What drives the diffusion of inclusionary zoning?

Social scientists offer competing theories on what explains the policymaking process. These typically include economic rationalism, political competition or power struggles, and policy imitation of the kind that diffuses across spatially proximate neighbors. In this paper, we examine the factors that have influenced a recent local policy trend in California: inclusionary zoning (IZ). IZ programs require developers to make a certain percentage of the units within their market-rate residential developments affordable to low- or moderate-income households. By 2007, 68 percent of jurisdictions in the San Francisco Bay Area had adopted some type of IZ policy. We test the relative importance of economic, political, and spatial factors in explaining the rapid diffusion of IZ, across 100 cities and towns in the Bay Area. Consistent with an economic efficiency argument, results of hazard models provide some evidence that IZ is adopted in places with less affordable housing. However, political factors, such as partisan affiliation and the strength of affordable housing nonprofits, are even more robust predictors of whether or not a local government adopts IZ. There is no evidence of spatial diffusion in the case of IZ adoption; jurisdictions are not, on average, responding to the behavior of their neighbors. © 2010 by the Association for Public Policy Analysis and Management.