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These results resonate with the recent increasing emphasis on the importance of individual differences in understanding and modeling behavior and dynamics in experimental games and decision problems. Here, we report the results of laboratory experiments that complement both theory and simulation results. We find that our subjects fall into three types, an individual's type is stable, and a group's cooperative outcomes can be remarkably well predicted if one knows its type composition. Reciprocal types, who contribute to the public good as a positive function of their beliefs about others' contributions, constitute the majority (63%) of players; cooperators and free-riders are also present in our subject population. Despite substantial behavioral differences, earnings among types are statistically identical. Our results support the view that our human subject population is in a stable, polymorphic equilibrium of types.<\/jats:p>","DOI":"10.1073\/pnas.0408759102","type":"journal-article","created":{"date-parts":[[2005,1,21]],"date-time":"2005-01-21T20:41:41Z","timestamp":1106340101000},"page":"1803-1807","update-policy":"https:\/\/doi.org\/10.1073\/pnas.cm10313","source":"Crossref","is-referenced-by-count":299,"title":["Experiments investigating cooperative types in humans: A complement to evolutionary theory and simulations"],"prefix":"10.1073","volume":"102","author":[{"given":"Robert","family":"Kurzban","sequence":"first","affiliation":[{"name":"Department of Psychology, University of Pennsylvania, 3720 Walnut Street, Philadelphia, PA 19104; and Interdisciplinary Center for Economic Science and Department of Economics, George Mason University, 4400 University Drive, MSN 1B2, Fairfax, VA 22030"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Daniel","family":"Houser","sequence":"additional","affiliation":[{"name":"Department of Psychology, University of Pennsylvania, 3720 Walnut Street, Philadelphia, PA 19104; and Interdisciplinary Center for Economic Science and Department of Economics, George Mason University, 4400 University Drive, MSN 1B2, Fairfax, VA 22030"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"341","published-online":{"date-parts":[[2005,1,21]]},"reference":[{"key":"e_1_3_2_1_2","unstructured":"Darwin C. 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