"Reptilian brain (Trust nothing) -> Mammalian brain (Trust family) -> Primate brain (Trust clan) -> Human brain (Trust all those who may help)

High trust societies are much more efficient because we don't all have to (metaphorically) squat in the dirt guarding our meat. This frees humans up to divide labor efficiently, spend time on higher order problems, etc.

What it is that leads to high trust societies (cultural homogeneity, common religious convictions) however, is an unpopular area of studies.

This also explains why otherwise highly educated people fall for advance-fee fraud scams--it's not just greed. Implicit trust in others is what has made them successful in the first place."