The firm’s demand reveals price-setting power

Market demand and firm demand are not the same.

A perfectly competitive firm’s demand is horizontal at market price because the firm is too small to affect price and sells an identical product. A monopolist faces market demand. Monopolistically competitive firms face downward-sloping demand because differentiation gives each a group of loyal customers. Oligopoly demand depends on expected rival responses and is not summarized by one universal curve.

The slope of firm demand reveals what happens if that firm changes output. A competitive firm can sell another unit at market price but cannot charge more because identical substitutes are available. A seller with downward-sloping demand must lower price to sell more, revealing price-setting power.

Perfect competition does not make the industry’s buyers perfectly responsive. Market demand still slopes downward. Only a single seller, too small to move the market price, faces a horizontal demand line. At the industry level, a fixed price would not produce unlimited purchases.

One event, two graphs

Market demand increases in a competitive industry, raising equilibrium price. Each existing firm’s horizontal demand line moves upward to that new price, and the firm expands along MC. The market demand curve shifted right. The firm’s demand remained horizontal rather than becoming market demand.

Differentiation gives some loyal demand but close substitutes keep it relatively elastic. Greater differentiation or fewer substitutes generally permits a larger markup. The four structures are benchmark categories along a broader continuum of market power.

A monopoly faces market demand only after the relevant market is defined. A sole brand can face close substitutes in a broader product market. Exam questions normally state no close substitutes and a barrier when monopoly is intended. Firm count alone is insufficient.

Once demand is identified, choose the correct revenue rule. A horizontal demand means every unit adds price to revenue. A downward-sloping demand means MR lies below price for a single-price seller. This selection affects output throughout the remaining chapters.

Average revenue equals price for any single-price firm because TR/Q=P. The structural difference is marginal revenue: it equals price for a price taker but lies below price when selling more requires a price reduction. This is more precise than saying one firm “sets price” and another does not.

Market power is a degree, not unlimited freedom. A differentiated seller can choose a modest markup but loses customers to substitutes. A protected monopolist may have more discretion but still faces market demand. In every case, buyers’ response constrains the price-output combination.

Elasticity connects the demand curve to markup. A seller facing many close substitutes loses a large share of buyers after a price increase and has limited power. A seller facing less elastic demand can sustain a larger price-cost gap. This refines the simple binary of price taker and price setter.

Always connect the graph’s shape to the alternatives buyers actually possess.