Similar to medicine, social science has historically grappled with controlling for chance and bias in assessing the effectiveness of interventions. Questions about what works and how to evaluate the evidence underlie all areas of policy intervention. In the early years of the 20th century, social science in North America developed an established tradition of quantitative sociology that included experimental studies. This was followed by a number of social experiments from the 1960's to the 1980's. The history of prospective experimental studies with control groups applied to the social domain contains important lessons for experimental social science in the 21st century. The fact that randomized controlled trials cannot be regarded historically as the exclusive property of medicine supports their increased use today as methods for evaluating social policy interventions. Progress toward more sustained use of experimental research in social settings is also likely to be driven by the lesson that social interventions, just like medical ones, may do more harm than good. As the methodologist Donald Campbell (1971/1988) argued, choosing social policy interventions on the basis of well-designed experimental tests is a prerequisite of a democratic and equitable society. 93 references