Economists who praised and publicized the study of the effects of the property titling program in Peru found it valuable not only for what it discovered about property rights but for the way in which it made the discovery: by using the titling program to carry out a sophisticated natural experiment (Krueger 2003). A study that merely compared the hours worked by people who had formal ownership of their houses with those who had not received title could not produce convincing evidence of the effect of formal ownership on employment. The extra hours that formal owners worked might be due to any number of unobserved factors.

Philip Mirowski & Dieter Plehwe (eds.), The Road from Mont Pèlerin, loc. 1937