The system of dual sovereignty known as federalism is a fundamental and cherished part of the American constitutional structure. Among other things, a system that respects the sovereignty of the 50 states serves as an engine of innovation, permitting individual states to serve as “laboratories” for social and economic experimentation. But as Justice Brandeis – the author of the “states as social laboratories” concept – himself acknowledged, the sovereign power of the states to experiment with social and economic innovations is a benefit only to the extent that a given state’s experiments can be conducted “without risk to the rest of the country.”