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Criminal cases

21

feb
2016

In Criminal Cases

By - Mr. Berndsen

Dna-match doesn't make a perpetrator yet*

On 21, Feb 2016 | In Criminal Cases | By - M. Berndsen

When someone's dna is on the handle of a murder weapon, the step to a conviction is quickly made. Dna traces at the crime scene are also sometimes conclusive evidence. However, recent research shows that such dna can be from an innocent third party. This raises the question of what exactly the value of a dna match is.

Dna evidence can remove doubt by supporting or disproving other evidence. Legal errors can thus be averted or - afterwards - still be rectified. But all this depends on the way dna evidence is used. Contrary to popular belief, a match in itself does not say much about the perpetrator.

What exactly does a dna-match prove? Strictly speaking, a match does not mean more than that the cell material found (in all probability) comes from a certain person. But that doesn't answer the most important questions. What really matters is what kind of material (skin cells, saliva, hair, etc.), how and when that cell material ended up there and - most importantly - whether that has something to do with the crime.

Thus, a dna-match only acquires meaning in the context of the other evidence. When a woman is found dead in the marital sponde, it will of course not be difficult to find a complete dna-match with her husband. After all, all kinds of biological material of her husband will be found on the woman's body. This material is therefore not necessarily suspect. Material from a stranger might be. Unless that material has an explicable, legitimate origin. But that's exactly what the lady in question can't tell us.

And that's how the dna traces found must be interpreted. By people. In a subjective way. On the basis of incomplete information. The more context information there is, the better the meaning of a match can be understood. But unfortunately this also applies the other way around.

That's where the shoe pinches. After all, the rapidly growing dna database logically leads to more 'cold hits' - dna-matches with people who weren't yet in the picture. Think of a case in which few traces are found. On the victim's clothing, however, there are some cool head hairs. One of them is according to a dna-match of yours. Then you will undoubtedly be arrested for questioning and further investigation. It is to be hoped that you have an explanation for that hair, and preferably also an alibi. So what's the significance of that match? That verdict is up to the judge who finds you.

Convictions based solely on found dna material unfortunately occur with some regularity. Neither the law nor the Supreme Court forbids this. If you are innocent but are unfortunately convicted, it is questionable whether your innocence will ever become plausible. After all, there is a dna match. This is an important task for counsel.

Recently, it has been shown that dna of innocent people can even be found in a suspicious place via a detour. The University of Indianapolis (US) investigated the transfer of dna material from third parties. In the study, subjects shake hands for two minutes. Then one of the two - let's call him 'the culprit' - took up a knife. The knives were then examined for the presence of dna. Most people, including judges, would expect the dna of 'the guilty' to be found on the knife. But what does it turn out to be? In 85% of cases, the person who had not touched the knife for a moment, but only shook the hand of 'the culprit', was found to have dna on the knife. This dna material was therefore transferred to the knife by shaking hands. In about 20% of the cases the dna of the innocent third party was even the only (or most important) dna profile on the knife. This research shows that the rock-solid confidence of many in the evidential value of dna needs some nuance.

As said, there are expected to be more and more dna-matches. Matches with innocent people, and matches with guilty people. But this does not answer the question of who belongs to which group. These developments call for a reconsideration of the value of a match. To prevent wrongful convictions, judges should be cautious when interpreting dna evidence. In particular, it seems to me advisable not to base a conviction on dna evidence without solid evidence of support. Only dna is too shaky a basis for a conviction. Cellular material can, as has been shown, originate from an unsuspected angle. It is always the context that determines the value of a dna match.

 

* This article is an adaptation of the article 'Je liegt, wat doen je dna dan op de PD?' by mr. M. Berndsen.

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