独占下の需要曲線

A monopoly demand curve slopes downward because the monopolist is the sole provider with no close substitutes available. Consumers are willing to purchase more units only if the price falls. This reflects diminishing marginal utility—each additional unit provides less satisfaction, reducing consumers' willingness to pay for extra quantities.

The steepness of a monopoly demand curve reflects the degree of elasticity. A steeper curve indicates relatively inelastic demand, meaning quantity demanded is less responsive to price changes. This occurs because consumers have limited choices and cannot easily switch to alternative suppliers, so they continue purchasing despite higher prices.

For a monopolist, average revenue equals the price at each quantity level, which is directly determined by the demand curve. Like perfectly competitive firms, AR = Price = Demand at any given output. However, unlike competitive firms, a monopolist's marginal revenue falls as quantity increases, creating a divergence between average and marginal revenue.

A monopolist's marginal revenue declines as quantity increases, while a perfectly competitive firm's marginal revenue remains constant. This occurs because the monopolist must lower price to sell additional units, reducing the addition to total revenue with each sale. The elasticity of demand varies along the monopoly demand curve, unlike in perfect competition.

A monopolist maximizes revenue by operating in the elastic portion of the demand curve, where a small price decrease generates a proportionally larger increase in quantity demanded. Since total revenue equals price times quantity, reducing price in the elastic range increases total revenue because the quantity increase outweighs the price reduction.

Barriers to entry—such as patents or government regulations—prevent competitors from entering the market, making the entire market demand curve the monopolist's demand curve. These barriers create the monopolist's unique position as sole provider, enabling full control over supply and price while facing the downward-sloping market demand.

De Beers, a major diamond supplier, demonstrates how a monopolist faces a steep demand curve. When De Beers raises prices, quantity demanded decreases minimally because consumers cannot easily switch suppliers. This limited responsiveness reflects the steepness of the curve and the monopolist's pricing power due to lack of close substitutes.