Economics begins with a simple fact: we cannot have everything we want. Time, money, land, materials and human effort are limited, while human wants are nearly endless. This gap between limited resources and nearly endless wants is called scarcity, and economics studies how people and societies make choices under it. The British economist Lionel Robbins gave a well known definition in 1932; in paraphrase, economics studies how people use scarce means, which could be put to different uses, to pursue their goals.

Scarcity does not mean poverty. A billionaire still has only 24 hours a day, and a rich country still has to decide whether to spend on hospitals or roads. Scarcity means choices are unavoidable.

Opportunity cost

Every choice has a cost, and the true cost is not always the price tag. The opportunity cost of a choice is the value of the next best alternative you give up. If you spend Saturday at a concert instead of working a paid shift, the opportunity cost includes the wages you did not earn, plus the ticket.

Thinking in opportunity costs avoids a common mistake: treating something as “free” because no money changes hands. A free event still costs your time. A government program funded by taxes is not free; its cost is what taxpayers and other programs give up.

Trade-offs

Because resources are scarce, getting more of one thing usually means less of another. Economists call this a trade-off. Individuals trade off leisure against income; firms trade off quality against cost; societies trade off consumption today against investment for the future. Seeing a trade-off does not tell you which choice is right, but it makes the choice visible.

Incentives

An incentive is anything that rewards or penalizes a choice and so makes it more or less attractive. Prices, wages, taxes, fines, praise and shame are all incentives. A central idea in economics is that people respond to incentives: when the cost of something rises, people tend to do less of it; when the reward rises, they tend to do more. A tax on cigarettes, for example, reduces smoking.

This does not assume people are selfish or care only about money. Incentives include social approval, convenience and moral satisfaction. The claim is simply that behavior shifts, on average, when costs and benefits change.

Thinking at the margin

Most real decisions are not all or nothing. They are about a little more or a little less: one more hour of study, one more worker, one more slice of pizza. Economists call this marginal thinking. The marginal benefit is the extra benefit from one more unit; the marginal cost is the extra cost. A sensible decision rule is: keep doing something as long as the marginal benefit is greater than the marginal cost.

Marginal thinking explains puzzles. Water is essential to life and diamonds are not, yet diamonds cost far more. Adam Smith noticed this “paradox of value.” The answer is that water is usually plentiful, so one more glass adds little, while diamonds are rare, so one more is highly valued by buyers. Price reflects value at the margin, not total usefulness.

Sunk costs

Sunk costs are costs already paid that cannot be recovered. Good decisions look forward, so sunk costs should not affect them. If you bought a nonrefundable film ticket and now feel ill, the ticket is gone whether you go or not; the only question is whether going now is worth it. People often fall into the “sunk cost fallacy,” continuing a failing project because so much has already been spent.

Worked example: one more hour

Maya has an exam on Monday and a chance to work extra shifts at a café for $15 an hour. She has already studied for three hours. Should she study a fourth hour? A fifth? A sixth?

Opportunity cost. Each hour of study costs her $15 in wages she could have earned, plus the leisure she gives up. That is the marginal cost of an hour of study, and it stays roughly the same each hour.

Marginal benefit. Her first hours of study helped a lot: she learned the main ideas. Later hours help less, because she is reviewing what she already knows and getting tired. Hour four still feels worth well over $15; hour five is roughly a toss-up; hour six adds very little. This pattern, where each extra unit adds less than the one before, is called diminishing marginal returns, and it is very common.

The marginal rule says: study through hour four, think hard about hour five, and stop before hour six. She does not ask whether studying is good; she asks whether one more hour is worth more than what that hour could otherwise buy.

A sunk cost. Maya paid $80 for a prep book that turned out to be poor. She feels she “should” use it to get her money's worth. But the $80 is gone either way; she should study from whatever materials help most from now on.

Incentives. If the café offered $40 an hour for a holiday shift, the opportunity cost of studying would rise, and she might reasonably study less.