The Affordable Care Act (“ACA”) is a controversial and historic statute that mandates people make insurance bargains. Unacknowledged is an innovative mechanism ACA uses to select the law that governs those bargains: opt-in federalism.

Opt-in federalism – in which individuals choose between federal and state rules – is a promising theoretical means to make and choose law. This Article explains why, and concludes that the appeal of opt-in federalism is independent of ACA. Whatever the statute’s constitutional fate, future policymakers should consider opt-in federalist approaches to answer fundamental but exceedingly difficult questions of health and retirement law.