"I'm starting to think that fitting in socially is one of the prime human imperitives, genetically based, probably. ..."
Exactly, Satanus...
Think "small tribal groups" and sabre-tooth cats... or dire wolves... or cave bears...
Or, in more modern times, think lions, cougars, leopards, tigers... or timber wolves; packs of them... or grizzles, or polar bears, or kodiak bears, or even a vicious mamma black bear...
And then there was all the inter-tribal warfare, after many of the large predators were pushed into more remote areas...
Travelling in groups - packs - herds - comes as natural to humanity as walking upright... And using opposable thumbs... (Which makes the old "Marlboro Man" image laughable in the extreme... A fantasy that really never existed...)
And in the past, expulsion from the 'herd' was often a death sentence.
That's why shunning is so effective - so vicious, so cruel... It goes against all human instinct. Shunning is basically a form of solitary confinement - or at least, it's supposed to FEEL like 'solitary confinement', if JWs are doing it the way "Mother" wants them to...
Zid