Zahavi’s Handicap Principle explaining honest signalling refuted in research led by ELKH researchers


Led by István Zachar and Dániel Czégel from the Institute of Evolution of the ELKH Centre for Ecological Research (CER) and Szabolcs Számadó from the ELKH Research Centre for Natural Sciences (TTK) a group of researchers investigated the conditions for the honesty of the signals used at evolutionary equilibrium. The researchers have devised a general formula that can be used to calculate the possible states of honest evolutionary equilibrium. In accordance with this, there exist honest equilibrium states in which the honest signal is not only without cost, but may even bring additional benefits to the individual giving the signal. This also provides an answer to Darwin’s question of why females prefer ornamental traits in males. The paper presenting the research was published in the journal BMC Biology.

Signals used in the animal kingdom, such as the male peacocks' ornate train, the antlers of deer and the many kinds of butterfly have always filled people with wonder and curiosity. At first sight, these astonishing ornaments are of little use. In the 19th century, Charles Darwin, the father of modern evolutionary thinking, came up with a revolutionary new theory to explain the phenomenon. In his opinion, males are ornamental because females prefer to breed with such individuals. He called this mechanism “sexual selection”. The essence of his theory is that females have a preference for certain traits, and this has since been shown in countless species. However, Darwin did not explain the reason why females prefer ornamental traits in males.

This issue has been suppressed for quite some time, instead the gene-centered approach came into focus. This view was shot to worldwide prominence by Richard Dawkins in his 1976 book The Selfish Gene. The idea is that individuals represent the interests of their genes, which raises the issue of deception and honesty. If an individual, such as a male, can gain an advantage for his genes by deceiving a female, why not do so? Dawkins describes communicative interactions as a duel between “manipulators” and “mind readers”, and comes to the not very optimistic conclusion that most signals are manipulative, i.e. not honest, because the signal does not make it possible to infer the unseen characteristics of the animal.

The 20th century evolutionary biologist Amotz Zahavi connected the two questions more than 100 years after Darwin. According to Zahavi, females will prefer males with ornate feathers when mating because these ornaments are wastefully expensive, and it is this wasteful cost that maintains the honesty of the signals. A trait that is so costly is not worth developing for an individual with inferior genes or in less good health. Zahavi called this mechanism the Handicap Principle. This has caused some confusion, because it is not that these traits have no utility, but that they are disadvantageous for survival, i.e. costly, but advantageous for reproduction, because this is again an example of Darwinian sexual selection.

Zahavi was surprisingly confident in extending the Handicap Principle to all signals. He argued that all the signals found in nature operate according to the Handicap Principle, i.e. they are honest and wasteful. Biologists have begun to measure the evolutionary equilibrium cost of signals. They very quickly came to the conclusion that Zahavi’s wasteful cost could not be observed in many cases. Cautious biologists, however, did not dismiss the principle, but suspected that these signals might have a cost that is yet to be measured.

At the same time, several theoretical models have shown that honest signals need not be costly at evolutionary equilibrium. In many cases, however, these models described specific situations and were based on specific assumptions. The general validity of the critique of the Handicap Principle was thus questioned by many, as it was not known what general critique could be drawn from these specific models.

The importance of the model developed by the researchers presented in this paper is that it is based on a general formula that can be used to compute all possible states of the honest equilibrium in any situation. It became clear from these solutions that honesty is not at all contingent on the cost paid in equilibrium, and thus the Zahavi Handicap Principle can be refuted. There can be honest equilibria where the signal used at the equilibrium is honest and costless, i.e. free, but there can also be equilibria where the honest signal brings an additional benefit to the individual using the signal. In the case of the peacock, for example, for a male with good genes and in good health, feather ornaments can be cheap because they are produced efficiently. Yet the signal will be honest if the ornament is costly to the potential cheat. It is the cost of the deception that is really of significance. The counterfeiting of banknotes is a good analogy. It is not the cost to the National Bank that needs to be high, but the cost to potential counterfeiters that should be higher than the profits from counterfeiting. It is beneficial for the National Bank to print money cheaply, in the quantities and with the technology that makes this possible. At the same time, the aim is to keep the cost to potential fraudsters as high as possible, so there needs to be an asymmetry between the two.

We already knew from previous models that it is the cost of cheating that really counts when it comes to honesty. In contrast to those that went before, the new model presents this knowledge in the form of a general formula. The significance of this is that it allows the state of honest equilibrium to be calculated in any context, which provides science with a very powerful tool.

The research helps us answer Darwin’s original question – why females prefer ornamental males – without assuming that males must pay a wasteful cost. Honesty is also about efficiency, as is Darwinian selection in general, i.e. honest individuals are efficient and not wasteful.


Számadó, Sz., Zachar, I., Czégel, D., Penn, Dustin J. (2023). Honesty in signalling games is maintained by trade-offs rather than costs. BMC Biology 21, 4. Doi: 10.1186/s12915-022-01496-9