ECON2000 Chap.4 Endogenous Growth and Inequality
Endogenous Growth and Inequality
Baseline Solow growth fades because capital has a diminishing marginal product. Endogenous growth models change that mechanism by introducing an accumulable factor that need not face diminishing returns at the aggregate level. In the AK model, output is proportional to capital, so saving can sustain per-capita growth when its productive effect exceeds population dilution and depreciation.
Ideas, research, quality improvements and creative destruction provide richer mechanisms behind that reduced-form result. The chapter also separates GDP growth from welfare, inequality from mobility and average outcomes from distributional positions. A faster-growing average does not by itself reveal who benefits, whether outcomes persist across generations or whether non-market welfare improves.
What this chapter covers
- 01
Diminishing returns
- 02
AK production
- 03
Sustained-growth threshold
- 04
Ideas and non-rivalry
- 05
Creative destruction
- 06
GDP and welfare
- 07
Inequality and mobility
Worked example · free
Checking the AK growth threshold
- +1The productive saving term is 0.24 times 0.40, equal to 0.096.
- +1Population growth plus depreciation is 0.07, so the growth threshold holds.
- +2The gross factor is (1 minus 0.05 plus 0.096) divided by 1.02.
- +2The factor is 1.02549, so per-capita growth is about 2.55% per period.
Key terms
- AK model
- An endogenous-growth model with output proportional to an accumulable capital composite.
- Non-rival idea
- Knowledge that can be used by one producer without preventing simultaneous use by others.
- Creative destruction
- Innovation that raises productivity while replacing older products, methods or firms.
- Intergenerational mobility
- The degree to which economic position changes rather than persists across generations.
- Diminishing returns
- A falling marginal product as more of an input is added with other relevant conditions held fixed.
Endogenous Growth and Inequality FAQ
Why does baseline Solow growth stop?
Per-capita production is concave in capital, so the marginal product falls as capital accumulates. Saving-financed investment eventually meets depreciation or break-even investment at a finite steady state. Without continuing exogenous technology growth, per-capita capital and output then stop growing.
What allows the AK model to sustain growth?
The accumulable capital composite has a constant marginal product rather than a diminishing one. If saving times productivity exceeds population dilution plus depreciation, capital and output per person grow at a constant positive rate. The result depends on that returns assumption.
How can ideas produce increasing scale effects?
Ideas are non-rival, so one discovery can improve production in many places without being used up. Research expands the stock of ideas, and adoption raises productivity across existing capital. Institutions and competition affect the incentive to create and diffuse those improvements.
Does faster GDP growth prove higher welfare?
No. GDP per person measures average market production, not its distribution, leisure, environmental quality or all non-market activity. Faster growth can expand feasible resources, but a welfare claim requires additional evidence about who gains and which omitted dimensions change.
Exam move
Compare Solow and AK on one pair of axes, then write the marginal product underneath each curve. The shape of that marginal product is the decisive difference. Practise the AK threshold with cases above, at and below the boundary, keeping population growth and depreciation in consistent period units. Add one paragraph that gives economic content to the capital composite through ideas, knowledge or quality improvements.
For distribution questions, keep three columns headed average income, inequality and mobility. A statement in one column does not automatically determine either of the others. End policy answers by separating the growth mechanism from the welfare and distributional evidence still required. Derive the gross AK growth factor before converting it to a percentage rate, and retain extra decimals when the threshold is close.
Stress-test the result by raising depreciation or population growth until the inequality reverses. Then compare the same parameter change in Solow, where diminishing returns create a level effect rather than permanent per-capita growth. Build a mechanism map linking research investment to ideas, ideas to productivity and institutions to innovation incentives.
Note that non-rivalry allows one idea to improve many production sites, while creative destruction creates winners and losses during adoption. For welfare questions, write four separate observations: average GDP, distribution, mobility and omitted non-market outcomes. Never infer one from another without evidence.
A strong policy answer specifies which innovation failure is being addressed, predicts the growth channel and then evaluates who receives the gains and costs. This structure keeps an endogenous-growth explanation from becoming a blanket claim that more investment or faster GDP growth is always socially optimal.
Use a final comparison card with four rows: returns to accumulable capital, long-run per-capita growth, source of productivity change and welfare evidence required. Fill it for baseline Solow, AK and an ideas-based mechanism. Then add a separate distribution note, since a model can explain the average growth rate without predicting inequality or mobility.
This card makes the theoretical fork and the empirical omissions visible at the same time.
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