# Joint and composite demand and supply

Book 5, Chapter 6

§1. A consumer demanding a good from a producer thereby creates derived demand for the inputs to produce that good. In most cases, inputs are not interchangeable (100% substitutes), which means they are joint (or complementary) with other inputs, with all supplied in stable proportions. Thus, the absence of one input can halt overall production, e.g., trying to make beer without hops.

§2. A missing factor (production input) will lead to increased prices for that factor if (1) the factor cannot be replaced (no substitutes), (2) consumers cannot do without that good (thus, they will pay more rather than go without), (3) the missing input plays only a small part in the price (meaning producers able to find some of that input can pay more for it without raising the product’s final price by much), and (4) other inputs can be acquired for lower prices, leaving extra space to pay for the scarcer input. In sum, a scarcer input will still be used if it’s an essential but small part of the production process.

§3. Composite demand for an input results from the summation of demands from all producers using that input for their consumer products.

§4. A joint product produces different goods for different markets (e.g., oil can be cracked into gasoline and lubricants). If the price of one product is significantly higher than that of the other, then producers will focus on it over the other. If demand for the more valuable product collapses, then the less-valuable one will be more scarce, driving up its price.

§5. Marshall uses many figures and mathematical logic to explain joint and separate contributions to supply and demand, but I find these uninteresting. It’s important to understand complements and substitutes on the supply or demand side, but I think it’s quite difficult to work out their exact (financial or mathematical) relations, even with the best data.

§6. Those who demand or supply one input might benefit from lower (or higher) demand or supply for substitutes or complements, and they will lobby for laws and regulations to favor themselves, thereby distorting broader markets.

This post is part of a series in the Marshall 2020 Project, i.e., an excuse for me to read Alfred Marshall’s Principles of Economics (1890 first edition/1920 eighth edition), which dominated economic thinking until Van Neumann and Morgenstern’s Theory of Games and Economic Behaviour (1944) and Samuelson’s Foundations of Economic Analysis (1946) pivoted economics away from institutional induction and towards mathematical deduction.

## Author: David Zetland

I'm a political-economist from California who now lives in Amsterdam.