Development with unlimited supplies of labour
A poor country holds a reserve of labour whose withdrawal from farming costs almost no output, so a modern sector can expand for decades at a flat wage, and everything the expansion earns above that wage is available to reinvest.
W. Arthur Lewis · Credits and sources ↓
What it says
Lewis divides a poor economy in two. There is a capitalist sector, which uses reproducible capital and hires labour for a wage, and a subsistence sector, which does not. In the subsistence sector, and this is the whole argument, an extra worker adds so little to output that removing one costs the sector almost nothing.
That fact sets the wage. Because a worker leaving the farm gives up very little, the modern sector can hire at the subsistence income plus a modest margin, and can go on hiring at that same wage as long as the reserve lasts. Its profits are therefore not competed away by rising wages. They accumulate, and if capitalists reinvest them the sector grows, hires more at the same wage, and earns more profit again.
Lewis put the point in a sentence that has outlived the model: the central problem in the theory of economic development is to understand the process by which a community which was previously saving four or five per cent of its national income converts itself into one saving twelve to fifteen per cent. The dual economy is his answer. The saving comes out of profits, and the profits exist because the wage does not rise.
The model has an ending built into it. When the reserve is used up, the next worker hired does cost the farm real output, wages start climbing, and the arithmetic of accumulation changes. Economists call that moment the Lewis turning point, and countries are still trying to work out whether they have passed it.
Drawn one step at a time
The theory as a graph, revealed a layer at a time. Use the buttons, or the left and right arrow keys. Each step adds the boxes that step introduces and the arrows into them.
- Starting condition
- Mechanism
- Outcome or policy
Four placements, and the reason for each
The four scores are editorial. They run from -3 to +3, they were assigned by the ImpactMojo editorial team from the theory's own texts, and each theory page shows the sentence that justifies its placement so the placement can be argued with. They are a way of arranging a shelf, not a measurement.
Lewis is not prescribing ownership. The capitalist sector can be private or a state corporation; what matters is that its surplus is reinvested rather than consumed.
Inequality is the engine, not a side effect. The model works because profits are large and wages are flat, and Lewis says so plainly.
The reserve of labour is treated as undifferentiated. Nothing in the model asks who is free to leave a village, which in India is a question with a caste answer and a gender answer.
Capitalists and, at independence, the governments Lewis advised. Workers in this account respond to a wage; they do not set one.
The theory against the record
Each entry takes one claim the theory makes and reports what the evidence says about it, with a named source and a year. This section is the reason the library exists; a catalogue of positions without it is a reading list.
The claim The marginal product of labour in peasant agriculture is near zero.
Theodore Schultz attacked this directly using the 1918–19 influenza epidemic in India, which killed a large share of the rural workforce in a single season. If those workers had been adding nothing, sown acreage and output should have held steady. Both fell. Schultz read that as evidence that poor farmers are efficient with what they have, and that the reserve Lewis assumed does not exist in the form he assumed it.
Theodore W. Schultz, Transforming Traditional Agriculture · 1964
The claim Labour leaving farming is absorbed by a modern wage-paying sector.
India's structural change went sideways. Agriculture's share of employment did fall, reaching roughly two-fifths of the workforce by 2018–19, but manufacturing's share stayed close to a ninth throughout, and most of those who left farming went into construction and informal services rather than into capitalist-sector jobs. The Periodic Labour Force Survey rounds after 2019 then recorded the share in agriculture rising again, the first sustained reversal of the transition in the series.
NSSO Employment and Unemployment Surveys and the Periodic Labour Force Survey, MoSPI · 2023
The claim Surplus is reinvested where it is earned.
Lewis stated this as a condition and not a prediction, and it is the condition that has failed most often. He wrote later, after advising Ghana, about capitalist classes that took the surplus abroad or into land and rent, and treated the failure as the ordinary case rather than the exception.
W. Arthur Lewis, The Theory of Economic Growth, and his later writing on West Africa · 1955
What this does not settle
Whether India has passed the turning point is genuinely open, and the answer differs by state. Real rural wages grew quickly between roughly 2007 and 2013 and then stalled, which is a pattern consistent with a turning point that arrived and then went into reverse, and equally consistent with a construction boom that ended. What is not in doubt is that the model's second half never happened here: the reserve emptied partway into an informal sector that pays a wage without providing the capital, so the surplus that was supposed to compound was never concentrated anywhere it could.
How it landed here
The model was built partly from Asian material and Lewis cited India directly, and its assumption maps onto something real: the joint family farm that absorbs a returning migrant without complaint. What it misses is who is free to leave. Withdrawal from the village is not a labour-supply decision alone when a Dalit household's alternative employment is set by who will hire it, and when women's withdrawal from the workforce in India rose as household incomes rose. The reserve is not one pool.
One that agrees, one that does not
Planning for the capital goods sector
Both make the reinvested surplus the engine and both are indifferent to consumption in the short run. Mahalanobis adds the question of what the surplus should be spent on.
Capabilities and human development
Lewis measures development by the saving rate. Sen and Haq built an index precisely to stop that number standing in for how people are doing.
Whose theory this is
W. Arthur Lewis 1915–1991
Born in Saint Lucia, taught at Manchester and Princeton, advised Ghana at independence. Shared the 1979 Nobel with Theodore Schultz, who had spent much of the previous fifteen years arguing that Lewis's central assumption was wrong.
What ImpactMojo added
The causal diagram, the four placements and the notes justifying them, and the evidence section: what each claim predicted and what the record shows, with a named source and year for every entry.
ImpactMojo · content CC BY-NC-ND 4.0 · code MIT
Start with these
- W. Arthur Lewis, Economic Development with Unlimited Supplies of Labour (1954). The Manchester School, May 1954. Short, readable, and clearer about its own assumptions than most of what was written about it.
- Gustav Ranis and John Fei, A Theory of Economic Development (1961). American Economic Review. The formalisation that made the turning point a thing you could look for in data.
- Theodore W. Schultz, Transforming Traditional Agriculture (1964). The case that peasant farmers are poor but efficient, and the influenza argument against zero marginal product.
Open access, in Development Discourses:
- World Employment and Social Outlook: Trends 2024 — ILO (2024)
- The Urban Informal Economy: Achieving Prospering and Territorial Equality — Martha Alter Chen, Caroline Skinner (2021)