: "It can evolve from small clusters of individuals who base their co-operation on reciprocity and have even a small proportion of
: their interactions with each other. But it cannot emerge if such individuals are too scattered and have a negligible proportion of
: their interactions with each other. "
: Thats what we have - the latter situation.
First of all, I don't necessarily agree with the (unwritten) premise that non-emergence of reciprocity requires a "scattered" population with "a negligible proportion" of interaction.
Take the Ik, for instance. In this case the sudden imposition of scarcity on a population culturally (and economically) conditioned to a particular (and now unsuitable) lifestyle, helped shatter social ties and thrust the individual into a situation of "steal or be stolen from" mentality.
Actually, IMO, we don't "have the latter situation" (scattered population with negligible interaction). We Do however have designed "scarcity", and this helps undermine the expression of the proposed tendency toward reciprocation.