Law of Demand and the Demand Curve

Demand is a central concept of market economics. Understanding demand is the key to understanding pricing, consumer behavior, firm strategies, and market dynamics. The law of demand is one of the few “regularities” in economics that works almost always.

What is demand

Demand is not just the desire to buy, but the willingness and ability to purchase a good at a given price. Desire without money is not demand. Money without desire is not demand either. Demand is the intersection of “want” and “can”.

Quantity demanded (Qd) is the amount of a good that consumers are willing and able to buy at a given price over a given period of time.

Important: demand is the relationship between price and quantity, not a single number. At different prices, there are different quantities. This relationship is what demand is.

Law of Demand

Law of Demand: ceteris paribus (all else equal), the higher the price of a good, the lower the quantity demanded, and vice versa. An inverse relationship between price and quantity.

Why does the law work?

Income Effect: when price rises, real purchasing power falls—you can afford less.
Substitution Effect: as price rises, the good becomes relatively more expensive compared to alternatives—you switch to substitutes.
Diminishing Marginal Utility: each additional unit provides less satisfaction—you are willing to pay less for more.

Exceptions? Theoretically, “Giffen goods” are possible—where a price increase raises demand (extreme poverty + necessity of the good). In practice, they are almost never encountered. Sometimes the “Veblen effect” (prestige goods) is mentioned, but this is more of a shift in the demand curve (change in perceived quality) than a violation of the law.

Demand Curve

The demand curve is a graphical representation of the law of demand. On the horizontal axis—quantity (Q), on the vertical—price (P). The curve slopes downward from left to right, reflecting the inverse relationship. Each point on the curve shows the quantity demanded at a given price.

Movement along the curve: change in Qd when the price of the good itself changes.

Linear demand function: $Qd = a - bP$, where $a$ is the maximum demand at zero price, $b$ is the price sensitivity. The greater $b$, the “flatter” the curve (demand is more sensitive).

Inverse demand function: $P = \frac{a - Qd}{b}$—expresses price as a function of quantity. Useful for revenue analysis and monopoly pricing.

Individual and Market Demand

Individual demand is the demand of a single consumer. Depends on their preferences, income, prices of other goods.

Market demand is the sum of all individual demands in the market. At each price, we add up the quantities all consumers wish to buy.

Horizontal summation: at $P = $10$ consumer A wants 2 units, B wants 3, C wants 5. Market $Qd = 2 + 3 + 5 = 10$ units. Repeat for each price to get the market curve.

The market curve is usually flatter (more elastic) than individual curves—when price drops, new consumers enter the market.

Factors Shifting the Demand Curve

The demand curve is constructed under “all else equal”. If these “other things” change, the curve shifts:

Shift to the right—increase in demand: at any given price, consumers want more.
Shift to the left—decrease in demand: at any given price, they want less.

Factors that shift the curve:

Income: for normal goods, income growth → shift to the right. For inferior goods—shift to the left.
Prices of substitutes: rise in price of a substitute → shift to the right (consumers switch to your good).
Prices of complements: rise in price of a complement → shift to the left (buy less of both).
Tastes and preferences: fashion, advertising, information → shift in any direction.
Expectations: expectation of rising prices → shift to the right today.
Number of buyers: population growth, new markets → shift to the right.

Movement Along vs. Shift of the Curve

Critical distinction:

Change in the price of the good itself—movement along the curve. Demand (the curve) does not change; only the quantity demanded (a point on the curve) does.
Change in other factors—a shift of the entire curve. At every price, the quantity is new.

A typical mistake: confusing “change in demand” (shift of the curve) and “change in quantity demanded” (movement along the curve). A price increase does not “reduce demand”—it reduces quantity demanded at unchanged demand.

Normal and Inferior Goods

Normal goods: demand increases as income rises. Most goods: restaurants, travel, quality clothing.

Inferior goods: demand falls as income rises. Consumers “grow out” of them: instant noodles, public transportation (under certain conditions), cheap brands.

Necessities vs. luxuries: income rises by 10%, demand for bread rises by 2% (necessity), demand for yachts by 20% (luxury). This is income elasticity (discussed later).

Substitutes and Complements

Substitutes: goods that fulfill similar needs. Coke and Pepsi, butter and margarine, taxis and subways. A price increase in one raises demand for the other.

Complements: goods consumed together. Cars and gasoline, printers and cartridges, hot dogs and mustard. A price increase in one reduces demand for the other.

For the investor: analyzing substitutes and complements helps understand competitive dynamics and pricing power. A company with a unique product without close substitutes has pricing power.

Practical Applications

Forecasting: how will changes in factors affect sales? Rising gasoline prices → falling demand for large cars, rising demand for hybrids.
Pricing: understanding the demand curve allows you to optimize price to maximize revenue or profit.
Marketing: advertising and branding aim to shift the demand curve right and reshape it (decreasing elasticity).
Policy: subsidies shift demand to the right (electric vehicle grants), taxes affect demand by changing price.

The law of demand is a starting point for the analysis of any market.