But to the South,Clay said, it was a hard social and economic fact.

VOA: special.2009.03.19

You can prove that, in fact, if you were writing a contract, if there was a social planner you'd work more.

事实上你会发现,如果有书面合同,或者有个规划师的话人们会超常发挥

耶鲁公开课 - 博弈论课程节选