Consumer Choice and Demand

Every day you choose what to buy, how much to work, and how much to save. These decisions follow a simple framework. It helps us predict how people respond to price changes, taxes, or new policies. In this chapter, we’ll build that framework step by step. You will see how individual choices create demand.

The Big Picture#

This chapter asks: how do people decide what to buy when money is tight but wants are many? We’ll explore the logic of rational choice—using ideas like utility, budgets, and trade-offs. We’ll show why demand curves slope down. By the end, you’ll understand consumer theory and see why it helps design better tax and welfare policies. Public economics starts with each person, so we begin there.

What Drives Our Choices — Utility and Preferences#

We choose things because they make us happy. Economists call that happiness utility. It’s not a real substance you can measure—it’s just a convenient way to show how much you like different combinations (bundles) of goods. If you prefer apples and bananas one way over another, we say that first bundle gives you higher utility.

Your preferences follow a few simple rules:

• Completeness: given any two bundles, you can say which you like better, or that you like them equally (you’re indifferent).
• Transitivity: if you like A more than B, and B more than C, then you like A more than C. Your choices stay consistent.
• More is better: if you get more of something you like, and nothing else changes, you won’t be worse off.

With these rules, we can draw indifference curves. An indifference curve shows all bundles that give you the same utility. You’re just as happy at any point on the curve. Picture apples on the bottom axis and bananas on the side. The curve slopes down: if you give up apples, you need more bananas to stay equally happy. The curve bows inward because of diminishing marginal utility: the more you have of something, the less you value an extra unit.

Term: Marginal utility (MU) is the extra utility from consuming one more unit of a good. For good , . Typically falls as rises.

The slope at any point on the indifference curve is the marginal rate of substitution (MRS). It tells you how much of the vertical-axis good you’d give up to get one more unit of the horizontal good, and still be just as happy. Mathematically:

The minus sign makes it a positive number (since you give up some to get more ). So the MRS is the ratio of marginal utilities. If is big compared to , you’d trade a lot of for a bit more , so the curve is steep. As you move down the curve, gets smaller, gets larger, and the MRS shrinks—the curve flattens.

MRS is the key to thinking about trade-offs. Soon we’ll connect it directly to prices.

📝 Section Recap: Utility describes our preferences; indifference curves show bundles that give the same happiness. The slope of these curves, the MRS, captures willingness to trade one good for another.

The Budget Constraint — What We Can Afford#

Your wants may be unlimited, but your money is not. Your budget constraint shows all combinations of goods you can buy given your income and the prices you face. Suppose you have income (per week, say) and you buy two goods: and , with prices and . The constraint is:

Rearranged, we get the budget line:

The vertical intercept is (if you spent all your money on ). The slope is —the rate at which the market lets you swap for . For example, if 2p_y = $1y1, which buys half a unit of . So the slope is : you must give up 2 units of to get 1 more .

Points inside the line are affordable (you’d have money left). Points outside are too expensive. Government policies often change your income (with taxes or benefits) or relative prices (with subsidies or taxes on specific goods). These changes shift or rotate your budget line. That’s why this simple line matters so much for policy.

📝 Section Recap: The budget constraint is the boundary of what you can afford. Its slope is the price ratio—the market’s trade-off between the two goods.

Finding the Best Bundle — Utility Maximization#

Now combine your wants (indifference curves) with your limits (budget line). You want the highest utility you can afford. The best bundle is where the highest indifference curve just touches the budget line—that is, where the two are tangent. At that point, the slope of the indifference curve (MRS) equals the slope of the budget line (the price ratio):

At the best bundle, your personal trade-off (how much you’d give up for ) matches the market trade-off. If , you value more than the market does—you’d benefit from buying more and less . As you adjust, the MRS falls until equality holds. If , you’d do the reverse.

We can also think “at the margin.” The last dollar spent on and the last dollar spent on should give the same utility boost:

This condition, called the equimarginal principle, is a handy rule for optimal choice. It works for many goods, not just two.

When your income goes up (prices unchanged), the budget line shifts outward. You can buy more, so you move to a higher indifference curve. That’s an income effect—the change in what you buy because your purchasing power changed. If a good’s price drops, the budget line rotates outward. You become effectively richer (income effect), and that good becomes cheaper compared to others (substitution effect). We’ll look at these effects in detail next.

