What Is Costly Signalling? Why an Expensive Message Is a Believable One
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A claim anyone could make conveys nothing, and a claim only certain people can afford to make conveys something. That single idea explains peacock tails, university degrees, engagement rings and a substantial amount of advertising.
The problem it solves
Where one party knows something the other does not, and where the first party has an incentive to claim the favourable version regardless of the truth, simply saying it conveys nothing, since a liar says the same thing. That is the credibility problem and it appears wherever information is asymmetric. Costly signalling solves it when the signal is expensive in a way that varies with the underlying quality, so that the cost is bearable for someone with the quality and prohibitive for someone without. A signal meeting that condition is believable not because the sender is honest but because dishonesty does not pay, and the mechanism therefore produces reliable communication between parties with conflicting interests, which is otherwise difficult to explain.
Where it appears
The same logic operates in biology and in economics and the examples are direct parallels:
- •Elaborate ornaments in animals, which are costly to grow and to carry and which only well-conditioned individuals can afford
- •Stotting, where a gazelle leaps conspicuously on spotting a predator, which advertises fitness and deters pursuit
- •Education as a signal of ability to employers, independently of anything learned, which is a substantial strand in labour economics
- •Advertising expenditure that conveys no information, which signals that the firm expects repeat business
- •Warranties and guarantees, which are cheap to offer only for a product that will not fail
- •Conspicuous consumption, which signals resources by wasting them visibly
The condition that makes it work
The mechanism requires more than the signal being expensive, which is where popular accounts go wrong. What is required is that the cost differs between senders in a way that tracks the quality being signalled, so that the same signal is cheaper for a high-quality sender than for a low-quality one. A cost identical for everyone deters everyone equally and separates nobody. A degree signals ability only if able people find it easier to obtain, which is the assumption the argument rests on and which is empirically contestable. An ornament signals condition only if a poorly conditioned animal genuinely cannot produce one. Where that differential cost fails, the signal stops carrying information and the arrangement collapses, which is why signalling equilibria can break down and why cheap imitation undermines them.
Signals that cost nothing
Reliable communication is also possible without cost, which qualifies the whole framework. Where the interests of sender and receiver align, there is no incentive to deceive and a cheap signal works perfectly well, which covers most communication within cooperating groups and much communication between species where both benefit, as with warning colours that are honest because a predator learning them serves the prey. Conventional signals work where deception is punished by other means, including reputation and repeated interaction, which is why a promise between people who will meet again carries weight that the same words between strangers do not. And some signals are cheap to produce and impossible to fake for physical reasons, including a call whose frequency is set by body size. The costly mechanism is therefore one solution among several rather than the general account of honest communication.
The awkward consequences
The arrangement produces outcomes that are individually rational and collectively wasteful, which is the uncomfortable part. Resources spent on signalling produce no value beyond the information conveyed, so if a degree functions purely as a signal, the years and money spent are a transfer of position rather than an addition of capability, and a society could in principle convey the same information more cheaply. The same reasoning applies to arms races in ornament, in advertising and in consumption. Whether education is primarily signal or primarily capability is a genuine and consequential dispute in economics, since the policy implications differ completely. And signalling arrangements tend to escalate, since the value of a signal depends on its relative cost, which means the standard rises as more people meet it, which is visible in credential requirements over recent decades.
The takeaway
A signal is believable when dishonesty does not pay, which requires the cost to be lower for senders who genuinely have the quality rather than merely being high for everyone. Ornaments, education, advertising and warranties all work this way. The arrangement conveys information and produces no other value, so it is individually rational and collectively wasteful, and it escalates as more people meet the standard.