A social experiment is an experiment with human subjects, which typically investigates effects on groups of persons. As an experiment, a social experiment evaluates effects of treatments, such as changes in a program or policy. For example, a study of influenza vaccination can evaluate the antibody counts of individual subjects; a social experiment can evaluate the effect of the vaccination on the health of family or friends.
Some experiments such as the Seattle-Denver Income Maintenance Experiment, the RAND Health Insurance Study have been large and highly publicized. Some of "the more recent experimental evaluations of state-run welfare-to-work programs and of programs run under the Jobs Training Partnership Act of 1982" have also been publicized. Others, however, have been small and obscure. Some "have pilot tested major innovations in social policy", some "have been used to assess incremental changes in existing programs", while some "have provided the basis for evaluating the overall efficacy of major existing programs. Most "have been used to evaluate policies targeted at disadvantaged population groups".
Social experiments have often been carried out because they enable to observe humans interact in unusual social settings. Nowadays, it has become more difficult to receive approval from ethics boards to conduct social experiments, especially after such experiments as the Milgram experiment or the Stanford prison experiment. These experiments have received criticism from the scientific community, and have been labelled as physically harmful and psychologically detrimental to the participants.