This is a teleological or 'functional' argument, of course, open to the objection that the need for the cohesion of social groups will not automatically engender that cohesion. In the absence of a designer and creator, the favourite logical substitutes are our old friends, natural selection and rational foresight. Presumably each of them operates, though no one really knows in what proportion.

QUESTION: The unresolved proportion question — how much social cohesion comes from natural selection versus rational foresight — seems genuinely open and unexplored here.

Diego Gambetta, Trust - Making and Breaking…, loc. 1934