Cost and conflict in animal signals and human language

The "costly signaling" hypothesis proposes that animal signals are kept hon est by appropriate signal costs. We show that to the contrary, signal cost is unnecessary for honest signaling even when interests conflict. We illust rate this principle by constructing examples of cost-free signaling equilib ria for the two paradigmatic signaling games of Grafen (1990) and Godfray ( 1991). Our findings may explain why some animal signals use cost to ensure honesty whereas others do not and suggest that empirical tests of the signa ling hypothesis should focus not on equilibrium cost but, rather, on the co st of deviation from equilibrium. We use these results to apply costly sign aling theory to the low-cost signals that make up human language. Recent ga me theoretic models have shown that several key features of language could plausibly arise and be maintained by natural selection when individuals hav e coincident interests. In real societies, however, individuals do not have fully coincident interests. We show that coincident interests are not a pr erequisite for linguistic communication, and find that many of the results derived previously can be expected also under more realistic models of soci ety.