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.