Collective action usually depends on a "critical mass" that behaves differently from typical group members. Sometimes the critical mass provides some level of the good for others who do nothing, while a t other times the critical mass pays the start-up costs and induces widespread collective action. Formal analysis supplemented by simulations shows that the first scenario is most likely when the production function relating inputs of resource contributions to outputs of a collective good is decelerating (characterized by diminishing marginal returns), whereas the second scenario is most likely when the production function is accelerating (characterized by increasing marginal returns). Decelerating production functions yield either surpluses of contributors or order effects in which contributions are maximized if the least interested contribute first, thus generating strategic gaming and competition among potential contributors. The start-up costs in accelerating production functions create severe feasibility problems for collective action, and contractual or conventional resolutions to collective dilemmas are most appropriate when the production function is accelerating.

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In this article, we examine the effects of classroom characteristics on the interracial friendliness of students in desegregated classrooms. We argue that social psychological processes that affect cross-race sociability are set in motion by the classroom climate, the organization of instruction, and the class racial composition. The effects of these variables on the likelihood of cross-race friendships are observed in longitudinal data from 455 students in 16 fourth- through seventh-grade desegregated classrooms. The results show that classroom characteristics associated with "status-leveling" effects operate differently for black and white students. Ability grouping influences the interracial sociability of white students more than that of blacks, while the academic status of peers affects the cross-race friendships of blacks more than of whites. Race differences in the effects of classroom climate on interracial friendships are also observed.

In this article, we examine the effects of classroom characteristics on the interracial friendliness of students in desegregated classrooms. We argue that social psychological processes that affect cross-race sociability are set in motion by the classroom climate, the organization of instruction, and the class racial composition. The effects of these variables on the likelihood of cross-race friendships are observed in longitudinal data from 455 students in 16 fourth- through seventh-grade desegregated classrooms. The results show that classroom characteristics associated with "status-leveling" effects operate differently for black and white students. Ability grouping influences the interracial sociability of white students more than that of blacks, while the academic status of peers affects the cross-race friendships of blacks more than of whites. Race differences in the effects of classroom climate on interracial friendships are also observed.

No abstract