This paper develops a unified model of growth, population, and technological progress that is consistent with long-term historical evidence. The economy endogenously evolves through three phases. In the Malthusian regime, population growth is positively related to the level of income per capita. Technological progress is slow and is matched by proportional increases in population, so that output per capita is stable around a constant level. In the post-Malthusian regime, the growth rates of technology and total output increase. Population growth absorbs much of the growth of output, but income per capita does rise slowly. The economy endogenously undergoes a demographic transition in which the traditionally positive relationship between income per capita and population growth is reversed. In the Modern Growth regime, population growth is moderate or even negative, and income per capita rises rapidly. Two forces drive the transitions between regimes: First, technological progress is driven both by increases in the size of the population and by increases in the average level of education. Second, technological progress creates a state of disequilibrium, which raises the return to human capital and induces parents to substitute child quality for quantity.

This paper presents a model in which a country's average propensity to save tends to rise endogenously over time. The paper uses a two-sector neoclassical framework to model the transition from agriculture to manufacturing which typically accompanies economic development. Key assumptions are that only the agricultural sector uses land and a simple version of Engel's law. When a country's income per capita is low, agricultural consumption is important; consequently, land is valuable and capital gains on it may account for most wealth accumulation, making the NIPA APS appear low. If exogenous technological progress raises incomes over time, Engel's law shifts demand to manufactured goods. Then land's importance in portfolios relative to reproducible capital diminishes and the measured average propensity to save can rise.