Handicap principle

The handicap principle is a hypothesis originally proposed in 1975 by biologist Amotz Zahavi  to explain how evolution may lead to "honest" or reliable communication between animals who have an obvious motivation to bluff or deceive each other. The handicap principle suggests that reliable signals must be burdensomely costly to the signaller, costing the signaller in the trait being signalled in a manner that an individual with less of that trait could not afford. For example, in the case of sexual selection, the theory suggests that animals of greater quality communicate this status through handicapping behaviour or morphology that effectively lowers their quality. The central idea is that sexually selected traits function like conspicuous consumption, signalling the ability to afford to squander a resource simply by squandering it. Receivers know that the signal indicates quality because inferior quality signallers cannot afford to produce such wastefully extravagant signals.

The generality of the phenomenon is the matter of some debate and disagreement, Zahavi's views on the scope and importance of handicaps in biology remain outside the mainstream. Nevertheless, the idea has been very influential,  with most researchers in the field believing that the theory explains some aspects of animal communication.

Criticism and support
Though the idea was initially controversial   (John Maynard Smith being one notable early critic of Zahavi's ideas   ) it has gained wider acceptance due to supporting game theoretic models, most notably Alan Grafen's signalling game model. Grafen's model is essentially a rediscovery of Michael Spence's job market signalling model, where the signalled trait was conceived as a courting male's quality, signalled by investment in an extravagant trait -such as the peacock's tail- rather than an employee signalling their quality by way of an expensive education. In both cases, it is the decreased cost to higher quality signallers of producing increased signal that stabilizes the reliability of the signal. Further formal game theoretical signalling models demonstrated the evolutionary stability of handicapped signals in nestling begging calls predator deterrent signals and threat displays.

The theory predicts that sexual ornaments must be costly if they accurately advertise biological fitness. Typical examples of handicapped signals include bird songs, the peacock's tail, courtship dances, bowerbird's bowers, or even possibly jewellery and humor. Jared Diamond has proposed that certain risky human behaviours, such as bungee jumping, may be expressions of instincts that have evolved through the operation of the handicap principle. Zahavi has invoked the potlatch ceremony as a human example of the handicap principle in action. This interpretation of potlatch can be traced to Thorstein Veblen's use of the ceremony in his book Theory of the Leisure Class as an example of "conspicuous consumption".

The handicap principle gains further support by providing interpretations for behaviours that fit into a single unifying gene-centered view of evolution and making earlier explanations based on group selection obsolete. A classic example is that of stotting in gazelles. This behaviour consists in the gazelle initially running slowly and jumping high when threatened by a predator such as a lion or cheetah. The explanation based on group selection was that such behaviour might be adapted to alerting other gazelle to a cheetah's presence or might be part of a collective behaviour pattern of the group of gazelle to confuse the cheetah. Instead, Zahavi proposed that each gazelle was communicating to the cheetah that it was a fitter individual than its fellows and that the predator should avoid chasing it.