Endogenous Growth Theory
What is Endogenous Growth Theory?
Endogenous growth theory models long-run growth as the result of choices inside the economy, such as research and human capital, not outside technical change.
In the neoclassical model capital runs into diminishing returns, so accumulation alone cannot sustain growth and the long run growth rate is handed to the model from outside as technological progress. Endogenous models make that progress the output of deliberate activity: research by profit seeking firms, learning by doing on the factory floor, and investment in human capital. Ideas are non rival, so one firm's discovery raises everyone's productivity, and those spillovers stop the return to the broad reproducible factor from falling. Because the marginal product no longer declines, the saving rate, the share of resources sent to research and the policy environment change the permanent growth rate rather than only the level of income. The theory strains where it implies scale effects, since large increases in the number of researchers have not produced proportional increases in growth, which suggests new ideas are getting harder to find.
Endogenous Growth Theory: a worked example
Take the simplest endogenous model, Y = A times K, with A equal to 0.4 units of output per unit of capital, a saving rate of 0.20 and depreciation of 0.05. Growth equals the saving rate times A minus depreciation: 0.20 times 0.4 is 0.08, less 0.05, which gives 3 percent a year, and it never fades because the marginal product of capital stays at 0.4. Raise the saving rate to 0.25 and growth becomes 0.25 times 0.4 minus 0.05, or 5 percent a year, permanently. Compounded over 30 years the first economy multiplies output by 1.03 to the power 30, about 2.43 times, and the second by 1.05 to the power 30, about 4.32 times, so the higher saving economy ends up roughly 78 percent richer. In the neoclassical model that same jump in saving would raise the level of income per worker and then leave the growth rate exactly where it started.
The mistake students make with endogenous growth theory
Students read endogenous as growth from internal or domestic sources and exogenous as growth from trade or foreign resources. The words describe where a variable is determined relative to the model, not where the cause sits geographically. The neoclassical model assumes a rate of technological progress and never explains it, which makes that rate exogenous; endogenous models write down the research and learning process that produces it. A second slip claims these models abandon diminishing returns altogether, when they only require constant returns to the broad reproducible factor once knowledge spillovers are counted, and an individual firm can still face a falling marginal product.
Endogenous Growth Theory questions
What is the difference between endogenous and exogenous growth theory?
Exogenous growth models, chiefly the neoclassical Solow model, assume a rate of technological progress and derive everything else from it, so the long run growth rate is an input rather than a result. Endogenous models explain that progress as the output of research, education and learning, which are choices firms and governments actually make. The practical consequence is that policy can raise the permanent growth rate in an endogenous model, while in the neoclassical model it can only lift the level of income and speed up the approach to a steady state.
Why are knowledge spillovers central to endogenous growth?
Ideas are non rival, so one firm using a design does not prevent another from using it, and a discovery raises productivity well beyond the firm that paid for it. Those spillovers keep the return to accumulated knowledge from diminishing, which is what lets growth continue indefinitely in these models. They also make the social return to research larger than the private return, so firms underinvest, and that gap is the standard economic case for research subsidies, public science funding and patent systems.
Does endogenous growth theory predict that poor countries catch up?
No, it allows income gaps to persist and even widen. If knowledge accumulates faster where there is already more of it, or where human capital and institutions support research, rich economies can keep growing at least as fast as poor ones indefinitely. The neoclassical model predicts the opposite, that capital scarce economies grow faster and converge, and the weak evidence for convergence across all countries is part of what motivated the endogenous approach.
Formula / Example
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