Bad apples ... scanning carrots as bananas and save $12 a kilo.

Bad apples ... scanning carrots as bananas and saving $12 a kilogram.

You wouldn't dream of shoplifting a litre of milk or a bunch of bananas from your local supermarket, would you?

That's stealing, after all - a disgraceful, juvenile habit you left back in high school, when the pack you ran with sported mullets and sucked on Winnie Blues.

I'm not proud with [the] banana swap but I don't steal; at least I don't do it now, as an adult 

But have you ever found yourself staring at one of the increasing number of self-service screens, with $15 in your pocket and the temptation to put your $13.98-per-kilo* lady fingers through as $4.98-per-kilo turnips?

Have you ever thrown a fist of garlic in with the onions - after you'd weighed the bag - or punched in green beans ($4.98/kg) instead of snow peas ($7.98/kg)?

Simone, a shopper leaving Coles at Pyrmont yesterday, admitted to frequent abuses of the self-service grocery lane.

"With all the fruit and vegetables I'll just put it [in] as the cheaper [variety]," she said.

"Or with a box of drinks or something I'll often put it on the floor, then take it after I've paid for all the other things."

The supermarket giants, Coles and Woolworths, deny their "shrinkage levels" - industry speak for theft or the loss of goods - have risen since rolling out the technology in 2003.

And while other shoppers we talked to said they had never called their royal galas ($5.98/kg) pink ladies ($2.98/kg), Simone is by no means alone.

A forensic psychologist who specialises in assessing shoplifters, suspects a "large chunk" of the population would do it once or twice.

"I think people who do it regularly would be different but some people will do it just because they're hard up on the day," Dr Christopher Lennings said.

"They've gone out with $20 in their pocket and they've got a $25 shopping bill so they're trying to squeeze."

Coles and Woolworths said they did not have figures on theft levels since the introduction of self-service checkouts so we consulted a few more shoppers.

A Sydney accountant told us he had, on more than one occasion, put a hand of bananas through as carrots since the price of bananas spiked.

"Roughly it's about a $7 to $8 difference, when bananas [were at their] peak," he said.

He would never consider hiding something in his pocket if he went through a regular checkout.

"That's stealing," he said.

"I'm not proud with [the] banana swap but I don't steal; at least I don't do it now, as an adult."

Fuzzy logic perhaps, but it is a very common justification and quite separate from a shoplifter's mentality, Dr Lennings said.

"It's called shame avoidance," he said.

"They don't want to be seen as a thief so, therefore, if they make some kind of contribution to the purchase, it's not as bad.

"It's a shame-based motivation for fudging the process ... They're minimisers rather than people who go out and steal."

Traditional shoplifters, on the other hand, steal for a thrill, for the hell of it, or because they have to.

"[Self-service cheats] don't like the idea of stealing or being thought of as a thief," Dr Lennings said.

"This idea is OK because they're substituting and paying part of the cost of the goods."

A Coles spokesman said theft or fudging at self-service checkouts was not a problem across the 190-odd stores that had been fitted with the technology to date.

"The vast majority are honest and do the right thing in regards to self-checkout," he said.

The retail giant prefers to trust its shoppers, he said.

"It's part of a broader philosophy that new management in Coles brought in when they joined the company a couple of years ago.

"They had security barrriers at the front of the store that you had to walk through before you could enter.

"Our new general manager took the view that 99 per cent of customers did the right thing and why would you treat the vast majority like criminals all for the sake of that 1 per cent?

"So he removed them ... we've seen no discernible affect on store theft as a result of doing that."

Similarly, Woolworths - with an average of five self-service checkouts each across 400 stores - is reluctant to admit any significant chunk of its hard-won customer base could be taking advantage of the new technology.

"It does happen occasionally but we have a number of loss-prevention measures in place to prevent it," a spokeswoman said.

One attendant is usually stationed in the self-service area and surveillance cameras are also in place.

"We trust that overall our customers will do the right thing."

Update at 2.30pm:

We have received a few reader comments regarding the machines' temperamental technology. 

Woolworths said it had been "working hard behind the scenes to improve IT systems".

"Over the past six months we have delivered several initiatives resulting in a 68 per cent reduction in the number of supervisor interventions, which is great news for customers," the spokeswoman said.

Coles said it continually fed back customer experiences to the manufacturers of the technology but it was not the sort of technology that could be upgraded every six months.

"We make sure the team members that are managing the checkouts are there to help," the company's spokesman said.

Do you use the self-service lane? Have you ever fiddled your bill or fudged your fruit varietals? Confess below.

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*Fruit and vegetable prices obtained from coles.com.au on August 3.