Law of diminishing marginal product

Also called: Law of diminishing returns, diminishing marginal returns · Topic: Production Function, Returns and Costs · NCERT: Class 12, Ch 3 "Production and Costs"

Meaning

The law of diminishing marginal product says this: when one input (usually labour) is increased and the other inputs stay fixed, the marginal product of that input starts to fall after a certain level of employment. Marginal product means the extra output from one more unit of the input. The law assumes technology does not change.

• Formula: MP_L = ΔTP / ΔL. For whole units, MP_L = TP_L − TP_(L−1).
• Why it matters: it explains why short-run costs rise and why crowded farms have low output per worker. It is also the basis of disguised unemployment, Ricardo's theory of rent and Malthus's population argument.

Explanation

Basic terms

• Production function: the link between inputs and output. It shows the most output a firm can get from given inputs, with a given technology.
• Short run: a period in which at least one input is fixed, such as land or a machine. This law belongs to the short run.
• Fixed factor: an input that cannot change in the short run, such as the farmer's 4 hectares.
• Variable factor: an input that can change, such as the number of workers.
• Total Product (TP): the total output from a given amount of the variable input.
• Marginal Product (MP): the extra output from one more unit of the variable input.
• Average Product (AP): output per unit of the variable input. AP_L = TP / L.

How it works: factor proportions

• Factor proportions are the ratio in which inputs are combined, for example workers per hectare. If land is fixed and workers are added, this ratio has to change.
• Why MP rises at first:
• The fixed factor is under-used. There is too much land for too few workers.
• Each new worker makes the mix better, so MP rises.

• Two causes: the fixed factor is indivisible (you cannot cut a tractor in half, so it gives its full benefit only when enough workers use it), and more workers allow specialisation (each worker does one task and does it better).

• Why MP later falls (this law):

• The fixed factor gets crowded. Each extra worker has less land or fewer machines to use.
• Factors are imperfect substitutes, meaning one input cannot fully take the place of another. Extra labour cannot make up for missing land.

• So each extra worker adds less output than the worker before.

• In everyday terms: a farmer with a fixed number of acres cannot get the biggest crop with too few workers. Adding workers raises output up to a limit. Past that limit, output starts to fall because there are too many workers and not enough land [2].

Worked example: NCERT Table 3.2 (land fixed at 4 hectares)

Labour (L)	TP	MP = ΔTP	AP = TP/L
0	0	–	–
1	10	10	10
2	24	14	12
3	40	16 (highest MP)	13.33 (highest AP)
4	50	10	12.5
5	56	6	11.2
6	57	1	9.5

• L = 1 → 3: MP rises (10 → 14 → 16). This is increasing returns to a factor.
• L = 4 onwards: MP falls (16 → 10 → 6 → 1). This is diminishing marginal product.
• TP still rises up to L = 6, but more and more slowly, because MP is still positive.
• NCERT error: the text says TP rises "by 10" and "by 12", but Table 3.2 gives 14 and 16. Go by the table.
• MP–AP–TP rules:
• MP > AP → AP rises. At L = 2 and 3, MP (14, 16) is above AP (12, 13.33).
• MP < AP → AP falls. At L = 4, MP (10) is below AP (12.5).
• MP cuts AP at the highest point of AP.
• MP positive → TP rises. MP = 0 → TP is at its maximum. MP negative → TP falls.
• The MP and AP curves are both inverted-U shaped.

Assumptions, stages and history

• Assumptions:
• Technology is fixed. A better seed or machine would shift the whole TP curve up.
• At least one input is fixed. If all inputs change together, the topic becomes returns to scale.

• Units of the variable input are homogeneous (identical). MP falls because of crowding, not because later workers are less skilled.

• Three stages (standard convention, beyond NCERT):

• Stage I: from the origin to maximum AP. MP > AP, and the fixed factor is under-used.
• Stage II: from maximum AP to MP = 0. AP and MP are both falling and both positive. This is the rational zone of production, the only range a sensible producer chooses.
• Stage III: MP < 0, and TP falls.
• Why Stage II: in Stage I, output per worker is still rising, so the firm should keep adding workers. In Stage III, an extra worker reduces output, so no firm would pay for that worker. The exact point inside Stage II depends on wages and the output price.

• In Table 3.2: Stage I ends near L = 3 (AP 13.33). Stage II runs from L = 4 onwards. MP never reaches 0, so Stage III is not reached.

