American economists Elinor Ostrom and Oliver Williamson, who study the way economic decisions are made outside markets, were awarded the Nobel Prize in economics Monday.

Ms. Ostrom, who teaches at Indiana University in Bloomington, Ind., is the first woman to win the economics prize, which had been awarded to 62 men since its launch in 1969. The judges cited her analysis of what happens when natural resources are shared commonly.

Mr....