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A social experiment is a research project conducted with human subjects in the real world. It typically investigates the effects of a policy intervention by randomly assigning individuals, families, businesses, classrooms, or other units to different treatments or to a controlled condition that represents the status quo. The qualifier "social" distinguishes a policy experiment from a "clinical" experiment, typically a medical intervention within the subject's body, and also from a laboratory experiment, such as a university psychology faculty might conduct under completely controlled conditions. In a social experiment, randomization to assigned treatment is the only element in the subject's environment that the researchers control. All other elements remain exactly what they were.
Social experiments are often referred to as "the gold standard" for program evaluation and reform processes. In measuring the impact of a social program, the researcher has to assess what the outcomes of the relevant population would have been in the absence of the program. Almost every naturally occurring comparison group, however, will differ from the composition of a non-random treatment group, usually because of selection bias (outside of an experiment, people choose to receive the treatment or choose not to). Randomization creates a control group that is statistically identical in large sample with the group that is assigned to receive the treatment, and in principle there is no selection bias.
The starting differentiation of the general field of psychology into physiological and social psychology as suggested by Wilhelm Wundt in 1862 lay way for the first social experiments. In 1895, American psychologist Norman Triplett constructed one of the first social experiments with the intention to study the influence of a group on racing performance. During the 1920s, Gordon Allport used experimental methods to study conformity, nonverbal communication, and social facilitation, shaping social psychology as we know it.
The social experiments that we commonly refer to today were conducted decades later; a famous example is Stanley Milgram's obedience experiment in 1963. Social experiments began in the United States as a test of the Negative Income Tax concept in the late 1960s and since then have been conducted on all the populated continents. 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".
During the 1970s, cristicism of the ethics and accusations of gender and racial bias led to a reassessment of both the field of social psychology and the conducted experiments. While experimental methods were still employed, other methods gained popularity.
Today, we often see the term social experiment on video sharing platforms like youtube, used for lay people's experiments during which actors attempt to provoke responses from e.g. passersby, usually filmed with a hidden camera. 
The HighScope Perry Preschool Project was evaluated in a randomized controlled trial of 123 children (58 were randomly assigned to a treatment group that received the program and a control group of 65 children that did not). Prior to the program, the preschool and control groups were equivalent in measures of intellectual performance and demographic characteristics. After the program the educational and life outcomes for the children receiving the program were much superior to outcomes for the children not receiving the program. Many of the program effects were significant or approaching significance.
The RAND Health Insurance Experiment was an experimental study of "health care costs, utilization and outcomes in the United States, which assigned people randomly to different kinds of plans and followed their behavior, from 1974 to 1982. As a result, it provided stronger evidence than studies that examine people afterwards who were not randomly assigned. It concluded that "cost sharing reduced "inappropriate or unnecessary" medical care ("overutilization), but also reduced "appropriate or needed" medical care. It did not have enough statistical power to tell whether people who got less appropriate or needed care were more likely to die as a result.
Oportunidades (now rebranded as "Prospera" ) is a government social assistance ("welfare) program in "Mexico founded in 2002, based on a previous program called Progresa, created in 1997. It is designed to target poverty by providing cash payments to families in exchange for regular school attendance, health clinic visits, and nutrition support. Oportunidades is credited with decreasing poverty and improving health and educational attainment in regions where it has been deployed.
Moving to Opportunity for Fair Housing was a randomized social experiment sponsored by the United States "Department of Housing and Urban Development in the 1990s among 4600 low-income families with children living in high-poverty "public housing projects.
The Stanford prison experiment was a study of the "psychological effects of becoming a "prisoner or "prison guard. The experiment was conducted at "Stanford University on August 14–20, 1971, by a team of researchers led by psychology professor "Philip Zimbardo using college students. It was funded by the "U.S. Office of Naval Research and was of interest to both the "U.S. Navy and "Marine Corps as an investigation into the causes of conflict between military guards and prisoners. The experiment is a classic study on the psychology of imprisonment and is a topic covered in most introductory psychology textbooks.
Sherif was a founder of modern social psychology, who developed several unique and powerful techniques for understanding social processes, particularly social norms and social conflict. Sherif's experimental study of autokinetic movement demonstrated how mental evaluation norms were created by human beings. Sherif is equally famous for the "Robbers Cave Experiments. This series of experiments, begun in Connecticut and concluded in Oklahoma, took boys from intact middle-class families, who were carefully screened to be psychologically normal, delivered them to a summer camp setting (with researchers doubling as counsellors) and created social groups that came into conflict with each other.