The Peacock, the Sparrow, and the Evolution of Human Language

How did human language arise, and what accounts for its present structure? Over the past decade, there has been great interest -- and impressive progress -- in using evolutionary theory to address to these and related questions. In particular, a number of studies have shown that several key features of language could plausibly arise and be maintained by natural selection when individuals have coicident interests. However, these models have largely ignored a vital strategic component of the social context in which language is employed: individuals in real societies, both past and present, do not have fully coincident interests. Simultaneously, theoretical models of animal communication have confronted these strategic issues head-on, but they have largely focused on costly (Zahavian) signals. We approach this problem directly, asking the following question: Can language evolve and be maintained under common biological scenarios of non-coincident interest? Using a trio of examples -- the peacock, the sparrow, and human language -- we show that an explicit connection can be drawn between "costly signalling theory" as developed for the study of animal communication, and the "cheap" signals employed in human language. We argue that coincident interests are not a prerequisite for linguistic communication, and explore the structural features to be expected of languages employed in societies with non-coincident interests. We find that many of the results derived previously by assuming coincident interests will also be expected under less restrictive models of society.