Economics · Ch 10 — The Theory of the Firm under Perfect Competition
Determinants of a Firm's Supply Curve
Determinants of a Firm's Supply Curve
The Firm's Supply Curve and Its Determinants
The supply curve of a firm under perfect competition is not a separate, independent curve. As we established in the previous section, it is simply that portion of the firm's marginal cost (MC) curve which lies above the average variable cost (AVC) curve. This means the firm's decision of how much to produce at any given market price is entirely governed by its marginal cost.
Consequently, any factor that shifts or changes the firm's marginal cost curve will directly shift its supply curve. The textbook identifies two such factors that are fundamental to understanding how a firm's supply responds to changes in its production environment.
1. Changes in the Price of Inputs
A firm's marginal cost is the cost of producing one additional unit of output. This cost is driven by the prices of the inputs it uses — labour, raw materials, energy, machinery, and so on.
- An increase in input prices (e.g., a rise in wages or the cost of steel) makes producing each additional unit more expensive. The entire MC curve shifts upwards (or to the left). Since the supply curve is a part of the MC curve, the supply curve also shifts upwards and to the left. At any given market price, the firm will now supply a smaller quantity than before.
- A decrease in input prices (e.g., cheaper electricity or a fall in the price of raw cotton) reduces the cost of producing each additional unit. The MC curve shifts downwards (or to the right). Consequently, the supply curve shifts downwards and to the right. At any given market price, the firm will now supply a larger quantity.
The relationship is direct: higher input costs → higher MC → lower supply at each price. Lower input costs → lower MC → higher supply at each price. This is a core reason why supply curves are not static.
2. Changes in the State of Technology
Technology determines how efficiently a firm can combine inputs to produce output. An improvement in technology allows the firm to produce the same output with fewer inputs, or more output with the same inputs.
- Technological progress (e.g., a more efficient machine, a better production process, or computerisation) lowers the firm's costs. The MC curve shifts downwards (or to the right). As a result, the supply curve also shifts downwards and to the right. The firm is now willing to supply more at every price.
- Technological regression (a deterioration in technology, which is rare but possible, e.g., due to a breakdown of key machinery or loss of skilled workers) would raise costs. The MC curve would shift upwards, and the supply curve would shift upwards and to the left.
In the real world, technological progress is the dominant force. This is why, over long periods, we often see industries able to supply vastly greater quantities of goods at prices that are stable or even falling, despite rising input costs.
Summary of the Two Determinants
The textbook's core message is that the firm's supply curve is not an independent law of nature. It is a derived relationship, entirely dependent on the firm's cost structure. The two key factors that alter this cost structure are:
- Input Prices: A change in the price of any factor of production shifts the MC curve and, therefore, the supply curve.
- Technology: An improvement in technology lowers the MC curve and shifts the supply curve outward; a deterioration does the opposite. …