Would you tell a shop assistant if they gave you too much change? How your answer will reveal if you - like most people - suffer 'unethical amnesia'

Done any good burglaries lately? Mugged anyone? Stripped some lead off the church roof in dead of night? Of course not! You're respectable. We're all respectable. Decent middle-class folk, appalled by the actions of criminals, dodgy banks, tax fiddlers and benefit cheats.

On the other hand, isn't that your firm's logo on that pen? And weren't you chuckling about the discount your builder gave for cash and the faulty gate at the train station that meant you didn't have to pay?

And was that lunch receipt really for entertaining a client? Looked like your friend Chloe to me. Come clean with the spouse about those daft saucy texts, did you? No? Ah well, least said soonest mended: we are, it seems, in an epidemic of 'unethical amnesia'.

A study has shown that even people who consider themselves honest cheat on taxes, steal from the workplace, illegally download music, have extramarital affairs, use public transportation without paying and lie

The phrase was coined by researchers led by Maryam Kouchaki of Northwestern and Francesca Gino of Harvard universities in the U.S. Their study, published in the Proceedings Of The National Academy of Sciences, looked at attitudes to common cheating and involved 2,100 people from different backgrounds.

'Many who consider themselves honest nevertheless cheat on taxes, steal from the workplace, illegally download music, have extramarital affairs, use public transportation without paying, lie and so on,' they concluded.

But, here's the fun of it, in various experiments they detected a psychological mechanism that enables people to forget the details of their bad behaviour.

They deduced that because even small acts of dishonesty cause us guilt and remorse and most of us value morality, we blank it out. The need for a positive self-image is so strong we are less likely to remember the bad stuff.

I love that theory. I recognise it in myself - more of that later - and in news stories. Take the absurdity of expenses fiddling by MPs and peers, who have sworn public oaths to serve the nation.

Or think of self-righteous celebrities talking up Left-wing virtue then signing up to legal, but unethical, tax fiddles. Or that wealthy hedge-fund manager who, in 2013, was caught defrauding the railways of £43,000 over five years by exploiting an Oyster travelcard loophole instead of buying a season ticket.

Libby Purves was brought as a convent girl from a Scottish Presbyterian background and says that she could no more have swiped a pick 'n' mix sweet than flown out through the window

Many such people have been publicly exposed and their inner moral compass pronounced faulty: a tough thing to face. But most of us, as the psychologists point out, just have a tiny private wince then mercifully forget and go on feeling that we are - as Tony Blair once put it - 'pretty straight sort of guys'.

The whole subject is fascinating, uncomfortable and provokes interesting reflections on how our ethical compasses work.

Of course, everyone is different. I am always amazed by famous people who cheerfully admit they shoplifted as teenagers and still think it a giggle.

As a convent girl from a Scottish Presbyterian background, I could no more have swiped a pick 'n' mix sweet than flown out through the window.

It was mainly fear of discovery; my husband concurs, shivering with horror 50 years later at the thought of what his Yorkshire granny would have thought if he'd disgraced the family that way.

But some normally virtuous friends disagree, often because of a vague Communistic feeling that 'property is theft' and that big corporations rip off cash-poor children and teenagers all the time. So they aren't sorry about those youthful peccadilloes.

'But, of course I'd never ever have pinched anything from the little corner shop,' says one. 'That would be real stealing.' Illogical, but emotionally understandable.

Even as a lifelong non-shoplifter, if Harrods gave me too much change I might not bother to notice, vaguely feeling the oligarchs' mecca could afford it. If the local shop did, I'd return it.

So the moral compass swings all over the place, because it is making judgments not only about its owner but about the world in general. Starbucks doesn't pay enough UK tax? So, if they give you an extra fiver in change you might just pocket it, muttering: 'Serves them right!'

Or suppose you're a student working in a sandwich bar and your employers used to offer a free lunchtime panini, but stopped when the minimum wage went up. Mean pigs! Somehow the paninis keep coming, unnoticed by the till, and your supervisor turns a blind eye as she feels just the same.

Libby Purves says that she could never shoplift, despite some of her friends freely admitting that they used to do so as teenagers (stock image)

Offices can be hotbeds of resentment, which may explain the wholesale pinching of stationery, folders and printer cartridges at the basic level and fiddled expenses farther up the ladder.

'I work all hours,' moans the wage-slave. 'No one offers me overtime pay, my boss is a bully and the HR chief doesn't listen.

'So who cares that I took the bus to that assignment? I've got a stack of blank taxi receipts, I'll just fill them in with my left hand and I'm twenty quid up.' Millions of such tiny frauds happen every day, adding up to extra costs for employers. And I suspect often it is because workers feel it is a deserved punishment.

One famous philosopher and moralist used to 'fine' the BBC whenever it had annoyed him by artful adjustment of his expenses.

Often, of course, it's just greed and a petulant sense of entitlement. The higher up you go, oddly, the more likely is that grandiose sense of entitlement.

Sometimes it is technically legal, but morally indefensible, such as the £250,000 boss whose £8.99 donation for a subordinate's leaving present is put on expenses.

Sometimes there's a culture too strong for individuals to resist: in the strange old days of high-rolling newspapers, I had a three-week journey through America for a Sunday supplement that equipped me with a fistful of dollar traveller's cheques.

When I got back and presented my accounts, I shamefacedly said they were 350 dollars adrift, and I had no idea what I had spent them on. The editor looked at me as if I were mad and snapped: 'Sundries!' So I wrote it down.

At the BBC (again, decades ago), a news editor sent me to record a report on buskers on the London Underground. When I put in my expenses, all Tube fares, he furiously said: 'I don't acknowledge that reporters should take the Tube. Put down some taxis.' I tried to explain, but gave in.

And I can tell of one reporter in those heady Seventies days who fiddled his expenses for years, but then suddenly 'found God' and tried to pay it all back. He was told there was no mechanism to do this. He debated whether to give it to charity or buy licence-fees for fake addresses. I am not sure how it turned out, but we all gathered round with suggestions.

I doubt there is anyone who has never taken advantage of that unethical amnesia the psychologists described.

Examining my own conscience, I can say that in the Nineties I printed out a copy of my third novel in someone's office, not caring that printer ink is as expensive as good brandy.

And that there have been times when trains have been so crowded, late and dirty that I have happily ignored the fact that the guard wouldn't come round and notice I was not on the service specified on my bargain ticket.

As for theft, in the Seventies, we all used to pinch the hard, nasty Bronco toilet paper at the BBC's Bush House headquarters, simply because every sheet was stamped 'Government Property'. We found this hilarious. A few years later, they changed to soft paper, and we stopped bothering.

In the car yesterday, musing on whether I had any more crimes to confess, I ended up driving away from a country petrol station without paying.

Two minutes later, a desperate U-turn and grovelling apology. But the moment of ghastly realisation and shame came straight from my Presbyterian forebears.