Robert Michael, from Victoria University, has found that witnesses' confidence in their memories can be affected by whether they are asked easy or difficult questions first.

Common police and court practice could be unwittingly manipulating witnesses' memories, new research suggests.

It has long been know that witnesses can be led to recall events incorrectly – such as by interviewers using leading questions – but the latest study by a Victoria University PhD student suggests just how easily a miscarriage of justice can happen.

Even subtle differences in interview practices, such as whether a witness receives easy questions followed by harder ones, or vice versa, can shape a person's confidence in their memory, Robert Michael has found.

"Typically what we find is the people who start with easy questions are more confident," he said. "On the one hand it might not seem surprising, but on the other hand it is, because everyone is answering the same questions.

"That was alarming.

"You're meant to be getting at the memory. You're not meant to be manipulating it in any way."

The confidence effect when beginning with easy questions also carried through to a mock juror who was asked to evaluate that witness's credibility, the psychology student said.

The unknown manipulation of witnesses' memories could lead to tragic consequences, both through innocent people being jailed, and criminals going free.

"There's a big problem in the justice system with wrongful conviction. It's coming to light more and more lately with all the DNA evidence," he said.

"A big part of the problem is eyewitnesses who mistakenly identify the wrong person. It's a big surprise to a lot of people, though not to those who work in the field of memory, because we know and have known for a long time that memory is fragile and is all too easy to distort."

In 2007, Christchurch man Aaron Farmer was freed after serving two years of his eight-year prison sentence after alibi and DNA evidence showed he had not committed the rape for which he was convicted. The witness testimony of the victim was central to the police case, with the woman telling the court she was "90 per cent sure" Farmer was her attacker.

Crown prosecutor Grant Burston said it was typical for witnesses in court to be introduced with "settling questions", whether they were a prosecution or a defence witness.

"You try to get them comfortable in the witness box ... It's simply polite and makes sense, and is a natural thing that everyone understands in the courtroom.

"I doubt that it would have any impact on the jury's perception of whether or not they should rely on the witness or not."

In his first experiment, Michael showed mock witnesses a six-minute video of the simulated crime of an electrician rummaging through a client's house.

After a short break, they answered 30 questions. Half were asked the easy questions – such the type of fruit the electrician picks up and eats, an event prominently shown in the clip – first.

"An example of a difficult question might be something like how many pillows were on the bed in the second bedroom he visited, something that's in clear view for a couple of seconds but is not something people pick up on," he said.

In a second experiment, Michael found "jurors" thought witnesses who started with easy questions got more of the 30 questions right than the others. These findings confirmed the adage that first impressions counted, he said.

"You don't even need suggestive or leading questions. All you have to do is change the order in which you ask a bunch of questions and suddenly you can have an eyewitness who is confident and persuasive ... It's something that no-one's really considered until this point."

Michael said although the findings were concerning for the judicial system, more work was needed to see if the effect lasted long-term or affected eyewitnesses' actions, such as confidently picking a person in a line-up.

A WITNESS'S EXPERIENCE

Nicola Boughtwood of Brooklyn, Wellington, witnessed a fatal crash, and was called to give evidence at a coroner's inquiry more than three years after the incident.

"I had a good view of what happened. The police must have contacted me within the day and I went in and gave quite a comprehensive witness statement to a police officer.

"That was definitely important because I then had that to refer to, when it came time, three years later, to give my evidence.

"Certain things were strong [in my memory], and I think they were important points, but I definitely needed to read that witness statement to really feel confident.

"I was quite conscious [of how memory can change]. Even over days it can change, especially if you haven't had it recorded.

"The details I remembered weren't so hard to get wrong, in a way. They weren't what someone was wearing or what someone looked like, or what the number plate was or the car height. It was directions people came from, so not so tricky."