In an experiment, a group of strangers was randomly divided in pairs to play a prisoners’ dilemma; this process was indefinitely repeated. Cooperation did not increase when subjects could send public messages amounting to binding promises of future play.
Camera, G., Casari, M., & Bigoni, M. (2012). Binding promises and cooperation among strangers. ESI Working Paper 12-27. Retrieved from http://digitalcommons.chapman.edu/esi_working_papers/62