This paper provides a unified growth theory, i.e. a model that explains the very long-run economic and demographic development path of industrialized economies, stretching from the pre-industrial era to the present-day and beyond. Making strict use of Malthus' (An essay on the principle of population. London, printed for J. Johnson, 1798) so-called preventive check hypothesis-that fertility rates vary inversely with the price of food-the current study offers a new and straightforward explanation for the demographic transition and the break with the Malthusian era. Employing a two-sector framework with agriculture and industry, we demonstrate how fertility responds differently to productivity and income growth, depending on whether it emerges in agriculture or industry. Agricultural productivity and income growth makes food goods, and therefore children, relatively less expensive. Industrial productivity and income growth, on the other hand, makes food goods, and therefore children, relatively more expensive. The present framework lends support to existing unified growth theories and is well in tune with historical evidence about structural transformation.