Understanding why local governments join regional planning networks is key for analyzing intergovernmental collaboration.

This article examines the role of political homophily—when similar communities collaborate—in California's regional planning systems. We surveyed planners and officials across California counties to test our hypothesis: politically alike areas are more likely to participate together in planning efforts.

Our findings reveal a clear pattern:

• Local governments with ideologically aligned residents show higher willingness to join collaborative networks

• Political alignment reduces transaction costs associated with collective action

Even in technical planning contexts where politics might be expected to play minimal roles, political similarity appears crucial for collaboration. This suggests long-standing economic theories about institutional cooperation may need refinement.

The study underscores how democratic processes at the local level shape governance structures and collaborative behaviors.