Monopsony Explained: Labor Markets, Wages, and the Minimum Wage

A monopsony is a market with a single buyer, and in AP Economics it almost always means a labor market with one dominant employer: a hospital that is the only large employer of nurses in a region, or the single factory in a mill town. Just as a monopoly is the whole selling side of a market, a monopsony is the whole buying side, and that buying power lets the employer pay a wage below the value a worker actually adds. The monopsonist hires where the marginal revenue product of labor equals the marginal resource cost (MRP = MRC), then reads the wage down on the labor supply curve, so it employs fewer workers and pays them less than a competitive labor market would. That one result is why monopsony sits directly opposite monopoly on the market-power map, and why, surprisingly, a well-set minimum wage can raise both the wage and employment at the same time.

You can build the factor-market diagram described below in the factor markets sandbox, pin down the vocabulary in the monopsony glossary entry, and see the full hiring model in the factor markets lesson. This guide walks through what a monopsony is, the wage-setting diagram explained in words, a fully worked numeric example, monopsony versus monopoly, and the minimum-wage twist that trips up even strong students.

What a monopsony is

A competitive employer is a wage taker. It is one of many firms hiring from a large pool of workers, so it can hire as many workers as it wants at the going market wage, and its labor supply curve is a flat horizontal line at that wage. A monopsony is the opposite. Because it is the only meaningful buyer of a particular kind of labor, it faces the entire upward-sloping market supply curve of labor directly. To attract more workers it has to offer a higher wage, exactly the way a monopolist has to cut price to sell more.

Monopsony power shows up wherever workers have few alternative employers: a single large hospital system, a lone auto plant in a small city, a regional grocery chain, or even a national employer of a narrow specialty. It does not require a literal single firm, only enough buying power that the employer sets the wage rather than accepting one. The defining feature to remember is that the monopsonist is a wage maker facing an upward-sloping labor supply curve.

The wage-setting diagram, explained in words

The monopsony graph has wage on the vertical axis and quantity of labor on the horizontal axis, and it holds four curves. Take them one at a time.

• Labor supply (S). This is the upward-sloping curve showing how many workers will accept jobs at each wage. It is also the monopsonist's average cost of labor, because whatever wage it posts is the wage every worker earns.
• Marginal resource cost (MRC). This is the extra cost of hiring one more worker, and here is the catch that mirrors monopoly. To hire the next worker the firm must raise the wage, and because it pays a single wage to everyone, it must raise the wage for every worker already employed, not just the new one. So the cost of the last worker is their own wage plus the raise given to all the others. That makes MRC lie above the supply curve at every quantity past the first. For a straight-line supply curve, MRC has the same vertical intercept but is exactly twice as steep, the same relationship monopoly's marginal revenue has to demand.
• Marginal revenue product (MRP). This downward-sloping curve is the firm's demand for labor, showing the extra revenue each additional worker generates (the worker's marginal product times the price of the output). It is covered in full in the factor markets lesson.

The hiring rule is the same as for any input: keep hiring while a worker adds more revenue than cost, and stop where MRP = MRC. That intersection sets the quantity of labor hired, call it Lm. Now comes the move that defines monopsony. You do not read the wage off the MRP = MRC intersection. You drop straight down from Lm to the supply curve to find the lowest wage that will attract exactly Lm workers, call it Wm. Because supply lies below MRC, that wage sits below the worker's marginal revenue product. The vertical gap between MRP and the wage at Lm is called monopsonistic exploitation: workers are paid strictly less than the value they add.

A worked example, step by step

Numbers make the diagram concrete. Suppose the labor supply curve a monopsonist faces is:

W = 4 + 2L (the wage needed to attract L workers)

Total labor cost is wage times workers, so total cost = (4 + 2L) x L = 4L + 2L squared. Marginal resource cost is the change in that total from one more worker:

MRC = 4 + 4L (same intercept of 4, but slope 4 instead of 2, so twice as steep)

Now suppose the firm's demand for labor, its marginal revenue product, is:

MRP = 28 - 2L

Step 1, find employment. Set MRP = MRC: 28 - 2L = 4 + 4L. Then 24 = 6L, so L = 4 workers. At that point MRP = 28 - 8 = 20 and MRC = 4 + 16 = 20, which confirms the two are equal.

