From Individual Choice to Demand

An individual's demand curve can be built by repeating the choice problem at different prices. Hold income, preferences, and other prices fixed; change the good's own price; record the utility-maximizing quantity each time.

These price-quantity pairs trace an individual demand curve. The curve usually slopes downward because a lower price creates a substitution effect toward the good and, for a normal good, a reinforcing income effect.

Reservation prices and marginal benefit

For discrete units, the demand curve can also be read as a schedule of reservation prices. The height at quantity $q$ is the willingness to pay for the marginal unit. The consumer buys another unit while its marginal benefit is at least the market price.

This connects consumer choice to welfare analysis: the area below demand and above price is consumer surplus, while movement along the curve reflects a changed own price.

Horizontal aggregation

Market demand combines many individual demands. At each shared price, add the quantities demanded by all consumers:

This is called horizontal summation because quantities are added along the horizontal axis while price is held fixed. Do not add people's prices vertically. Everyone in the simple competitive market faces the same price, even though they choose different quantities.

Why market demand can have kinks

At a high price, only consumers with high willingness to pay participate. As price falls, existing consumers buy more and new consumers enter. Each entry can change the slope of market demand, creating kinks even when each individual demand segment is straight.

Market demand shifts when something other than the good's own price changes across consumers. Population growth usually shifts it right. An income increase shifts demand for normal goods right but may shift demand for inferior goods left. Advertising, seasons, expectations, and prices of substitutes or complements may also change individual choices and therefore the aggregate curve.

Aggregation does not erase differences

Two neighborhoods can have the same total demand but very different distributions of need, income, and access. The aggregate curve predicts total market purchases; it does not reveal who consumes or whose unmet need matters most. Policy analysis often needs both aggregate behavior and subgroup evidence.

Consumer theory now completes a chain: preferences rank bundles, budgets restrict them, marginal reasoning selects a basket, price variation traces individual demand, and horizontal addition builds the market demand used in supply-and-demand analysis.

Assemble a market at one price

At price 2, suppose four households demand 4, 3, 2, and 1 rides. Market demand is 10. We do not add their reservation-price intercepts; every household faces the same market price, so quantities are the dimension that adds.

Raise price to 4. Some households reduce rides and one may reach zero. The new market quantity is found by repeating the horizontal sum. When a household enters or exits, the aggregate slope changes, which is why market demand can have kinks even when individual schedules are simple.

Derive demand from optimized choices

An individual demand curve is not an unexplained line. Each point comes from solving the same budget-and-preference problem at a different own price while holding other factors fixed. The substitution and income effects explain why the chosen quantity usually rises when price falls.

This also explains a movement along demand. If population rises, a new individual schedule is added at every price, shifting market demand. If only the ride price changes, the set of consumers is unchanged and the market moves along its existing aggregate curve.

Aggregation hides important differences

A market total of 10 rides could come from one household demanding 10 or ten households demanding one each. These patterns have different implications for access, congestion, and targeted policy. Aggregate demand is essential for price prediction, but subgroup data remain important for distributional questions.

The completed chain is now visible: preferences rank baskets, the budget limits them, marginal reasoning selects a basket, price changes trace individual demand, and horizontal summation builds market demand.