Solow growth model
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Why do poor countries often grow faster than rich ones, and why can no country keep growing fast just by building more factories? In the 1950s the economist Robert Solow built a simple but powerful story that answers these questions, and it earned him a Nobel Prize. Told in words, the Solow growth model explains how an economy accumulates capital, why that process slows down on its own, and why lasting growth must come from somewhere else.
The model imagines output being produced from capital and labour, with one crucial assumption: capital has diminishing returns, so each extra machine adds less than the one before. Every year the economy saves and invests part of its output to build new capital, but capital also wears out (depreciates). Early on, when a country has little capital, investment easily outruns wear and tear, so capital and output grow quickly. But as capital piles up, the gains from each new machine shrink while depreciation keeps rising, until eventually new investment only just replaces what wears out. At that point, called the steady state, capital per worker stops growing and so does output per worker, unless something shifts the picture. The model's famous punchline is that saving more raises the level of income but cannot raise the long-run growth rate; only improvements in technology (total factor productivity) can keep output per person rising forever.
The Solow model matters because it reshaped how economists think about growth and development. It predicts conditional convergence: countries with less capital should grow faster and catch up to similar richer ones, which partly fits the data. It explains why pouring in capital alone eventually fizzles, pointing instead to technology and ideas as the true long-run engine. Its main limitation is honest and important: it treats that technological progress as falling from the sky, unexplained from outside the model, which later endogenous growth theories tried to explain from within.
Picture a farm that keeps buying tractors. The first tractor hugely boosts output, the second helps a lot, the fifth barely helps, and meanwhile tractors keep wearing out. Eventually all new spending just replaces worn-out tractors, and output per worker stops rising, the steady state in miniature.
Diminishing returns to capital push the economy toward a steady state.
A key takeaway often missed: in the Solow model, a higher saving rate makes a country richer in level but does not raise its long-run growth rate. The model also leaves technological progress unexplained, treating it as exogenous.