Yesterday’s U.S. Supreme Court decision in United States v. Windsor struck down Section 3 of the federal Defense of Marriage Act (DOMA) as unconstitutional, and held that the federal government must recognize and accept same-sex marriages recognized under state law. As a result of the Windsor decision, the definition of “spouse” under any federal law governing employee benefits must now be interpreted to include same-sex spouses recognized under state marriage laws.

The decision raises the threshold question of which state’s marriage law will control: a couple’s state of residency or the state in which the same-sex marriage was performed. Although the Internal Revenue Service generally recognizes the law of the state of residency for tax purposes, it may adopt a “state of celebration” rule to ensure consistent implementation of Windsor nationwide. In either case, the decision will trigger significant changes in the way employee benefits are delivered and administered for thousands of employees.