A peacock's tail and a public promise do the same job

In the example Michael Spence built for his 1973 paper on job markets, there are two kinds of worker. One has a productivity of 1, the other a productivity of 2, and an employer cannot tell them apart by looking. Education, in the example, adds nothing to productivity. It is merely available at a price: each unit costs the less productive worker 1 and the more productive worker 0.5. Spence then showed that a market can settle into a state where the employer pays 2 to anyone holding at least some minimum amount of schooling, pays 1 to everyone else, and is correct about both groups every time. The schooling teaches nothing. It works because it is cheaper to acquire for the people who were going to be productive anyway.

Two years after that paper, and in a different field, a biologist was saying roughly the same thing about birds. Amotz Zahavi, at Tel Aviv University, argued in a 1975 paper in the Journal of Theoretical Biology that traits chosen by mates, the standard case being a peacock’s tail, are credible precisely because they burden their owner. As secondary summaries of the paper put it, nearly all such characters seem to impose a handicap on survival, and the choosing sex benefits by reading the handicap as a quality test. A hen cannot inspect a cock’s immune system or foraging skill. She can see whether he carries a tail that makes escape harder and is still alive.

Costly signaling is the use of an expensive or hard-to-fake action to convey hidden information, and it works when the action costs more for a sender who lacks the claimed quality than for one who has it. Zahavi (1975, Journal of Theoretical Biology) proposed the idea for animal displays, Spence (1973, Quarterly Journal of Economics) formalized it for hiring, and Grafen (1990, Journal of Theoretical Biology) built an evolutionary game model in which truthful signaling is stable when low-quality senders pay a greater marginal cost per unit of signal than high-quality ones. Applied to promises, the same rule answers why public commitments sometimes work and sometimes do not: a commitment becomes believable to an audience when breaking it would be costly to someone who will not keep it and cheap to someone who will.

That last sentence is an extension of the three papers, not a quotation from them. The rest of this piece tests it.

What each source actually claims

Reading the three papers side by side shows how much narrower each claim is than the folk version (“costly things are trustworthy”).

Spence’s paper states the condition in a section titled “A Critical Assumption”: a signal will not distinguish one applicant from another unless the costs of signaling are negatively correlated with productive capability, because otherwise everyone invests in the signal in the same way and the signal cannot tell them apart. That is a claim about a gap between groups, not about the size of the cost. In his numbers, the schooling level that separates the groups can be any value above 1 and below 2. At 1.5 units, a low-productivity worker would pay 1.5 to raise a wage from 1 to 2 and net 0.5, which is worse than staying put. A high-productivity worker pays 0.75 and nets 1.25, which is better. Spence also notes an awkward consequence: if half the population is in each group, the high-productivity group nets less than the 1.5 it would earn if employers simply paid the average wage, and he concludes that everyone would prefer a world without signaling.

Zahavi’s claim is looser and more ambitious. Secondary summaries of the 1975 paper say that sexual selection is effective because it improves the choosing sex’s ability to detect quality, and that probably all characters shaped by mate preference are handicaps. I could not open the original text, only those summaries and a first-page citation, so the quotation marks in this piece stay with Spence and with the review literature.

Grafen’s contribution was to convert the story into a model in which a strategy can be checked for stability. The summary line that matters here, as quoted in a later critique, is that reliable signals can evolve when “low-quality males pay greater marginal fitness costs for signalling.” Again the operative idea is a difference in cost between types.

The part of the theory that is disputed

The handicap principle has a contested record, and any use of it should say so. Penn and Számadó, in a 2020 review in Biological Reviews, conclude that “There is no theoretical or empirical support for the Handicap Principle.” Their reasoning is that Zahavi’s original claim, that signals are reliable because they are wasteful, is circular, and that Grafen’s models support something else: Zahavi’s separate idea that signal cost depends on the sender’s condition. They also cite John Maynard Smith as holding that “the process envisaged by Fisher is overwhelmingly more important than any kind of handicap effect,” which is a statement that arbitrary female taste may explain ornaments better than cost does. I read the first half of their paper, about 100,000 of 206,000 characters, so I am not representing their treatment of the empirical studies in the second half.

For commitments, the dispute has a useful reading. The part that survives the critique, differential cost between types, is exactly the part that predicts which vows work. The part that is under attack, that waste in itself certifies reliability, is the part that would predict an expensive vow is always good. The critique therefore sharpens the model rather than demolishing it: an expensive promise that costs a keeper and a quitter the same amount is just an expensive promise.

Scoring a commitment

Four questions turn the theory into something that can be applied to an ordinary promise.

1. Who pays? The cost has to land on the sender, in something the sender values.
2. Who verifies? Someone other than the sender has to be able to observe whether the commitment was kept, or the cost is never imposed.
3. Can a non-committer afford it? This is the single-crossing question. The cost should be low in expectation for someone who will keep the commitment and high in expectation for someone who will not.
4. Does the audience care? A signal sent to a receiver who ignores it separates nobody.

A vow passes when all four hold. Two of them fail more often than people expect, and the third is easiest to misread. The cost to the non-committer is an expected cost, which means it depends on the chance of failing as well as the size of the penalty. A refundable deposit costs a reliable person almost nothing, since they get it back, and costs an unreliable person the full amount on a high fraction of occasions.

