Plate i in print

The Convention

Lewis; a regularity that holds itself in place, where each conforms because the others do and prefers to conform so long as they do — the unwritten rule in its purest, self-enforcing form.

A convention is a regularity that holds itself in place: each keeps to it because the rest do, and expects the rest to keep to it because each does — drive on the right, call the coin money, let this sound mean that — so that an arbitrary practice, once settled, needs no law and no agreement to endure, only the expectation, held in common, that it will.

— the argument of this plate, after David Lewis, 1969

The volume's subject is the unwritten rule — the trust, the reciprocity, the done thing that no contract specifies and every market silently needs. To understand the unwritten rules, begin with the simplest of them, the convention, and with the philosopher who gave it its exact definition. David Lewis showed that a great deal of social order consists of regularities that hold themselves in place: practices no one legislated, no one agreed to, and no one enforces, which persist purely because everyone expects everyone else to keep to them. The convention is the unwritten rule in its purest form, and it reveals how order can arise and endure with no authority above it at all.

A convention, Lewis proposed, is a regularity in the behavior of a population, arising in a situation that recurs, and meeting three conditions together. Nearly everyone conforms to it; nearly everyone expects nearly everyone else to conform; and nearly everyone prefers to conform on the condition that the others do. The third condition is the key, for it marks out the kind of situation a convention solves: a coordination problem, in which what each person wants is simply to do what the others do — to drive on the same side of the road, to speak the same language, to accept the same money. Given that everyone else conforms, each does best by conforming too, and no one has any reason to break ranks. The regularity is therefore self-enforcing, an equilibrium that, once reached, holds itself in place without anyone intending to maintain it and without any rule being written down.1

1. Lewis's definition: a convention is a regularity in a recurrent situation where nearly everyone conforms, expects the others to conform, and prefers to conform provided the others do. The third condition marks a coordination problem — each just wants to do what the rest do (same side of the road, same language, same money). Given everyone else conforms, each does best conforming, so no one breaks ranks. The regularity is self-enforcing, an equilibrium that holds itself in place with nothing written down.

A convention is arbitrary, in the precise sense that some other convention would have served as well: a people could as easily drive on the left as on the right, take gold for money or take silver, make one sound or another carry the same meaning. What singles out the convention actually in force is not its merit but its history — the precedent that happened to be set, and the focal salience, in the sense the volume of the game gave the term, that made one solution the obvious one for everyone to expect everyone to expect. But once a convention has settled, its arbitrariness is no weakness. It binds as firmly as if it had been the only possibility, because for a coordination problem each person's expectation that the others will conform is entirely self-confirming. The convention needs no foundation in reason or in justice; the bare fact that it is established is reason enough to keep to it, and the attempt to overturn it on the ground that another arrangement would have been just as good founders on the difficulty that everyone would have to switch at once.2

2. A convention is arbitrary — some other would have served (left or right, gold or silver). What selects the one in force is history and focal salience (the volume of the game), not merit. But once settled, arbitrariness is no weakness: it binds as firmly as a sole possibility, because each person's expectation that others conform is self-confirming. The mere fact of being established is reason enough to keep to it, and to overturn it everyone would have to switch at once.
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What holds a convention in place is written nowhere; it lives in the structure of mutual expectation. Each person conforms because he expects the others to; and he expects the others to because he knows that they expect him to, and knows that they know he expects them to, in a tower of shared expectation that Lewis analyzed under the name of common knowledge. The convention is, in this sense, a fact about what everyone believes everyone believes — a shared understanding, held quietly in common, requiring no document and no enforcer to sustain it. This is why conventions can be at once so stable and so invisible: they are nowhere recorded, no one consciously maintains them, and most of the time no one even notices them — until a stranger or a newcomer, ignorant of the local rule, violates one, and the unwritten regularity becomes suddenly, sometimes painfully, visible in the breach.3

3. A convention lives in mutual expectation, not on paper. Each conforms because he expects the others to; and expects them to because they expect him to, and so on — the tower Lewis called common knowledge. The convention is a fact about what everyone believes everyone believes, needing no document or enforcer. Hence its stability and its invisibility: nowhere recorded, no one maintaining it, unnoticed until a stranger violates it and the rule shows itself in the breach.

Once one sees the form, conventions appear everywhere, carrying an enormous share of the work of social and economic life. Language is a vast system of conventions, the regularities by which sounds and marks come to bear meaning. Money, treated in its own volume, is at its root a convention: a paper or a token has value because everyone expects everyone else to accept it, a self-confirming expectation that can hold firm for generations or, should it fail, collapse into worthlessness within days. Weights and measures, the side of the road, the queue, the meaning of a handshake, the hours a market keeps, the small gestures of courtesy — all are conventions, unlegislated regularities sustained by mutual expectation alone. Markets in particular are dense with them: the conventions of how a deal is opened and closed, how a price is quoted, what a silence in a negotiation signifies, what counts as fair dealing and what as sharp practice. Strip the conventions away and a market would not merely slow; it would seize, for the parties could no longer anticipate one another at all, and exchange depends at every step on being able to guess what the other will do.4

4. Conventions do an enormous amount of work. Language is a system of them; money (its own volume) is one at root — paper has value because all expect all to accept it, self-confirming until it isn't. Weights, the road's side, the queue, the handshake, market hours, courtesies — all unlegislated regularities held by expectation. Markets teem with them: how a deal opens and closes, how a price is quoted, what a silence means. Strip them away and the market seizes, for the parties can no longer anticipate one another.

But the convention, in Lewis's pure sense, is the easy case, and the volume cannot rest there. A convention solves a problem of pure coordination, in which no one is tempted to defect because conforming is simply in everyone's interest. Many of the most important unwritten rules are not like this. They govern situations with the structure of the prisoner's dilemma met in the volume of the game, where each is tempted to break the rule for private gain even as all benefit from its being kept — to lie when honesty is expected, to take without giving, to cheat a partner who has trusted him. For such rules, mutual expectation alone does not suffice, because expecting others to keep the rule does nothing to make it in one's own interest to keep it; if anything, a world of trusting rule-keepers is the easiest to exploit. These rules need something more — the trust that others will keep faith, the reciprocity that answers a kindness, the sanction that falls on a cheat, the reputation that rides on a man's conduct. The volume turns first to the most important of them, the trust on which exchange depends. The next plate is the faith.5

5. The pure convention is the easy case — pure coordination, no temptation to defect. But many crucial unwritten rules have the prisoner's-dilemma structure (the volume of the game): each is tempted to break the rule for private gain though all gain by its keeping — to lie, to take without giving, to cheat a trusting partner. Mutual expectation cannot hold these, since expecting others to keep the rule does not make keeping it one's own interest. They need trust, reciprocity, sanction, reputation. Next: the faith.

The unwritten rule in its simplest form is the convention, and David Lewis gave it its definition: a regularity that holds itself in place because each conforms, expects the rest to conform, and prefers to conform so long as they do. It is arbitrary — some other convention would have done as well — but, once settled by precedent and salience, it binds as firmly as law, sustained by nothing but a shared expectation held quietly in common. Conventions are everywhere: in language, in money, in measures, in manners, in the thousand unspoken regularities that let the parties to a market anticipate one another. They need no authority and no enforcement, for in a problem of pure coordination it is in everyone's interest to conform. But the unwritten rules that matter most are not all so easy; many must hold against the standing temptation to defect, and for those, mutual expectation must be reinforced by trust, by reciprocity, by sanction, and by reputation. The next plate is the faith.