Economists David Card, Joshua Angrist and Guido Imbens won the 2021 Nobel economics prize on Monday for pioneering "natural experiments" to show real-world economic impacts in areas from minimum wage increases in the US fast-food sector to migration from Castro-era Cuba.

Unlike in medicine or other sciences, economists cannot conduct rigidly controlled clinical trials. Instead, natural experiments use real-life situations to study impacts on the world, an approach...