The Death of Costly Signalling?

How do organisms communicate honestly despite conflicts of interest? Over the past quarter-century, the "costly signalling" hypothesis -- that signal honesty can be ensured by appropriate signal cost -- has emerged as the dominant explanation for this puzzle. First proposed by Zahavi [1, 2] and formalized by Grafen [3] and Godfray [4], this hypothesis has led to a proliferation of theoretical models [5, 6, 7, 8, 9, 10, 11, 12, 13, 14] and empirical tests (reviewed in [15, 16, 17, 18]). Unfortunately, these empirical studies suggest that honest signalling is not always accompanied by the predicted signal costs (reviewed in [12]), and some signalling systems (including human language) appear not to require signal cost at all. In response to these difficulties, researchers have attempted to identify special mechanisms by which signalling can be honest even with low or zero signal cost (also reviewed in [12]). Here, we show that no special mechanism is necessary. While the cost of out-of-equilibrium signals plays an important role in stabilizing honest signalling, the signals actually employed at equilibrium need not be costly. Therefore, even unrelated individuals with conflicting interests can communicate honestly using cost-free or very cheap signals; contrary to the "handicap principle," waste is not required to ensure honest signals. We illustrate this by constructing examples of cost-free signalling equilibria for the two paradigmatic signalling games of Grafen [3] and Godfray [4]. Our findings (1) significantly revise previous theoretical conclusions regarding the requirement for signal cost in honest signalling systems, (2) explain the discrepancy between empirical signalling studies and theoretical predictions, (3) suggest why some animal signals use cost to ensure honesty while others do not, and (4) provide ways in which signalling theory can be used to address the "problem of deception" in the evolution of human language.