• Older CBSE convention: the phases are increasing, diminishing and negative returns to a factor, and the first phase ends at maximum MP. In general MP peaks before AP does. In Table 3.2 both peaks fall at L = 3. Say which convention you are using.

• History:
• Turgot first stated diminishing returns on land.
• Ricardo's theory of rent: as more work goes onto the same land, or farming spreads to poorer land, extra output falls, so the better land earns rent.
• Malthus: food output meets diminishing returns while population keeps growing.

In India

• Disguised unemployment means people who look employed but add nothing to output, because their MP is zero. It is the extreme end of this law.
• Example: 5 family members work 2 acres and produce 20 quintals of wheat. One member leaves for a city job. The other 4 still produce 20 quintals.

• The 5th worker's MP = 20 − 20 = 0. This is the Stage II/III boundary, where TP is at its maximum.

• Who measures it: the Periodic Labour Force Survey (PLFS), run by the NSO, measures the workforce. The Economic Survey (Ministry of Finance) analyses the data.

• Latest figures:
• More than 45% of India's workforce is in agriculture. The rest: 11.4% in manufacturing, 28.9% in services and 13.0% in construction. The total workforce is about 56.5 crore (2022-23) [3].
• Agriculture's share of the workforce was 45.8% (2023). The Economic Survey assumes it will slowly fall to one-fourth by 2047 [3].
• Agriculture produced 17.7% of GVA at current prices in FY24. Industry produced 27.6% and services 54.7% [4].

• Agriculture and allied sectors grew at an average of about 5% a year (FY17–FY23) [5].

• What this means:

• About 46% of workers produce under one-fifth of output.

• So output per farm worker is low, and many farm workers have very low MP.

• Policy response: move workers out of farming. India needs about 78.5 lakh non-farm jobs every year until 2030 to absorb its growing workforce and the workers leaving agriculture [3].

Don't confuse with

• Law of variable proportions: covers the whole path of MP, first rising and then falling. The law of diminishing marginal product covers only the falling phase. NCERT uses the two names as synonyms, so read the question carefully.
• Diminishing returns to scale: a long-run idea in which all inputs grow together. Diminishing marginal product is a short-run idea in which at least one input is fixed.
• Negative marginal product (falling TP): under diminishing MP, TP still rises, only more slowly. TP falls only when MP becomes negative (Stage III).
• Law of diminishing marginal utility: a consumer idea (extra satisfaction from one more unit of a good falls). Diminishing marginal product is a producer idea (extra output from one more unit of an input falls).

Prelims Hooks

• It is a short-run law and needs at least one fixed input. It also assumes fixed technology and homogeneous units of the variable input.
• MP = ΔTP/ΔL; AP = TP/L. MP cuts AP at the highest point of AP. TP is at its maximum when MP = 0.
• Trap: "diminishing returns means total output falls". This is wrong. MP falls while TP keeps rising.
• Rational zone = Stage II, from maximum AP to MP = 0. Convention trap: in the standard convention Stage I ends at maximum AP. In the older CBSE convention it ends at maximum MP.
• Disguised unemployment = MP of labour is zero. This is linked to the Lewis (1954) surplus-labour model.
• Turgot first stated the law. Ricardo (rent) and Malthus (population) built on it. Agriculture had 17.7% of GVA in FY24 [4] but more than 45% of the workforce [3].

Mains Points

• Structural transformation (moving workers from farms to industry and services):
• With about 46% of workers on farms but only 17.7% of GVA (FY24) [3][4], many farm workers have near-zero MP.
• Moving them into factory and service jobs raises national output without lowering farm output. This is the Lewis (1954) argument.

• This is why the Economic Survey calls for about 78.5 lakh non-farm jobs every year until 2030 [3].

• Technology offsets diminishing returns:

• The law assumes fixed technology. HYV seeds, irrigation and the Green Revolution shifted the whole TP curve up.
• This is why Malthus's forecast of food shortage failed. Farm R&D and extension services (teaching farmers new methods) are GS-III priorities.

• Land consolidation, rural non-farm jobs, agro-processing, and more capital per worker (irrigation, machines) can raise the MP of labour on small, crowded plots.

• Caveat for answers:

• Zero MP is an extreme case. In peak seasons such as sowing and harvest, farm labour is often scarce.
• So Indian farm labour is better described as seasonal underemployment with low MP than as strictly zero MP all year.

Related concepts

• Law of variable proportions
• Factor proportions

Read more

• Law of variable proportions and diminishing marginal product