Consumer choice and demand - Microeconomics AP Study Notes

Consumer choice examines how individuals make decisions about purchasing goods and services given their limited income and unlimited wants. The fundamental principle is that consumers aim to maximize utility (satisfaction) subject to their budget constraint.

Key Terms:

Utility is the satisfaction or benefit derived from consuming a good or service. Total utility (TU) measures overall satisfaction from all units consumed, while marginal utility (MU) is the additional satisfaction from consuming one more unit.

The Law of Diminishing Marginal Utility states that as consumption of a good increases, the marginal utility derived from each additional unit decreases, holding all else constant. This foundational principle explains why demand curves slope downward.

Budget Constraint represents all combinations of goods a consumer can afford: Income = (Price of Good A × Quantity of A) + (Price of Good B × Quantity of B)

Consumer Equilibrium occurs when a consumer allocates income to maximize total utility. The condition is: MUₐ/Pₐ = MUᵦ/Pᵦ — meaning the marginal utility per dollar spent must be equal across all goods.

The Income Effect describes how quantity demanded changes when purchasing power changes. The Substitution Effect explains how consumers switch between goods when relative prices change. Together, these effects explain movement along the demand curve when price changes.

Rational Consumer Assumption: consumers are assumed to make logical decisions, have complete information, and seek to maximize utility within their budget constraints.

Think of your monthly entertainment budget as a budget constraint in action. With £100, you must choose between cinema tickets (£10 each) and streaming subscriptions (£8 each). Your budget line shows all possible combinations you can afford.

Diminishing Marginal Utility in Practice: Your first slice of pizza might bring immense satisfaction (high MU), but by the fifth slice, you're barely enjoying it (low MU). This explains why all-you-can-eat buffets work—restaurants know you'll stop eating when marginal utility falls to zero, even though you could eat more.

Consumer Equilibrium Example: Imagine you're choosing between coffee (£3) and pastries (£2). If coffee gives you 30 utils and pastries give you 24 utils, you calculate: MU/P for coffee = 30/3 = 10 utils per pound; for pastries = 24/2 = 12 utils per pound. You're not in equilibrium—you should buy more pastries and less coffee until the ratios equal.

Real-World Income and Substitution Effects: When petrol prices rise, the substitution effect causes consumers to drive less and use public transport more (substituting away from the now-expensive good). The income effect means your real purchasing power has fallen—you're effectively poorer, so you might reduce consumption of many goods. For normal goods, both effects work together to reduce quantity demanded when price rises.

Behavioral economics challenges the rational consumer assumption: people use mental accounting, face decision fatigue, and exhibit loss aversion—real consumers aren't always perfectly rational maximizers.