Romer Growth Model and R&D-Based Endogenous Growth

Core Idea

The Romer model endogenizes technological progress through R&D spending. The economy has a manufacturing sector producing output and an R&D sector producing designs. Sustained growth occurs because knowledge accumulation increases productivity of all future capital, creating a positive externality that drives perpetual growth.

Explainer

From your study of endogenous growth theory, you know the core dissatisfaction with the Solow model: long-run growth depends on an exogenous technology parameter that the model itself cannot explain. The Romer model attacks this gap directly by asking where new technology actually comes from. The answer is purposeful R&D investment — firms devote real resources (researchers, labs, funding) to producing new ideas, and those ideas become the engine of growth.

The model splits the economy into three sectors. The final goods sector uses labor and intermediate capital goods to produce output, much like a standard production function. The intermediate goods sector produces differentiated capital inputs — each one protected by a patent purchased from innovators. The R&D sector employs researchers who produce new designs (blueprints for novel intermediate goods). When a researcher invents a new design, it enters the stock of knowledge, expanding the variety of intermediate goods available. The critical insight is that this stock of knowledge is nonrival: one firm's use of an idea does not diminish another firm's ability to use it. This nonrivalry is what makes sustained growth possible.

The positive externality works through knowledge spillovers. Each new design not only earns its inventor a patent (the private return) but also raises the productivity of all future researchers (the social return). The existing stock of ideas A makes the next idea easier to discover — standing on the shoulders of giants. Because of this externality, the decentralized market outcome underinvests in R&D relative to the social optimum. The private innovator captures only a fraction of the total benefit their discovery creates, so too few resources flow into research.

In equilibrium, the growth rate of the economy depends on parameters the model pins down: the size of the research workforce, the productivity of the R&D process, and the degree of knowledge spillovers. Unlike the Solow model, policy matters for long-run growth — R&D subsidies, patent protection, and education investments that increase the number of researchers can permanently raise the economy's growth rate. This is the model's most important policy implication: growth is not manna from heaven but the result of incentives, institutions, and deliberate resource allocation toward the production of ideas.

The Romer framework also highlights a deep tension. Nonrival ideas require some form of monopoly power (patents) to incentivize their production, since competitive pricing would drive the price of an idea to its zero marginal cost. But monopoly power creates static inefficiency — the intermediate goods are underproduced relative to the social optimum. This tradeoff between dynamic incentives for innovation and static efficiency in the use of ideas is fundamental to the economics of growth and intellectual property, and it recurs throughout the innovation policy literature that builds on Romer's foundation.