Introduction to Market Structures

1Why this matters

The same table can sell for very different prices depending on the market the seller is in. A bean trader at Nyabugogo who charges more than the stall beside her sells nothing; the single company licensed to pipe water to a town can raise its price and customers have nowhere else to go. The difference is not the product — it is the market structure: how many firms there are, whether their products are alike, and whether new firms can enter. This lesson compares the two extremes, perfect competition and monopoly, and shows how structure decides both the price and whether the outcome is good for society.

2Perfect competition

Picture the beans section of Nyabugogo market: dozens of traders, each selling beans that look the same, and buyers who wander from stall to stall. This is close to the economist’s model of perfect competition, which rests on four assumptions:

1. Many buyers and many sellers, each so small that one alone cannot move the market price.
2. A homogeneous product — every firm’s good is identical, so buyers have no reason to prefer one seller.
3. Free entry and exit — no barriers stop a new firm starting up or an old one leaving.
4. Perfect information — everyone knows the prices being charged and the product on offer.

These assumptions have a sharp consequence. Because the products are identical and buyers know every price, a trader who charges even a little above the market price sells nothing — buyers simply step to the next stall. And because each trader is tiny, she can sell all she wants at the going price without ever having to lower it. So each firm is a price taker: it accepts the market price and only chooses how much to supply. Its demand curve is horizontal at that price, and, as we saw in the last lesson, AR = MR = price.

What sets the price, then? The market does — the interaction of total supply and total demand for beans. The individual trader reads that price off the market and decides her quantity by producing where MC = MR.

3Monopoly

Now the opposite extreme. A monopoly is a market with a single seller of a product that has no close substitutes, protected by high barriers to entry that keep rivals out. Those barriers can be legal (a patent, or a sole licence), technical (one firm owns the only pipeline or the only usable ore deposit), or the sheer scale needed to compete. A single firm supplying piped water to a town is a natural example: laying a second network of pipes would be wasteful, so one provider serves everyone.

A monopolist is not a price taker but a price maker. Since it is the whole market, it faces the market’s downward-sloping demand curve directly: to sell more, it must lower the price — and not just on the extra unit, but on all the units it sells. That single fact changes everything. Watch what happens to revenue for a firm that is the only seller of bottled cooking gas in a town (prices in Frw per cylinder):

To sell the 3rd cylinder the firm drops the price from 900 to 800 Frw. It gains 800 Frw from the new cylinder but loses 100 Frw on each of the 2 it could have sold at 900 — so marginal revenue is only 800 − 200 = 600 Frw, well below the 800 Frw price. This is the key result: for a monopolist, MR is less than price (AR) at every unit beyond the first, and MR falls faster than the price does. Because the monopolist still maximises profit where MC = MR, and MR sits below the demand curve, it ends up producing less and charging more than a competitive industry would.

4Comparing the two — price and efficiency

Between these two extremes sit other structures — monopolistic competition (many firms selling slightly different products) and oligopoly (a few large firms that dominate) — but the two ends of the range show the mechanics most clearly. Put them side by side:

The last row needs its reasoning. In perfect competition, if firms are earning supernormal profit, the free-entry assumption lets new firms pour in; supply rises, the price falls, and the profit is competed away until only normal profit remains. The long-run price is driven down to the lowest average total cost — exactly the break-even price from the last lesson. In monopoly, the barriers block that entry, so a monopolist can hold on to supernormal profit indefinitely.

This is why economists judge perfect competition the more efficient structure, using two tests:

• Allocative efficiency means price equals marginal cost (P = MC): the price a buyer pays for the last unit just equals what it costs society to produce it, so exactly the right amount is made. A perfectly competitive firm produces where price = MC, so it is allocatively efficient. A monopolist sets price above MC, so it produces too little — society would value more units than the monopolist chooses to make.
• Productive efficiency means producing at the lowest average total cost. Free entry pushes the competitive firm to the bottom of its ATC curve in the long run. A monopolist, shielded from competition, faces no such pressure and need not produce there.

One honest qualification, the kind that earns marks in an evaluation. Monopoly is not automatically worse in every respect. A single large firm may reach economies of scale that a host of tiny firms could not, and its protected profit can fund research and innovation. This is why some monopolies — a water or an electricity network — are allowed to exist but are regulated: the state permits the single supplier and then caps its price to hold back the abuse. ‘Monopoly is always bad’ is too blunt; ‘monopoly restricts output and raises price, but scale and innovation can offset this, so we regulate it’ is the Advanced-Level answer.

Practice

Solution. She sells nothing: buyers move to a stall charging 500 Frw for beans that are exactly the same. The assumptions of a homogeneous product and perfect information explain it — because the beans are identical and buyers know every price, no one will pay her the extra 50 Frw. This is precisely why each trader is a price taker.

Solution. TR at 4 units = 700 × 4 = 2,800 Frw. TR at 5 units = 600 × 5 = 3,000 Frw. MR of the 5th unit = 3,000 − 2,800 = 200 Frw, far below its 600 Frw price. The price had to fall on all 5 units, not just the new one, which is why MR (200) is so much less than the price (600).

Solution. Many buyers and sellers, each too small to affect the price; a homogeneous (identical) product; free entry and exit; and perfect information.

Solution. In perfect competition there is free entry: supernormal profit attracts new firms, supply rises, the price falls, and profit is competed down to normal. A monopoly is protected by high barriers to entry, so no new firms can come in to compete the profit away — it persists.

Solution. Allocative efficiency is producing where price = marginal cost, so the value buyers place on the last unit just equals the cost of making it, and the right quantity is produced. A monopolist sets price above MC, which means the value of another unit to buyers exceeds its cost, yet the monopolist chooses not to make it. Output is held below the efficient level, so too few resources are allocated to the good.