The handicap principle is a hypothesis proposed by the biologist Amotz Zahavi to explain how evolution may lead to "honest" or reliable signalling between animals which have an obvious motivation to bluff or deceive each other. It suggests that costly signals must be reliable signals, costing the signaller something that could not be afforded by an individual with less of a particular trait. For example, in sexual selection, the theory suggests that animals of greater biological fitness signal this status through handicapping behaviour, or morphology that effectively lowers this quality. The central idea is that sexually selected traits function like conspicuous consumption, signalling the ability to afford to squander a resource. Receivers then know that the signal indicates quality, because inferior-quality signallers are unable to produce such wastefully extravagant signals. History The handicap principle was proposed in 1975 by Israeli biologist Amotz Zahavi. The generality of the phenomenon is the matter of some debate and disagreement, and Zahavi's views on the scope and importance of handicaps in biology have not been accepted by 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. Handicap models Though the handicap principle was initially controversial,—John Maynard Smith was a notable early critic of Zahavi's ideas—it has gained wider acceptance because it is supported by game theoretic models, most notably Alan Grafen's signalling game model. This is essentially a rediscovery of Michael Spence's job market signalling model, where the job applicant signals their quality by declaring a costly education. In Grafen's model, the courting male's quality is signalled by investment in an extravagant trait—such as the peacock's tail. In both cases, the signal is reliable if the cost to the signaller of producing it is proportionately lower for higher-quality signallers than it is for lower-quality signallers (Fig. 2). Having read Grafen's mathematical modeling, Smith accepted the validity of the Handicap Principle. A series of papers by Thomas Getty shows that Grafen's proof of the handicap principle depends on the critical simplifying assumption that signalers trade off costs for benefits in an additive fashion, the way humans invest money to increase income in the same currency. This is illustrated in the figures to the right, from Johnstone 1997. The validity of this assumption has been contested, in its application to the survival cost–reproduction benefit trade-off that is assumed to mediate