How to Learn Economics

If you take one idea from economics, take this one. The cost of anything is not what you paid; it is the best alternative you gave up. A free afternoon spent one way costs every other way you could have spent it, and the money price is often the smallest part.

Applied honestly it changes decisions. A degree costs the fees plus several years of earnings. A cheap flight with two connections costs the fare plus a day. Once you are asking what else this could have been, you are doing economics whether or not any money is involved.

Almost no real economic decision is all-or-nothing. The question is rarely whether to produce, consume or work at all; it is whether to do slightly more or slightly less. That is what marginal means, and it is the analytical move the whole discipline runs on.

It also resolves puzzles that look paradoxical. Water is essential and cheap while diamonds are useless and expensive, which seems to contradict the idea that value relates to usefulness. It does not, once you ask about the value of one more unit rather than of the whole category.

This model is the most useful thing in the subject and the most abused. It describes competitive markets with many buyers and sellers and reasonable information, and it gets deployed confidently in situations meeting none of those conditions.

The technical thing to get right is the difference between moving along a curve and shifting it. A price change moves you along; almost anything else - income, tastes, input costs, the price of something related - shifts the whole curve. Confusing these produces answers that are exactly backwards, and it is the single most common error in the subject.

The judgement thing to get right is knowing when the model does not apply. Healthcare, education and housing all break its assumptions in different ways, which is why they generate the arguments they do.

Macroeconomics is not microeconomics scaled up, and treating it that way produces confident wrong answers. Behaviour that is sensible for one household can be damaging if everybody does it simultaneously - saving more in a downturn being the classic case.

This is also where economics stops having consensus. Micro is relatively settled; macro has genuine schools of thought that disagree about mechanisms and remedies. Presenting one as established fact is a sign that somebody is selling something.

This is the least intuitive important idea in the subject and worth the time it takes. Two countries both gain from trade even when one is better at producing everything, because what matters is what each gives up to produce a thing rather than how fast it can produce it.

It generalises beyond countries. It is why a surgeon who types faster than their assistant still should not do the typing. Once you see it, a lot of arguments about trade look different.

Economics runs on indicators, and every indicator is a decision about what counts. GDP measures output regardless of whether it is useful, so rebuilding after a disaster raises it and unpaid care work does not appear at all. GDP per capita is an average that says nothing about distribution.

The strongest answers in this subject are the ones that use a statistic and then say what it hides. That is not scepticism for its own sake; it is what the indicator's definition actually implies.

The most practically valuable unit here is personal finance, and it is the one most often skipped as too simple. Compound interest works identically on debt and on savings, which is why the same mechanism that makes long-term investing powerful makes high-interest debt so difficult to escape.

Learn the model, then learn its limits. Somebody confidently applying supply and demand to everything is doing the equivalent of using a hammer on a screw.