Efficiency and legitimacy in collaborative public management: mapping inter-local agreements in England using social network analysis

English councils have long aspired to be “self‐sufficient,” providing services within single jurisdictions with limited inter‐local collaboration. However, by 2017 almost all local councils (97%) participated in one or more frontline or back‐office “shared service” involving 338 distinct partnerships. We analyse this new‐found enthusiasm for inter‐council collaboration by performing exploratory social network analysis on organizational and financial data for all 353 English councils. We examine factors predicting collaboration and the characteristics of the service networks that result, focusing on resource, organizational and political considerations. Propensity to collaborate was found to be unpredictable, but partner choice was rational, driven by geographical proximity and similarity in organizational and resource characteristics. We argue that, according to the institutional theory of organizations, both efficiency and legitimacy influenced reform choices, and the risks of collaboration were mitigated by careful partner selection. We highlight implications for future quantitative research into symbolic (non‐instrumental) forms of collaboration