Biologists and sociologists tell us that our brains evolved in small groups or tribes.

As one example of how profoundly the small-group environment affected our brains, Daily Galaxy points out:
Research shows that one of the most powerful ways to stimulate more buying is celebrity endorsement. Neurologists at Erasmus University in Rotterdam report that our ability to weigh desirability and value doesn’t function normally if an item is endorsed by a well-known face. This lights up the brain’s dorsal claudate nucleus, which is involved in trust and learning. Areas linked to longer-term memory storage also fire up. Our minds overidentify with celebrities because we evolved in small tribes. If you knew someone, then they knew you. If you didn’t attack each other, you were probably pals.

Our minds still work this way, giving us the idea that the celebs we keep seeing are our acquaintances. And we want to be like them, because we’ve evolved to hate being out of the in-crowd. Brain scans show that social rejection activates brain areas that generate physical pain, probably because in prehistory tribal exclusion was tantamount to a death sentence. And scans by the National Institute of Mental Health show that when we feel socially inferior, two brain regions become more active: the insula and the ventral striatum. The insula is involved with the gut-sinking sensation you get when you feel that small. The ventral striatum is linked to motivation and reward.

In small groups, we knew everyone extremely well. No one could really fool us about what type of person they were, because we had grown up interacting with them for our whole lives.

If a tribe member dressed up and pretended he was from another tribe, we would see it in a heart-beat. It would be like seeing your father in a costume: you would recognize him pretty quickly, wouldn't you.

As the celebrity example shows, our brains can easily be fooled by people in our large modern society when we incorrectly ascribe to them the role of being someone we should trust.