Increasing supply of dentists induces their geographic diffusion in contrast with physicians in Japan.

The geographical distribution of health professionals reflects behavioral characteristics of such professionals and of the health system in which they work. The spill-over hypothesis asserts that their oversupply leads to a more even geographic distribution. The current surplus of dentists in Japan is a suitable opportunity to observe such situations. This study demonstrates the transition of the geographic distribution of dentists from 1980 to 2000 in comparison with that of physicians. Using data from the Population Census and the Physician, Dentist, and Pharmacist Census, we calculated the ratio of dentists working in clinics and hospitals per population in 1980, 1990, and 2000 and the Gini coefficients according to the municipality boundaries at the end of 2000. We also plotted the municipalities on a graph, which illustrated the ratios of the dentists by population. We did the same analysis with physician data. The number of dentists increased by 71% during the 20 years studied. The ratios of dentists/100,000 population were 44.1, 58.3, and 69.7 in 1980, 1990, and 2000, respectively. The Gini coefficients for dentists by municipality were 0.270, 0.213, and 0.197, excluding the municipalities with a dental university or its hospital. In contrast, the Gini coefficients for physicians barely changed while the number of physicians increased by 60% during the same periods. The graphs for dentists appeared to indicate the ceiling of those ratios (approximately 100 dentists/100,000 population), but such a ceiling was not seen for physicians. The supply of dentists might have reached a level that generated the geographic diffusion and redistribution of dentists in Japan, in contrast with the situation involving physicians. This supports some results from other countries suggesting that saturation of local markets for health professionals may result in geographical redistribution, producing a more equal pattern of provision across the national space.