This study establishes a socio-demographic theory of international development derived from selected classical and contemporary sociological theories. Four hypotheses are tested: (1. population growth's effect on development depends on age-structure; (2. historic population density (used here as an indicator of preindustrial social complexity) boosts contemporary economic performance; (3. ethnic polarization impairs economic growth; and (4. a nation's degree of sociolinguistic integration positively influences economic performance. Investigating annual changes in real gross domestic product per capita from 1970 to 2000, our pooled time-series analyses of 101 developed and developing countries generally support these hypotheses net of common alternative explanations, suggesting that the etiology of economic growth could benefit from the reintroduction of classic and contemporary sociological theories. (Contains 2 tables, 2 figures and 4 notes.)