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Endogenous growth models explain persistent per-capita growth through mechanisms endogenous to the economic system—human capital accumulation, R&D, spillovers, or learning-by-doing—rather than exogenous technological progress. Unlike Solow, these models show how policy (education subsidies, R&D incentives, IP protection) and private innovation incentives directly affect long-run growth rates. They highlight the role of increasing returns, knowledge externalities, and monopolistic competition in sustaining endogenous growth.
The Solow growth model, which you studied as a prerequisite, identifies three sources of economic growth: capital accumulation, labor force growth, and technological progress. However, Solow treats technological progress as exogenous — it is assumed to grow at some constant rate A, determined outside the model. The critical question Solow leaves unanswered is: where does technological progress come from, and what determines its rate? Endogenous growth theory was developed specifically to answer this question by building the sources of technological change directly into the economic model.
The simplest endogenous growth model is the AK model, where aggregate output is Y = AK and A is a broad productivity parameter that encompasses both physical and human capital. Unlike Solow's model with diminishing returns to capital, the AK model assumes constant returns — an extra unit of capital always produces A units of output. This eliminates the convergence to a steady state: the economy grows indefinitely at a rate determined by the saving rate and A. While stylized, this captures the core insight that if capital (broadly defined to include knowledge and human capital) does not exhibit diminishing returns, growth can persist without any exogenous input.
Richer models (such as those of Romer and Lucas) specify the microeconomic mechanisms through which knowledge accumulates. In Romer-style models, profit-seeking firms invest in R&D to develop new varieties of goods or new technologies. Each successful innovation generates a temporary monopoly profit for the innovator, which is the private return to R&D investment. Crucially, innovations also generate knowledge spillovers — they add to the public stock of ideas that other researchers and firms can draw on for free. This means the social return to R&D exceeds the private return, leading a market economy to under-invest in innovation relative to what would maximize social welfare. This externality is the central market failure motivating public R&D subsidies and science policy.
Human capital models, associated with Lucas, place education and skill accumulation at the center of growth. Workers who invest in education become more productive, and interactions between skilled workers can generate further spillovers — a highly educated workforce makes everyone around them more productive. Like knowledge spillovers in R&D models, human capital externalities mean that private investment in education under-shoots the social optimum, justifying education subsidies and public schooling.
The most important practical implication of endogenous growth theory is that long-run growth rates are policy-responsive. In the Solow framework, policy can raise the level of output (by changing the steady-state capital stock) but cannot permanently accelerate growth. In endogenous models, a sustained increase in the fraction of resources devoted to R&D or education raises the long-run growth rate permanently. This means the choice of institutions, incentive structures, and public investments is not just about distributional fairness — it is a first-order determinant of how fast the standard of living rises over decades.