One theory predicts exactly when sharing a currency will end in tears

Robert Mundell argued a shared currency only works well across regions that can absorb shocks together — through labor mobility, wage flexibility, or transfers between members, since none of them can devalue on their own anymore. Decades later, the Eurozone's struggles during its debt crisis matched almost exactly the failure modes his 'optimal currency area' theory predicted.

— Robert A. Mundell, A Theory of Optimum Currency Areas — American Economic Review, 1961