Studies of twins are among social scientists' most powerful tools. And everyone knows that North and South Korea differ dramatically in just about every way. Twenty-five years ago, the same could be said about East vs. West Germany, and Taiwan-Singapore-Hong Kong vs. mainland China. Culture was held constant, so performance differences could be pinned to policies. The simple conclusion was always that socialism does not work.
In "A Tale of Two Islands", the authors compare the performance of Barbados and Jamaica. Similar history and background, but different outcomes and contrasting policies. Statism, again, comes up way short.