The measured findings in this group range from near-laws that have held for two centuries to recent discoveries about how shoppers read a price tag. What unites them is that each describes a pattern people keep running into whether or not they believe in it. Economics has few laws in the physicist's sense, but this group is as close as it comes.
Old regularities
Some findings have survived every change of economic system since they were first stated. Diminishing Marginal Returns holds that each extra unit of input eventually yields less. Comparative Advantage shows that trade benefits both sides even when one is better at everything. Engel's Law finds the share of income spent on food falling as income rises, in every country measured. Wright's Law finds costs falling by a steady fraction with each doubling of cumulative production, which is why solar panels and batteries became cheap. Gresham's Law records bad money driving out good, and Thiers's Law records the reverse when a currency collapses.
Measures that stop measuring
A second cluster is the most widely quoted in the ledger outside economics. Goodhart's Law holds that a measure made into a target ceases to be a good measure; Campbell's Law says the same of social indicators; the Ratchet Effect shows managers holding back output because meeting a target raises the next one. Together they explain why every performance-management system eventually needs reforming.
Supply that creates its own demand
A third set shows capacity filling itself. Induced Demand finds new road lanes filling with new traffic. Roemer's Law found hospital beds filling regardless of need, and Parkinson's Second Law finds expenditure rising to meet income. Marchetti's Constant suggests the underlying reason for the transport case: people keep roughly an hour a day for travel, and spend faster transport on living further away.
How people actually pay
The newest findings come from behavioural economics and marketing science. The Left-Digit Effect, the Zero Price Effect, the Pain of Paying and Drip Pricing show that how a price is presented changes what people do. The Ehrenberg-Bass findings, Double Jeopardy and Repertoire Buying among them, show that most buyers of a brand are light and disloyal, and that big brands have more buyers rather than more devoted ones.
Where it applies
The practical uses are wide. Any organisation that sets targets meets Goodhart's Law. Transport planners meet induced demand every time a bypass is proposed. Pricing teams use the behavioural findings daily, and regulators in several countries have moved against drip pricing because it works. Marketers who absorb the Ehrenberg-Bass findings spend on reaching more buyers rather than on deepening loyalty among existing ones, which reverses much conventional advice. Countries with a resource boom meet Dutch Disease and the Resource Curse.
Where it is argued over
The macroeconomic entries carry the live disputes. The Phillips Curve broke down in the 1970s and has looked flat for long periods since. The claim that inflation is always a monetary phenomenon holds over long periods but explains recent episodes less well than supply shocks and expectations. Hauser's Law and Wagner's Law are argued over along political lines as well as statistical ones. The Great Gatsby Curve, linking inequality to lower mobility, is robust as a correlation and disputed as a cause.
Why it is worth knowing
These regularities shape prices, wages, commutes and budgets whether anyone notices them or not. Knowing them gives a person something economists call priors: a sense of what usually happens before looking at the particular case. That matters most when someone claims a new road will end congestion, a target will improve performance, or a price cut to zero will simply sell a little more. The evidence says, in each case, what to expect instead.
The entries behind this essay: Evidence in Economics & Incentives. Every subject has its essays on the essays page.