Human syntactic language has no close parallels in other systems of animal communication. Yet it seems to be an important part of the cultural adaptation that serves to make humans the earth’s dominant organism. Why is language restricted to humans given that communication seems to be so useful? We argue that language is part of human cooperation. We talk because others can normally trust what we say to be useful to them, not just to us. Models of gene-culture coevolution give one plausible explanation for how language, cooperative institutions, and the genetic basis for both could have evolved. Why did the coevolutionary process come to rest leaving a huge space for the cultural evolution of language? We argue that language diversity functions to limit communication between people who cannot freely trust one another or where even truthful communications from others would result in maladaptive behavior on the part of listeners.