Beyond Salary: Habitable-Area Population Density and Postgraduate Residency Match Outcomes at Japanese University Hospitals—A Nationwide Hospital-Level Study

Background University hospitals play a central role in postgraduate clinical training in Japan, yet filling residency positions remains challenging. Geographic conditions may be associated with recruitment outcomes independently of institutional characteristics and financial incentives. This study examined the association between habitable-area population density and residency position filling at Japanese university hospitals. Methods This preregistered nationwide hospital-level cross-sectional study included all 81 main university hospitals in Japan. Official results from the 2026 Japan Residency Matching Program were analyzed using quasi-binomial logistic regression. The primary adjusted model included habitable-area population density, hospital bed count, and university ownership. An additional model incorporated estimated annual resident remuneration for 79 hospitals. Matching outcomes from 2025 were examined in a historical analysis. Results In 2026, 2,473 of 3,310 residency positions were filled (74.7%). Each doubling of habitable-area population density was associated with higher odds of position filling after adjustment for hospital bed count and university ownership (odds ratio [OR], 1.56; 95% confidence interval [CI], 1.24–1.95). This association remained similar after additional adjustment for remuneration (OR, 1.54; 95% CI, 1.21–1.96). Estimated remuneration was not statistically associated with position filling (OR per 1 million JPY, 0.86; 95% CI, 0.54–1.39). The 2025 analysis showed an association in the same direction (adjusted OR per doubling, 1.73; 95% CI, 1.39–2.16). Conclusions Higher habitable-area population density was associated with greater residency position filling at Japanese university hospitals. This association was not materially attenuated by adjustment for measured institutional characteristics, including remuneration. Geographic context should be considered when interpreting residency recruitment outcomes and planning postgraduate clinical training. These hospital-level findings do not establish causality or identify the factors underlying individual graduates' residency choices. Trial registration Not applicable