when group members have opportunities to punish low contributors. Cooperation may also be supported if individuals are … punishment costs. The negative effect of altruism on cooperation and material payoffs is greater the stronger is the reciprocity …

support near e¡Ë cient levels of contributions to a public good. Cooperation may also be supported if altruistic individuals … cooperation may be less than additive. Using a utility function embodying both reciprocity and altruism we show that unconditional … contribution costs and punishment costs. The range over which altruism inhibits cooperation and reduces material payo¢´s is greater …