Step 2, find the wage. Read down to the supply curve at L = 4: W = 4 + 2(4) = 12.

So the monopsonist hires 4 workers and pays each 12, even though the fourth worker adds 20 in revenue. The gap of 8 is the exploitation per worker.

Compare to a competitive labor market. A competitive market hires where the wage equals the value of the last worker, that is where supply meets MRP: 4 + 2L = 28 - 2L, giving 24 = 4L, so L = 6 workers at a wage of W = 4 + 2(6) = 16. The verdict is clean: relative to competition, the monopsony employs fewer workers (4 versus 6) and pays a lower wage (12 versus 16). Restricting hiring to hold the wage down is the labor-market echo of a monopoly restricting output to hold price up.

Monopsony vs monopoly

The two are mirror images: a monopoly has market power on the selling side, a monopsony on the buying side. Laying them side by side is the fastest way to keep the mechanics straight, and it is a common exam comparison (see the monopoly vs monopsony breakdown).

Notice the symmetry in the two-step procedure. A monopolist finds quantity at MR = MC and then goes up to demand for a high price. A monopsonist finds quantity at MRP = MRC and then goes down to supply for a low wage. Both distort the market away from the efficient competitive point, and both create deadweight loss because mutually beneficial trades (extra units of output, extra jobs) go unmade.

The minimum wage in a monopsony

Here is the result that feels backwards until you see the diagram. In a competitive labor market, a minimum wage set above equilibrium causes unemployment: it is a price floor, so quantity of labor demanded falls while quantity supplied rises. In a monopsony, a minimum wage set in the right range can raise the wage and increase employment.

The reason is what the minimum wage does to marginal resource cost. Once the government sets a wage floor, the firm can hire additional workers up to that floor without raising anyone's pay, because everyone is already at the legal minimum. Over that stretch the wage floor is flat, so the firm's marginal cost of labor is simply the minimum wage itself, a horizontal line, instead of the steep MRC. The exploitation wedge that made MRC shoot above the wage disappears for those workers.

Use the worked numbers. The monopsony outcome was L = 4, W = 12; the competitive outcome was L = 6, W = 16. Now impose a minimum wage of 16. The firm's marginal cost of labor is a flat 16 until it runs out of workers willing to work at 16, which happens at L = 6 (since 16 = 4 + 2L gives L = 6). The firm hires where MRP meets this new marginal cost: 28 - 2L = 16, so L = 6. Employment rises from 4 to 6 and the wage rises from 12 to 16, exactly the competitive result. A minimum wage set anywhere between the monopsony wage of 12 and the competitive wage of 16 raises both pay and jobs; a minimum wage set right at 16 pushes the market all the way to the efficient competitive outcome.

The usual caution still applies at the top end. Push the minimum wage far above the competitive wage (say to 20) and the floor now bites like an ordinary price floor, and employment falls back. So the monopsony case is not a claim that any minimum wage raises jobs, only that within a specific range a floor offsets the employer's wage-setting power. That nuance, correctly bounded, is exactly what free-response questions reward.

Where to practice

Monopsony rewards the same habit monopoly does: draw the diagram until the two-step sequence is automatic. The firm faces upward-sloping supply, MRC sits above it and twice as steep, employment comes from MRP = MRC, and the wage comes down on supply. Build and drag the model in the factor markets sandbox, and cement the labor-demand foundations, marginal product and the marginal revenue product and marginal resource cost definitions, in the factor markets lesson. Since the whole structure mirrors selling-side market power, reading the monopoly guide right after this one locks in the symmetry that AP graders love to test.