A new conceptual framework for explaining physician distribution and forecasting regional physician supply is presented. Unlike existing efforts at explaining physician distribution, which rely on models of individual physicians making choices regarding location of practice (e.g., economic and psychological models), this paper develops an environmental selection model which views populations of physicians as the unit of analysis and identifies the environmental conditions under which these populations grow and decline. Nine dimensions of the environment affecting populations of physicians are described and hypotheses regarding the effect of four dimensions-residential population size, density, growth, and health system development are pooled cross-sectional and time-series data for counties in New York state. A modified generalized least-squares routine is used to estimate the coefficients. Results reveal substantial support for fundamental aspects of the model: a carrying capacity varying with density of...