Sen is credited worldwide for his invaluable contributions to research on fundamental problems in economics and philosophy. His theoretical and empirical work encompasses a range of issues, from famine and poverty to social choice theory, decision theory, and the demands of rationality and freedom. Sen is widely recognized for his ability to join economics and philosophy, reflected in his work through ethics and a sense of common humanity. In 1998, he won the Nobel Prize in Economics for his work in "social choice theory," the philosophical and mathematical inquiry into links between individual values and collective choice.