Lewis held that a disparate impact claim accrues upon the application of an employment practice or policy - in other words, the limitations period does not begin solely upon the initial adoption of the practice, but resets upon the application of that practice.
By contrast, Ledbetter held that the statute of limitations for a disparate treatment claim does not reset upon the application of a prior discriminatory act. Specifically, the limitations period does not reset upon the issuance of a paycheck that is lower - effectively tainted - because of past discrimination. It was a controversial decision which was essentially mooted by the enactment of the Fair Pay Act.
So why did Arthur Lewis win, while Lilly Ledbetter lost? Some (apparently including Congress) would argue that Ledbetter was just wrong. The Supreme Court, however, offered this explanation in Lewis:
For disparate-treatment claims—and others for which discriminatory intent is required—that means the plaintiff must demonstrate deliberate discrimination within the limitations period. But for claims that do not require discriminatory intent, no such demonstration is needed.In short, the Court points out that the gravamen of a disparate treatment claim is intent; so, subsequent effects are irrelevant if there's no discriminatory intent (such as upon the routine issuance of a paycheck). Meanwhile, a disparate impact claim is based on effects. Hence, disparate impact claims can occur upon the mere application of a practice, even absent discriminatory intent, because it's the effects that matter.
Posted by Philip Miles, an employment lawyer with McQuaide Blasko in State College, Pennsylvania.