By Simon Johnson and Niklas Pollard STOCKHOLM (Reuters) - 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 U.S. 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 that has spread to other social sciences. Their research has substantiall...