@Radical_EgoCom As for solidarity, humans are not egoistical homo economicii. But it's a useful approximation. Kin selection promotes familial solidarity... and also sibling rivalry. Imagined communities promote solidarity, but are built on familial concepts, and blood generally trumps religion and nationality. (Imagined communities also have a nasty habit of defining themselves in opposition to The Other.)
@Radical_EgoCom Economic incentives at least have the advantage of being able to bind agents who *are* egoistic together.