Solow growth model
The Solow growth model explains long-run growth through capital accumulation, labour, and technological progress.
Why do economies grow, and why does growth from investment alone eventually peter out? The Solow model gave the first clear answer.
The Solow growth model, developed by Robert Solow, explains long-run economic growth through the accumulation of capital, the growth of the labour force, and technological progress. Its central and surprising conclusion is that capital accumulation alone cannot sustain growth indefinitely; only technological progress can.
Diminishing returns to capital
The engine of the model is diminishing returns to capital. As an economy adds more capital per worker, output per worker rises, but by less and less with each addition, because the same workers can only do so much with ever more machines. Eventually new investment only just covers the wearing-out of existing capital, and growth in output per worker stops. An economy can grow rich by investing, but the gains from investment alone run down.
The role of technology
If capital accumulation hits a ceiling, what keeps living standards rising in the long run? In the Solow model, the answer is technological progress, which raises the productivity of both capital and labour and so keeps shifting the ceiling upward. Crucially, the model treats this progress as exogenous, arriving from outside the model rather than being explained by it, which is both its honesty and its limitation: it identifies technology as the key without explaining where technology comes from.
Convergence and its puzzle
The model predicts convergence: poorer economies, with little capital, should earn high returns on investment and grow faster, catching up with rich ones. This happens in some cases but not others, and explaining the exceptions, why some poor countries fail to converge, pushed economists toward institutions, human capital, and the endogenous growth theories that tried to explain technological progress rather than assume it.
The Solow model remains the foundation of growth economics because it cleanly separated the sources of growth and showed that the lasting one is productivity, not mere accumulation. Its very gap, leaving technology unexplained, defined the agenda for the growth theory that followed.