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I have reacted to the Telegraph's daily feed of different MPs' detailed expense claims with the usual mix of amusement, shock, disappointment and anger.

But from the beginning of it, I have had a guilty voice in my head asking me how I would myself behave if I was offered a temptation similar to that of our parliamentarians? A system that apparently encouraged large claims, validated the general sense that the money was there to be taken and which was hidden from public view.

It's hard for each of us to know how we would behave given the chance because we are not offered such opportunities very often. We might tell ourselves we would be very different to those scoundrel MPs - especially now we have seen the MPs being held to account.

Well it was this week that I remembered there was a lot of evidence on this subject. Behavioural economist has performed a number of tests on groups of people, offering them chances to benefit from cheating.

The evidence suggests that our MPs are not abnormally mischievous.

The message is that most ordinarily moral people will give themselves the benefit of the doubt on moral questions. They wouldn't dream of stealing even 10p, because they know stealing is wrong, but they would happily bend some rules to give themselves $100, if they could justify it to themselves.

We are more likely to cheat if we see others doing so. We tend to conform to accepted norms of reasonable behaviour, rather than adhere to strict rules. You probably won't have to think hard to find examples in your own life (from claiming on insurance, to paying in cash for a small building job).

The evidence is not that a few bad apples cheat a lot, it's that the vast bulk of us cheat a little.

And we have an amazing capacity to tell ourselves it is all right.

Professor Ariely - whose book, Predictably Irrational is an amusing and informative read on this and other human traits - makes the point that we should be tolerant of individual weakness in this situation, but harsh on the system that encouraged that weakness.

Unfortunately for the MPs, it was the MPs who made the system.

But the implication of Professor Ariely's work is that it is collective failure which is more significant than the personal failings of individual members.