It's impossible to rely on DNA evidence to prove one identical twin committed a crime. New research is hoping to change that. Photo: Getty Images

In today’s world of forensic science, the evidence usually points clearly to a single suspect. But what happens when the DNA fingers two people?

There are some classic alibis in criminal history.

Being overseas, or at a friend’s house, or at work late have all been used, alongside blaming it on the butler.

But how about “my evil, identical twin did it”?

Turns out it can be a pretty solid defence.

For most of us, our DNA sequences are unique, with half coming from our mother and the other half from our father.

Identical, or monozygotic, twins are different.

Created when the same egg splits in half inside the womb, their genetic profiles are almost impossible to tell apart.

While their fingerprints are individual, from a legal standpoint it is impossible to prosecute one twin for a crime that relies solely on DNA evidence.

You simply cannot be sure which twin the sample came from.

“It pops up surprisingly often, I wouldn’t say daily but take, for example, we’ve got over 190,000 DNA profiles on the databank and a few hundred of those are identical twins, and that’s sets of twins.

Rebecca Richards, an Auckland University PhD student, is trying to solve the dilemma with the help of ESR’s forensic biology department.

During the next three years she will research DNA methylation, a modification of the DNA molecule that can change how genes are expressed but not their underlying DNA sequence.

It’s believed that, as they age, a person’s lifestyle, diet and living conditions, along with other environmental and geographical factors can subtly change their methylation markers and therefore potentially allow each twin’s DNA to be identified.

After doing some preliminary work as part of her masters degree, Richards’ next step will be to find volunteer sets of identical twins who will provide swab samples.

Then she will decide on a set of lifestyle markers, which could range from smoking through to more minor differences such as allergies or medication, and test them for variation in their methylation levels.

“We’ve found from previous research that they don’t even need to be that extreme in different lives. Even the fact that they’re living in different places, eating different things, over time they start to differ in these markers of their DNA.”

So how often does this problem come up in New Zealand?

Police couldn’t say exactly how often they’ve dealt with a twin DNA conundrum, but Kate Stevenson, a senior scientist in ESR’s forensic biology department, said it’s more common than you’d think.

“It pops up surprisingly often, I wouldn’t say daily but take, for example, we’ve got over 190,000 DNA profiles on the databank and a few hundred of those are identical twins, and that’s sets of twins.

“We don’t always find out if a case gets dropped, but I’ve certainly had conversations with police where they’ve said DNA is the one bit of evidence that we’ve got and if we can’t differentiate there’s nothing further we can do, but that’s not to say they haven’t gone on (and solved it)”

Auckland University PhD student Rebecca Richards is hoping to develop a method to tell the DNA of identical twins apart. Photo: Keshni Rasanayagam

Currently the methods used to differentiate twins’ DNA are not robust enough to stand up in court, which makes this research important.

If police are unable to find any evidence apart from DNA in such a case, they were essentially out of luck, she said.

There was other research being conducted globally where they were looking to find individual differences in identical twins DNA sequence, but that was like looking for a “needle in a haystack”.

“The research we were wanting to do was ‘is there a more targeted approach where we can apply it to casework we encounter?’

“Although there’s research going on it’s really important for ESR and other forensic organisations around the world, in order to meet our high quality standards, that whatever method we use it has to be robust enough to stand up in court.”

Until then, police will be hoping the country’s twins behave themselves.

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