Several require the factor to be job-related and consistent with business necessity, to account for the whole difference rather than part of it, and to be applied reasonably. That is a materially harder standard than the federal text and it is the reason state claims are often stronger.
Two things that are not defenses
That she did not ask. That budgets were tight in a particular year. Both explain how a difference arose and neither is a permitted justification for continuing it once identified.
Common questions
When is a pay difference lawful?
Under a seniority system, a merit system, a system measuring production, or a genuine factor other than sex.
Who has to prove it?
The employer. These are affirmative defenses, so the burden of establishing one sits with them.
Can prior salary justify a difference?
Increasingly not. Several states and some courts have held it cannot, either at all or on its own, because it carries earlier discrimination forward.
Is 'she didn't negotiate' a defense?
No. It may explain how a difference arose, and it does not justify continuing it once identified.
How do states tighten the fourth defense?
By requiring the factor to be job-related, consistent with business necessity, and to explain the entire difference rather than part of it.