The lost-letter technique (Milgram et al., 1965) has been used for almost 60 years to measure social attitudes and helping behavior in psychological, sociological, and economic research. (In this procedure, unmailed but stamped letters are placed or dropped in an area. People who discover such a letter have the choice of sending it or ignoring it.)

The paper https://www.sciencedirect.com/science/article/pii/S0167487024000679 by Asanov, Schirmacher & Bühren provides a meta-analysis of lost-letter experiments to summarize the evidence. The authors analyze 78 studies with an overall sample size of 53,504 letters from 18 countries on five continents and find an average return rate of 50 percent across all countries. Their meta-analysis shows that the return rate is lower for political or deviant issues. Stamped letters are also more likely to be returned, but letters with money are not more likely to be returned. A high socio-economic environment increases the chances of the return. The authors conclude that in line with the lost-letter paradigm, the technique allows capturing citizens’ attitudes toward the issue communicated. However, citizens do not act selflessly but react differently depending on the type of incentives.

link to article: https://www.sciencedirect.com/science/article/pii/S0167487024000679

link to I. Asanov's personal webpage: https://www.igorasanov.com/