In the Solow model, long-run growth in output per worker eventually halts unless an external factor is assumed. Romer's model avoids this fate primarily because:
ARomer assumes a higher savings rate, which sustains capital accumulation indefinitely
BIdeas are non-rival — using a design doesn't deplete it — so including knowledge in the production function creates increasing returns to scale that offset diminishing returns to capital
CRomer assumes population growth, which keeps labor supply expanding and prevents stagnation
DRomer eliminates the steady state by assuming constant marginal product of capital
The Solow model hits diminishing returns because doubling physical inputs (capital, labor) exactly doubles output — constant returns — and capital eventually accumulates to its steady-state level where investment just replaces depreciation. Romer's key insight is that ideas are non-rival: a design used in one factory can simultaneously be used in another. Including the stock of knowledge in the aggregate production function creates increasing returns to scale, which sustains long-run growth indefinitely as the knowledge stock expands.
Question 2 Multiple Choice
In Romer's model, patents create temporary monopoly rights for innovators. A critic argues this is welfare-reducing because monopoly prices restrict output. Which response best captures the Romer-model counterargument?
AMonopoly prices are actually efficient because they reflect the true social value of knowledge
BPatents are unnecessary — innovators are motivated by fame and professional recognition, not profit
CWithout partial excludability through patents, no firm could earn a return on R&D investment and innovation would collapse; the static deadweight loss of monopoly is the price of sustaining dynamic innovation
DPatents are bad policy in the model; the optimal solution is public ownership of all research
Romer's model acknowledges the tension: ideas are non-rival (should be freely shared) but require excludability (to give innovators an incentive to create them). Patents are a second-best solution — they create static monopoly distortions but solve the appropriability problem that would otherwise make private R&D investment zero. The model actually predicts *too little* innovation even with patents, because innovators cannot capture all the social spillovers their ideas create. The correct policy is not to eliminate patents but to supplement them with R&D subsidies.
Question 3 True / False
In the Romer model, the economy exhibits increasing returns to scale at the aggregate level even though individual sectors have constant or diminishing returns.
TTrue
FFalse
Answer: True
Each individual production function (final goods, intermediate goods) exhibits constant returns to scale in its physical inputs. But when knowledge is included alongside capital and labor in the aggregate picture, doubling all inputs — including the stock of ideas — more than doubles output. This is because ideas are non-rival: the same blueprint can be replicated across all producers simultaneously, unlike physical capital. The increasing returns at the aggregate level is precisely what sustains long-run growth in the Romer model.
Question 4 True / False
The Romer model predicts that the market equilibrium produces the socially optimal level of innovation, since private R&D firms are rewarded through patent monopoly profits.
TTrue
FFalse
Answer: False
Even with patents, the market produces *too little* innovation relative to the social optimum. This is because each new idea generates positive spillovers for future researchers — it expands the knowledge base from which new innovations are produced — but the innovating firm cannot capture this social value through its monopoly profits alone. The divergence between private return and social return to R&D is a market failure that Romer's model makes precise, providing a rigorous justification for R&D subsidies and public basic research funding.
Question 5 Short Answer
Why does the non-rivalry of ideas imply that private markets will underinvest in R&D, even when innovators can patent their discoveries?
Think about your answer, then reveal below.
Model answer: Non-rivalry means a new idea immediately becomes part of the public stock of knowledge that all future researchers build on — each idea raises the productivity of all subsequent R&D. The innovating firm captures only the private monopoly profit from its specific patent, not the value it creates for the entire future innovation stream. Since the social return to innovation exceeds the private return by the amount of these knowledge spillovers, private markets equate private marginal benefit to marginal cost and produce less R&D than would maximize social welfare.
This is the core market failure in Romer's model: R&D has positive externalities that are not internalized by the market. Patents improve appropriability but cannot capture the full social spillover value of new knowledge. The implication is that even perfectly competitive R&D markets with strong patent protection will systematically underinvest, justifying public subsidies to close the gap between private and social returns.