Indifference Curves in Plain Words

How economists represent people's preferences using indifference curves, and how combining them with the budget constraint explains choices.

The budget constraint shows what a person can buy. To explain what they will buy, economists describe preferences using indifference curves.

What an indifference curve is

An indifference curve connects all the combinations of two goods that give a person the same level of satisfaction. The person is “indifferent” between these combinations.

For example, a person might be equally happy with:

4 mangoes and 1 banana.
2 mangoes and 3 bananas.
1 mango and 6 bananas.

These combinations lie on the same indifference curve.

Key features

Higher curves are better: combinations with more of both goods lie on higher indifference curves, representing greater satisfaction.
Curves slope downward: to keep satisfaction the same, getting more of one good means accepting less of the other.
Curves are usually bowed inward: people typically value variety. When someone has lots of mangoes and few bananas, they will give up many mangoes for one more banana. As bananas become more plentiful, they will give up fewer mangoes for each additional banana.

The marginal rate of substitution

The rate at which a person is willing to trade one good for another while staying equally satisfied is called the marginal rate of substitution. It reflects diminishing marginal utility: the more you have of something, the less you value an extra unit.

Putting it together

A person makes the best choice by reaching the highest indifference curve they can afford. This happens where an indifference curve just touches the budget line. At that point, the rate at which the person is willing to trade the goods equals the rate at which the market lets them trade, the relative price.

Choosing a lunch combination

A student has money for lunch and can buy samosas or juice. She considers all affordable combinations and picks the one she likes most. If samosas get cheaper, she may buy more samosas and less juice, moving to a new best point. Indifference curves and budget lines are a way of drawing this everyday reasoning.

Thinking economists claim people draw these curves in their heads

Indifference curves are a model of how preferences work, not a claim that people consciously draw graphs. They help predict how choices respond to prices and income.

Key takeaways

Indifference curves show combinations that give equal satisfaction.
Higher curves represent greater satisfaction, and curves usually bow inward.
The marginal rate of substitution is the willingness to trade one good for another.
The best choice is where the highest affordable indifference curve touches the budget line.