account for and explain observed associations. These causal models include interactions such as the one just noted where the effect of one independent variable on the dependent variable changes with the value of a third variable. For example, Bardhan and Dayton-Johnson (Chapter 3) theorize that the effects of heterogeneity may follow several causal paths. “Olson effects” (Olson, 1965; see causal path (a) in Figure 13-2) operate when certain resource users have enough at stake, and enough wealth, to maintain the resource on their own even though there are free riders. Two alternative causal paths, (b) and (c), have negative effects on resources and, according to Bardhan and Dayton-Johnson, are more often consistent with the evidence on the functioning of irrigation systems.
Another example of causal models comes from experimental research attempting to understand why communication within a resource user group fosters cooperative outcomes. This research suggests three possible causal mechanisms. Communication may increase group identity or solidarity, create the perception of a consensus to cooperate, or result in actual commitments to cooperate, which function as shared norms to which members adhere (see Kopelman et al., Chapter