AbstractHigh courts are widely believed to influence the criminal justice system, yet judicial impact varies widely across political and institutional contexts. Here, we seek to identify the factors that constrain judicial influence on state incarceration rates. We find that the preferences of justices on state courts of last resort and the US Supreme Court influence incarceration; however, high-court impact is conditional on two factors. Judicial influence is stronger in states with divided partisan governments and occurs more quickly in states that lack intermediate appellate courts. Our findings suggest that legislative gridlock and institutional thickness significantly constrain judicial impact.