The Critical Mass in Collective Action: A Micro-Social Theory, by Gerald Marwell and Pamela Oliver. Cambridge University Press, 1993, 206 pp.

Since the publication of Mancur Olson's Logic of Collective Actionin 1965, social science has brought forward a mountain of cross-disciplinary analysis concerning various facets of the "paradox" of cooperation among rational individuals in large groups in pursuit of a collectively desired good. Most of it has accepted Olson's conclusion that such cooperation is almost always impossible without the provision of selective incentives (private benefits or cost exemptions that are available only to those who contribute towards the collective good), and has moved on to examine various ways in which such incentives can be provided, particularly by centralized hierarchy.

More recently, a newer body of work, primarily in sociology and psychology, has questioned the premises of this analysis and has sought to examine the conditions under which selective incentives may not be necessary for cooperation even in large groups. However, unlike other criticisms of rational choice collective action theory, this literature has maintained the assumption that individuals are purposive actors and that they will seek to maximize the net benefits that they gain from their choices.

Marwell and Oliver are two of the major figures in this newer literature, and this wide-ranging book is an amalgamation and reworking of several of their earlirer papers. After presenting a critique of Olson's theory and describing the basic building blocks of their own theory, they go on to examine the effect on collective action of a range of variables ignored by the original theory and most subsequent elaborations.

These variables include heterogeneity among group members in their resources and in their interest in the collective good, the jointness of the good (i.e. the extent to which an individual can enjoy its benefits without reducing the benefits to others), and the extent to which provision of the good is characterized by an accelerating or decelerating production function (i.e. increasing or decreasing returns to contributions by members). Among the major conclusions they reach is that, given high jointness and member heterogeneity in interest and resources, group size will have a positive effect on the extent of cooperation, contrary to the implications of Olson's theory. Another is that, given sequential decisions by members on whether or not to contribute to the collective good, the nature of the production function for the good will have a crucial effect on the extent of cooperation and how it is affected by variables such as order of decision\-making and the number of would-be willing contributers.

The book also focuses on the effects of social networks on recruitment to collective action. Variables examined here include network density and centrality, the costs to organizers for recruiting members, as well as the different strategies that potential organizers might use. This analysis, linked closely to sociological network theory, challenges the conventional rational choice assumption that recruitment is costless.

This book is a useful addition to recent efforts to stretch rational choice collective action theory beyond its original assumptions. Though some may disagree in places with the new assumptions that the authors use to replace those provided by Olson, this contributes to a healthy reawakened debate on the conditions surrounding collective action. The book's analysis might be extended by linking it more with recent work in public goods theory that covers some of the same issues. Also, it is important not to overly deemphasize the role of selective incentives. Ultimately, theory ought to seek to examine the combined effects of both public and private incentives in facilitating or hindering cooperation.