Why tell the truth? There are possible explanations based on repeated social interaction and social status, as well as explanations based on relatedness (if honesty benefits the receptive communicative partner, and that partner shares genes with the transmitting partner, then genes for honesty can benefit themselves in another body). However these accounts fail to explain honesty between unrelated, transient contacts. It might be suggested that honesty in towards unrelated, transient contacts is a mere ‘mis-firing’ of the evolved within-group or within family honesty, but I think this is an unsatisfactory answer. Humans already have behaviours which are specifically directed towards in-group and family members, and so must already have some apparatus to distinguish those groups. It would be strange if honesty simply failed to tie into those existing cognitive systems. Furthermore, these accounts provide only very poor explanations for inter-species honesty/honest-signals.
Why communicate at all? Communication is an exchange of information. In this context it is best analyzed as unidirectional; any bidirectional communication can be construed as merely two simultaneous instances of unidirectional communication. So to communicate is to give information to another entity. Communication is only even potentially useful if the receptive partner’s behaviour may be changed by the received information. Communicating with a rock does not meaningfully (at this level of analysis) change the world, and does not produce evolutionarily relevant effects. So, communication, if it is evolved, must have evolved to be directed towards agents, and in particular evolved to modify the behaviour of those agents; this is, in essence, Krebs & Dawkins ’76.
So (modulo some sloppiness regarding the difference between the organismal and the genetic levels of explanation) the point of communication is to cause other agents to act in ways which are beneficial to the communicator. This reduces our original question “Why be honest?” to “Why does honesty make others useful?”, or “Why is honesty the best policy?”. To answer this question we need to explore what we mean by ‘honesty’. Temporarily neglecting considerations of intent, honest communication is true communication, which is communication which tends to produce true beliefs in its recipient. Where the evolutionarily relevant notion of truth is that a belief is true if it produces behaviours ‘as-if’ it were true. That is, if it produces behaviours which correspond with the behaviours that an idealized agent would perform under the same goals. Under this use, it is ‘true’ that jumping off a cliff will hurt, even if in fact it will kill you instantly; because pain is merely a tag classifying events as harmful, and in general dying is harmful from the point of view of evolving entities. (Aside: It is in this sense that useful fictions, like Newtonian mechanics are ‘true’.)