Endogenous growth theory
Endogenous growth theory explains growth as arising from forces inside the economy, such as innovation and human capital, rather than external shocks.
If technological progress drives long-run growth, leaving it unexplained is unsatisfying. Endogenous growth theory tried to bring it inside the model.
Endogenous growth theory is a body of work, associated with Paul Romer and others, that explains long-run growth as arising from forces within the economy, especially the deliberate creation of knowledge and investment in human capital, rather than from technological progress that simply falls from outside. It seeks to explain the engine that earlier models assumed.
Making technology a choice
In the Solow model, technological progress was exogenous, a gift from outside. Endogenous growth theory makes it a result of decisions inside the economy: firms and people invest in research, education, and innovation in pursuit of returns, and that investment generates the new knowledge that drives growth. Growth becomes something economies produce through their choices, not something that merely happens to them, which means policy can influence the long-run growth rate, not just the level of income.
The special economics of ideas
The theory rests on the peculiar nature of knowledge as an economic good. Ideas are non-rival: one person using an idea does not stop another from using it, so an idea, once created, can raise productivity everywhere at little extra cost. This means knowledge can yield increasing returns and spill over to others, escaping the diminishing returns that limit physical capital. An economy that keeps producing ideas need not run into the Solow ceiling.
Implications and debates
If knowledge drives growth and spills over, then investment in research, education, and the institutions that support innovation can raise long-run growth, which gives a strong rationale for public support of science and learning. The theory also implies that leaders can stay ahead by continuing to innovate, complicating the simple convergence story. Critics note that the mechanisms are hard to measure and that the theory can be flexible to the point of being difficult to refute.
Endogenous growth theory matters because it took the most important driver of prosperity, the growth of useful knowledge, and tried to explain rather than assume it. Whatever its unresolved debates, it put innovation, human capital, and the economics of ideas at the centre of how economists think about why some economies keep pulling ahead.