The table below applies the four questions to five common commitments. The scores are my own judgments, and the numbers in the last column are assumptions chosen to illustrate the logic, not measured rates.

Commitment	Who pays	Who verifies	Affordable to a non-committer?	Audience cares?	Illustrative cost ratio (non-committer : committer)	Prediction
A degree	Tuition, years, effort	Employer, via the institution	Costlier for those who struggle with coursework (Spence’s 1 vs 0.5)	Yes, in hiring	About 2 : 1 in Spence’s example	Works, as long as the cost gap holds
A tattoo of a name or a date	Pain, money, removal cost	Anyone who sees it	Equal at the parlor, unequal later: only the person who changes their mind pays for removal or regret	Only if the viewer knows what it means	Depends on the chance of regret	Works for loyalty claims to a group that can read it, weak for private goals
A New Year’s post	A few seconds, a mild chance of embarrassment	Followers, in theory	Yes, near zero for everyone	Rarely, after the first week	About 1 : 1	Predicted failure
A refundable deposit	The deposit if forfeited	The holder of the deposit	Costly in expectation only if the non-committer’s failure rate is high	Holder must care enough to enforce	Assume forfeit chance 70 percent vs 10 percent on 100 units: 70 : 10, or 7 : 1	Works if the holder actually keeps the money
A forfeit to a friend	Money or a chore	The friend	Same expected-cost logic as the deposit, plus the friend’s opinion of you	Yes, if the friend will collect	Similar to the deposit, usually higher because of the social cost	Works if the friend is willing to be the bad guy

The row that fails is the third. A New Year’s resolution posted to a feed costs the same few seconds whether the poster intends to follow through or not. The expected cost of breaking it is also low, because the audience rarely checks in February. In Spence’s terms, both groups send the same signal, and the receiver learns nothing. The vow is cheap talk with a larger audience. Self-reported progress has its own treatment, a piece on why recorded proof outperforms a claim, and its argument is not repeated here. What the signaling frame adds is a diagnosis of why a public post fails: it is a signal with no gap in cost between types.

The same test applied to public visibility

A public commitment can fail in two opposite ways. It can cost nothing, like the post. It can also cost too much for anyone to be willing to make it, which is Spence’s point about signal levels near the top of the range. A forfeit set so high that even a fully committed person declines to sign gives no information about anyone, because almost no one sends it.

Visibility is the second dial. A public record changes the verification question, which is why a contribution graph or a donor wall can work with no fee attached: the cost is the observer’s opinion, and the audience is built into the page. This is a weaker form of single-crossing, because it depends on whether the sender’s reputation is worth anything to them. A person with no friends who read the profile pays nothing for a gap in the graph.

The forfeit to a friend works for the reason the table gives, and fails for the reason the table gives. A well-specified friend bet makes the terms checkable, which handles verification, but the friend has to be willing to collect. If the friend would wave it through, the expected cost to a non-committer falls toward the cost to a committer and the signal collapses.

Applying the test to a consequence-based app

DontSnooze attaches a consequence to a missed morning check-in: an embarrassing photo from the user’s camera roll goes to friends the user chose. Under the four questions, it is a costly signal only to the extent that two conditions hold. The friends have to exist and care, and the user has to mind the exposure. Both are weaker than they look. A user who shrugs at embarrassment, or picks friends who would find the photo funny and move on, has set the non-committer’s cost near the committer’s. And a determined user can ignore or disable the app, which moves the cost of defection from the penalty to a button. For someone who would mind the exposure, though, a quitter pays and a keeper does not, which is the shape the theory asks for, and that is why it is worth trying.

Can the audience be you?

A commitment made to oneself is a harder case, and here the argument is my own extension. The receiver in a signaling model is someone who lacks information and updates after seeing the signal. A person making a private resolution is both sender and receiver, and the information at stake is their own future behavior. The same four questions apply, with an unusual twist on the second. Nobody else verifies, so the only verifier is the sender’s later memory. A promise to oneself with no external cost is the cleanest example of cheap talk, and its failure rate matches the pattern the model predicts: the cost of breaking it is zero for committers and quitters alike.

The self-audience case does suggest a prediction. If self-signaling works at all, it should work when the sender observes their own costly action and updates on it. Someone who pays a nonrefundable fee for a course they then attend has learned something about themselves, and someone who skips it has learned something as well. I have not found a study I could verify that tests this, so it stays a hypothesis.

Where the argument runs out

Three things limit the conclusions above. First, the cost ratios in the table are illustrative assumptions, and the framework cannot say what real failure rates are. Second, the biology is contested: if Penn and Számadó are right that the wasteful-handicap reading is wrong, the useful content of the theory is the narrower claim about differential cost, and that is what this piece has relied on. Third, Crawford and Sobel’s 1982 paper on strategic information transmission is the usual reference for why cheap talk can still carry information when the parties’ interests line up, and I did not read it beyond its abstract, so the claim that promises to oneself are pure cheap talk is a simplification. Separately, reasoning experiments on social-contract rules suggest that a promise with a named creditor gets audited differently from a private rule, as in the four-card selection task. The cleanest summary of the theory is a rule of thumb with a test attached: ask what the vow would cost a person who was never going to keep it, and compare that to what it costs a person who will. If the two numbers are close, the vow is decoration.