the evolution of sexually selected signals. It can be reasoned that as fitness depends on the production of offspring, this is therefore a multiplicative function, not an additive function, of reproductive success. Further formal game theoretical signalling models demonstrated the evolutionary stability of handicapped signals in nestlings' begging calls, in predator-deterrent signals and in threat-displays. In the classic handicapped models of begging, all players are assumed to pay the same amount to produce a signal of a given level of intensity, but differ in the relative value of eliciting the desired response (donation) from the receiver. The hungrier the baby bird, the more food is of value to it, and the higher the optimal signalling level (the louder its chirping). Counter-examples to handicap models predate handicap models themselves. Models of signals (such as threat displays) without any handicapping costs show that conventional signalling may be evolutionarily stable in biological communication. Analysis of some begging models also shows that, in addition to the handicapped outcomes, non-communication strategies are not only evolutionarily stable, but lead to higher payoffs for both players. Mathematical analyses including Monte Carlo simulations suggest that costly traits used in mate choice by humans should be generally less common and more attractive to the other sex than non-costly traits. While Grafen’s model was claimed to support the Handicap Principle it soon turned out that honest signals need not be costly at the honest equilibrium not even under conflict of interest. This conclusion was first shown in discrete models then continuous models as well. Similar result was obtained in conflict models: threat displays need not be handicaps to be honest and evolutionarily stable. In all of these models signals at the honest equilibrium can be cost-free or even beneficial for honest signallers (independent of the receiver’s response). Generality and empirical examples The theory predicts that a sexual ornament, or any other signal, such as visibly risky behavior, must be costly if it is to accurately advertise a trait of relevance to an individual with conflicting interests. Typical examples of handicapped signals include bird songs, the peacock's tail, courtship dances, and bowerbird bowers. 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