📝 Section Recap: The best bundle is where the indifference curve and budget line have the same slope: MRS = price ratio. That’s the happiness-maximizing point.

From Choice to Demand — How Price Changes Shift Decisions#

A demand curve shows how much of a good you want to buy at different prices, other things unchanged. We can build it from our utility-maximizing model. Change the price of , keep income and price of fixed. Find the new best bundle. Record the quantity of . Plot price against quantity. Connect the points—that’s the demand curve for .

Why does the demand curve usually slope down? That’s the law of demand, and it’s driven by two forces.

1. Substitution effect: When price of drops, is cheaper compared to . You switch from to , buying more , if we keep your happiness the same (as if we gave you just enough money to stay on the same indifference curve). This substitution effect always makes you buy more when price falls: quantity moves opposite to price.

2. Income effect: A lower price makes you effectively richer—your real income rises. For a normal good, higher real income boosts demand, so the income effect reinforces the substitution effect: price down → quantity up even more. For an inferior good, higher income reduces demand, so the income effect works against the substitution effect. Usually the substitution effect dominates, so the demand curve still slopes down. If the income effect for an inferior good is so strong it outweighs the substitution effect, we get a Giffen good—a theoretical oddity where demand rises when price rises. (Imagine very poor people who eat mostly bread. If the price of bread spikes, they can’t afford meat and actually buy even more bread. Such cases are extremely rare.)

Picture the budget line rotating when price changes. The substitution effect is a slide along the original indifference curve until the slope matches the new price ratio. The income effect is the jump to a new indifference curve after the budget line shifts.

Our demand curve is just price vs. quantity. It’s the building block for market demand—the total demand from all individuals. We’ll use market demand to study taxes, subsidies, and welfare.

📝 Section Recap: Demand curves come from solving the best bundle again and again at different prices. A price change has a substitution effect (always pushes quantity opposite price) and an income effect (depends on whether the good is normal or inferior). These forces shape the demand slope.

When Budgets Have Kinks — Welfare Programs and Incentives#

In the real world, budget lines aren’t always straight. Programs to help low-income families often create kinks—sharp bends—that change behavior in unexpected ways. Understanding these kinks is key to designing good policy.

Consider a simple income guarantee with a benefit reduction rate (sometimes called a phase-out). A welfare program might promise: if your earned income is zero, you get a grant . For every dollar you earn, the grant is reduced by some amount (say, G/\tau$, the grant disappears entirely.

Imagine a graph with leisure on the bottom axis and consumption (income) on the side. Without any program, your budget line is a straight line. It goes from the point where you take all leisure (work 0 hours, consume 0) to the point where you work all available hours at wage .

Now add the grant. If you don’t work, you get . If you work hours at wage , your earnings are . But the grant is reduced by per dollar earned. So your total income is . Your effective net wage is until the grant runs out. After that, your net wage returns to . The budget line has a kink where the grant phases out completely.

A kinked budget can discourage work. For someone not working, the grant raises income without working—an income effect that reduces the need to work. For someone already working, the lower effective wage (due to the phase-out) makes leisure cheaper (substitution effect), so they might work less. Together, these effects can trap people in poverty or shrink the labor supply.

Other programs like food stamps or housing vouchers also introduce kinks. For instance, food stamps might be a fixed amount if income is below some threshold, and then drop to zero abruptly. That creates a cliff—a vertical drop in the budget line—where a small increase in earnings can cause a huge loss in benefits, producing a strong disincentive to earn just above the threshold.

Policymakers face tough trade-offs: a more generous grant helps the needy but dulls work incentives; a lower phase-out rate (lower ) reduces the disincentive but makes the program costlier and may reach higher incomes. The kinked budget model is the main tool for evaluating welfare reforms.

📝 Section Recap: Real budget constraints often have kinks due to welfare programs with income limits or phase-outs. These kinks can create strong work disincentives by changing effective wages and imposing high implicit taxes on earnings.

Summary#

We traced the path from personal tastes to market demand, then to real-world budgets with policy twists. You now have a toolkit: indifference curves show what we want, budget lines show what we can afford, and the tangency point shows the best choice. Price changes trigger substitution and income effects that shape demand. And when governments add transfers, the budget line gets kinks that can have unintended side effects. This microeconomic foundation is the first step to thinking clearly about taxes, subsidies, and safety nets.