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The handicap principle is an idea proposed by biologist Amotz Zahavi^[1][2][3]. It concerns the way in which animals communicate through their behaviour and anatomy, and makes the counterintuitive claim that certain forms of sexually selected behaviour, and anatomical features supporting them, may have evolved because they apparently act to reduce the chances of individual survival of the animal exhibiting the behaviour.

The reasoning supporting this claim depends on considering the question as to how an animal that is the recipient of communication can be assured that the information conveyed is accurate (that the signal is "honest"). The 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. Traditionally, zoologists had believed 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 (see Group selection). 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. If honest, this claim benefits the cheetah which avoids the wasted energy of a fruitless chase after a healthy animal. The difficulty for the cheetah is to figure out whether it should trust such an interpretation of the behaviour of stotting. Zahavi's answer is that the signal is reliable precisely because only a fit gazelle can afford to grant the cheetah such an advantage and hope to survive.

Though this idea was initially controversial^[4][5][6][7] (John Maynard Smith being one notable early critic of Zahavi's ideas^[8][9][10]), it has gained wider acceptance due to supporting game theoretic models, most notably Alan Grafen's signalling game model^[11]. Zahavi's views on the scope and importance of handicaps in biology remain outside the mainstream; nevertheless, most researchers in the field believe the theory explains some aspects of animal communication. It also suggests that sexual ornaments should be costly to be attractive, so that they can accurately advertise biological fitness: typical examples are 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.

Economist Michael Spence proposed that in the job market, education played the role of a handicap or signal^[12], work that inspired an enormous literature in contract theory, and eventually lead to a Nobel Prize for Spence in 2001. Although the ideas were essentially the same, for more than a decade economists and biologists remained unaware of each other's work.^{[citation needed]}

• Johnstone, R. A. (1995) Sexual selection, honest advertisement and the handicap principle: reviewing the evidence" Biological Reviews 70:1-65.
• Johnstone, R. A. (1997) The evolution of animal signals, In Behavioural Ecology: an evolutionary approach 4th ed., J. R. Krebs and N. B. Davies, editors. Blackwell. Oxford, pp:155-178.
• Maynard Smith, J. and Harper, D. (2003) Animal Signals. Oxford University Press. ISBN 0-19-852685-7

• "The Theory of Honest Signaling"