Endogenous Growth Theory

Endogenous growth theory is a family of economic models in which long-run growth is driven by forces inside the economy, such as investment in knowledge, human capital, research and development and innovation, rather than by technical progress that is simply assumed to arrive from outside. It was developed from the mid-1980s, mainly by Paul Romer (1986 and 1990) and Robert Lucas (1988), and extended by Gene Grossman and Elhanan Helpman and by Philippe Aghion and Peter Howitt.

The theory grew out of dissatisfaction with the Solow model of neoclassical growth, in which long-run growth in income per person equals the rate of technological progress but nothing in the model explains that rate. Romer shared the 2018 Nobel memorial prize for integrating technological change into long-run analysis, and Aghion and Howitt shared half of the 2025 prize. In British politics, the name became famous through Gordon Brown's 1994 reference to "post-neoclassical endogenous growth theory", which has its own short page under that phrase.

Compare a farm with a library. Each extra field a farmer ploughs tends to be poorer than the last, so adding land brings smaller and smaller gains. Knowledge is different. Once a useful idea is written down, everyone can use it at once, it never wears out, and each new idea makes the next one easier to find.

Endogenous growth theory says that economies can keep growing because they keep adding to that library. How fast they add to it depends on choices people and governments make, such as how much they spend on schools, research and new businesses.

In the Solow model, capital is subject to the law of diminishing returns: each extra machine adds less output. Higher saving raises the level of income but not its long-run growth rate. The simplest endogenous model, the "AK" model, removes diminishing returns by treating capital broadly to include knowledge and skills:

• $g$ is the long-run growth rate of capital and output.
• $s$ is the share of output saved and invested.
• $A$ is output per unit of capital, assumed constant.
• $\delta$ is the rate at which capital wears out.

Worked example: with $s = 0.20$, $A = 0.30$ and $\delta = 0.05$, growth is $0.20 \times 0.30 - 0.05 = 0.01$, or 1% a year. If the saving rate rises to 0.25, growth becomes $0.075 - 0.05 = 0.025$, or 2.5% a year, permanently. Unlike in the Solow model, investment choices change the long-run growth rate itself.

• Spillovers and learning by doing. Romer's 1986 model, drawing on Kenneth Arrow's 1962 work, lets firms' investment raise the knowledge available to all firms, so returns to the economy as a whole do not diminish.
• Human capital. Lucas (1988) placed the accumulation of skills, through education and experience, at the center of growth.
• Ideas and market power. Romer's 1990 paper "Endogenous Technological Change" treated ideas as non-rival goods produced by profit-seeking research. Because firms need some reward to invent, the model gives a role to intellectual property and temporary monopoly.
• Quality ladders. Grossman and Helpman (1991) and Aghion and Howitt (1992) modelled growth as a sequence of improvements in which new products replace old ones, a formal version of creative destruction.

If growth depends on choices inside the economy, policy can affect the long-run growth rate, not just the level of output. Endogenous growth models have been used to support public spending on education, R&D subsidies and tax credits, protection of property rights, openness to trade and ideas, investment in infrastructure, and active competition policy. Some also point to business clusters, where knowledge spills over between nearby firms. The theory gives no single prescription, however: models differ on whether there is too little or too much research, and on how strong patent protection should be.

On 27 September 1994 Gordon Brown, then Labour's shadow chancellor, said in a speech that his economic approach drew on "post neo-classical endogenous growth theory". The phrase was widely attributed to his adviser Ed Balls, who later wrote that he had tried to remove it from the draft. Michael Heseltine mocked it at the 1995 Conservative conference with the line "It's not Brown's, it's Balls'". After 1997, Labour governments introduced R&D tax credits for small firms in 2000 and for large companies in 2002, policies consistent with the theory's emphasis on research.

Charles Jones (1995) pointed out that early models predicted that more researchers should mean permanently faster growth, yet US growth did not speed up as research employment rose sharply after the 1950s. This led to "semi-endogenous" models in which policy affects growth for long periods but not forever. Critics also argue that the models are hard to test, that broad "capital" in the AK model is vague, and that cross-country evidence fits conditional convergence, in which poorer economies catch up given similar institutions, as well as it fits endogenous growth. Persistent gaps, including the middle-income trap, keep the debate open.

• Nobel Prize: Paul M. Romer, facts (2018)
• Nobel Prize: Economic Sciences 2025 press release