People measure their behaviour and beliefs by those around them, so could MPs have thought that their now discredited expenses system was entirely reasonable?
Of course you have your own moral code, of course you do, your own sense of right and wrong, just as MPs do.
So imagine you park your bike in a bike shed. A sign says: no graffiti. On your return, you find a leaflet stuck to the handlebars. What do you do with it? Chuck it in the street, or bin it elsewhere?
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That depends, says Ramsey Raafat from University College London, who describes a set of curious experiments in Holland.
"When the riders or owners returned to their bike, 33% of the people chucked the flyer, littered, broke a norm.
"But when there was a slight manipulation, everything's the same - we have our bike shed, bikes, prominent 'no graffiti' sign - but now there's graffiti in the area, so a norm has been violated. Now interestingly in this situation, a whopping 69% of the riders when they returned chucked the flyer. And so in this instance when one norm's violated - the graffiti violation - there's a massive effect on another norm of littering."
Is that a surprise? We've always known that behaviour is sometimes easily influenced. How else, you might have found occasion to ask, does the nice lad from the nice family next door become a lout in a mob, lurching, swearing, singing offensively down the road? Because his frame of moral reference temporarily stops at those around him. He sees no-one else.
Us and them
But group-think goes further than conformity. It also means rationalising decisions and behaviour that you might normally reject. Littering comes to feel not only convenient, but also - so you tell yourself - right in the circumstances.
Littering is bad, but can seem OK if everyone else has done it
Is it this sort of group-think that has allowed MPs to think their expenses system - in recent weeks shown to have been widely abused - was reasonable? Could it also distance them in other ways from the electorate?
In the exclusive neo-gothic henhouse that is Westminster, is it any wonder they flock together? When the rules - from tradition or otherwise - are idiosyncratic, where they arrive mystified and dependent on the kindness of the party whips to show them the ropes, where the career path depends on doing as others do
what would you do if not conform, and then, being clever, find smart reasons for doing so?
Much about Westminster conspires to make it separate. Much about the expenses saga suggests that MPs stopped seeing their world as it might look from outside the group. As many have observed, when they saw others doing it that made it feel all right. What was odd became normal.
Cash in post box
Danny Dorling, a professor of geography at Sheffield University, has been observing the parliamentary expenses scandal. With MPs' sense of reward and entitlement in mind, he says it's sometimes hard to get out of your head the idea that the privilege enjoyed by your group is anything other than normal.
It's entirely understandable, he says, that MPs think like this.
"It's not helpful to think like this, but you have to work very hard not to do it," says Mr Dorling.
If that's true, it reminds us that MPs are not only representatives, but people with group interests of their own who sometimes do lose touch precisely because they are a group. It also reminds us that in this respect they are much like everyone else. Hearing about the problems, the misery, the grind of the constituency cases that MPs are asked to take on, I suspect that if they can lose touch given their experiences, so can we all.
For a final, bizarre illustration of where our immediate social influences may lead us, Ramsey Raafat cites another experiment in group-think.
"Now stealing - if one was to steal, that's a powerful norm violation. We learn at a very early age not to steal. So how do they do this? This is very elegant. They had a post-box and sticking out of it was an envelope with a five-euro note attached. Now in the control condition - no litter and no graffiti - only 13% of people stole, took the envelope.
"However when there was graffiti or litter surrounding the post-box, a whopping 25% or 27% of people stole. That's more than a doubling of norm violation. And, again, it's a powerful effect of how when one norm is broken, we become more likely to break another, or essentially the spreading of disorder."
Today expenses, tomorrow your hubcaps? No. Not least because the world beyond Westminster has reminded MPs of the view from elsewhere.