Whether it is a stash of diamonds or a sum of cash, the law says you are required to surrender valuable finds to the police.

So why are you more likely to hand in something worth thousands than the $50 note you find at the bus stop?

Earlier this year a Cairns woman cleaning her apartment at the end of her lease came across a stash of diamonds worth thousands of dollars. She handed them in to police and the rightful owner was found.

Last year a Nanango man found tens of thousands of dollars inside a timber cabinet he purchased from a recycling market and handed the find to police.

What drives these people to hand in their finds — what is the science behind their honesty?

A stash of diamonds was this year handed in to police after a Cairns woman found them while cleaning her apartment. ( Rio Tinto Diamonds )

"Honesty can be understood as the practice of telling the truth, or in other words, to avoid lying, cheating etcetera," said Dr Wendy Li, a senior lecturer in psychology at James Cook University.

"But one of the reasons people lie is to benefit in some way.

"So when we look at cases [where people find valuables] and don't hand them in, the benefits seem obvious — if you can quietly keep them."

The sticking point of the seemingly obvious benefit, Dr Li said, is being able to keep quiet about your find and do so with a clean conscience.

Theft by finding

Laws regarding your obligation to hand in valuables to police vary in all Australian states and territories, but the central tenet is that 'finders, keepers' does not fly with the law.

In Queensland, Victoria and South Australia failing to hand in a valuable find could see you charged with theft, in New South Wales it is classed as larceny, and in Western Australia the charge is stealing.

The threat of being caught acting illegally is one of the deciding factors for many people's level of honesty. ( ABC News: Demi Lynch )

Senior Constable Russell Parker said handing in valuables to Queensland Police had two benefits: you avoid breaking the law and you may eventually get to keep what ever you have found.

"You must hand found property in and allow the police a reasonable time to try and identify and locate an owner," Senior Constable Parker said.

"After about eight weeks, if we haven't located the owner, then in most cases the finder will be the new owner."

Weighing up the cost of (dis)honesty

According to Dr Li, most people who find valuable items, whether consciously or subconsciously, conduct a sort of mental cost benefit analysis.

"The reward or benefit of handing valuables in may include avoiding being charged, which is kind of a big deal for most people," Dr Li said.

"Then there's the self appraisal of being a good person, feeling good and sleeping well every night because you're not worried about being caught in the future."

At the opposite end of the same scenario, the cost of being dishonest can wrack some people with guilt, and has potentially devastating consequences.

Psychology lecturer Dr Wendy Li says people are more likely to be honest if they know there is a chance their actions are being monitored. ( ABC News: Sam Ikin )

"Maybe there were CCTV cameras monitoring the area you where found the item, maybe there were people around," Dr Li said.

"And also you need to tell your family members. You need to tell the story of how you found a large amount of money, for example, and whether your family supports your decision.

"Ultimately if you are worried about being caught one day then you could develop an anxiety disorder."

What about the 'pineapple'?

Dr Li said the reason many people would simply pocket a $50 note found at a bus stop or on the street, again, comes back to their psychological cost-benefit analysis.

"People may think the chances of being charged over $50 are much less than in the case of thousands of dollars," she said.

"And having to spend time going to the police station to hand it in means the cost of being honest may be outweighed by the reward of keeping it.

"If nobody knows, there are no witnesses, no cameras and you don't have to explain your find, you're more likely to be dishonest."