DNA Evidence: a silver bullet for convictions?


It’s common to hear the media report that the Crown has DNA evidence that means that DNA found is “4 billion times more likely to have come from the accused than a random person” or similar. Often, people mental equate this position to one of clear inferred guilt. After all, four billion to one are long odds indeed. Jae Gerhard, principal at Independent Forensic Services, reminds us that such calculated statistics are not, in fact, the “odds that the person did it”- rather,like any evidence, it is capable of being interpreted in a variety of ways. The key aspect to DNA in terms of it’s potential in court is the effect that hyperbolic sounded calculated statistics and complex science can provide a veneer of irrefutable objectivity, when the reality is that like any evidence tendered by the Crown, it can and often ought be subject to testing by defence counsel for various subjective biases, misinterpretation, contamination and simple error. We will address two of the more significant issues identified by Independent Forensic Services in relation to the current situation with DNA evidence as utilized in criminal prosecutions.

The Type of DNA is Crucial:

The mere presence of the DNA purported to be that of the accused at a crime scene will often be incapable of giving rise to any inference of guilt; this is especially true where the DNA sample is irrelevant to the scene- while blood may be relevant where the crime is a violent struggle, or semen if the crime is sexual in nature, other forms of DNA may well be entirely irrelevant. Indeed it may not even be able to be taken as far as demonstrating that the accused was present. Transference of DNA can occur; this may be where two people hold the same glass at a party, and person (2) later opens the door to a house and leaves a sample of person (1)’s DNA on the door knob. It is essential, in that sort of instance, to get evidence with respect to transference before a jury as, on the basis of the common “four billion to one”, that the jury will otherwise here, it sounds tremendously likely that person (1) was, in fact, at that house.

The Number of Contributors:

DNA, particularly insofar as a violent, high-traffic, crime-scenes are concerned, will often result in what is called a “mixed profile”: that is a profile containing DNA from two or more contributing people. Significant difficulty can arise when this evidence is assessed and the number of contributors ought to be the subject of rigorous verification when such evidence is produced by the Crown. In the first instance, Independent Forensic Services have found that 76% of “4 person” mixtures, will appear to be 2-3 person mixtures in the context of the current police DNA assessment procedures. The importance of this cannot be understated.

Take the example that a murder had been committed. For arguments sake, assume the innocence of the accused, but the police having identified him as a key suspect very early due to known anger towards the victim. The combination of cognitive bias and weak forensic procedure may well result in evidence as follows:

  1. Mixed profile DNA with two contributors was found at the crime scene;
  2. There is a clear suspect, being the accused;
  3. It is 4 billion times more likely that the accused was a contributor than a random person;
  4. It is 7 billion times more likely that the victim was a contributor than a random person.

On the basis of the above, were the evidence allowed to be presented as such unchallenged, then logically, one would be defending only on the basis of alternate hypotheses as to how the defendant’s DNA ended up at the scene. The weakness, obviously, being the lack of other DNA from a potential “alternate killer”- it’s a case theory that will be difficult from the start. The logical assessment of that evidence, taken unchallenged as is often the case with DNA, appears to be a clear inference of guilt.

Imagine, however, that the number of contributors is re-checked by an independent analysis and determined to be four contributors rather than two. In the first instance, the innate scientific credibility attached to Crown DNA evidence is injured in the eyes of the jury. More importantly, however, you have two additional persons with DNA at the crime scene; as long as those remain ‘unknown persons’, then you indeed have a number of immediate and simply hypotheses consistent with innocence.

Verifying Crown Evidence: A Worthwhile Exercise. 

It is not uncommon that the Crown case appears insurmountable and incontrovertible at first blush. It is equally common, that the seemingly watertight declarations and identification begin to fall apart at the seams once a thread is pulled and scrutiny is applied; either being inconsistent or entirely consistent with a hypothesis consistent with innocence. DNA evidence can be treated similarly; the frightening complexity of some aspects of the science can be dispensed with by way of engaging the appropriate experts at an early stage. Items can be subject to independent testing, and reports can be prepared for the defence. As DNA becomes more commonly the linchpin of criminal prosecutions, then it becomes commensurately more important to subject that evidence to the same dispassionate cynicism with which any declaration is treated; feeling as though you need to plead guilty because the “police have DNA” ignores the fundamental unreliability and inbuilt biases within the reports as they are tendered to the court.

Culshaw Miller Criminal Lawyers are available to provide a case review at no cost. Contact us on 8464 0033 or book a review online for a response within 12 hours.