Classical nucleation theory has long guided our understanding of how crystals form, relying on the idea that bulk properties determine the outcome. 

But is it falsifiable? In this study, we put the theory to the test, literally, by designing a system where all crystal polymorphs have identical thermodynamic properties. According to the theory, the polymorphs nucleate at the same rate. They don’t. 

This controlled “fair race” exposes a fundamental flaw in the theory’s assumptions and offers a falsifiability test, revealing the overlooked role of structural fluctuations in the liquid.