Courts generally accept DNA evidence these days. Defence lawyers have attempted to get the evidence excluded for time to time, but without a great deal of success.
Reports this week of the great success of DNA testing of Victorian prisoners have revived questions about civil liberties and also questions about whether the ACT is doing as good a job with similar legislation.
Since compulsory testing of prisoners began in Victoria in 2000, authorities have linked 99 prisoners with 238 previously unsolved crimes, including violent rape and murder. Many are yet to be charged tried and sentenced.
The question is whether people (even prisoners) should be forced to give DNA samples. On this question, intrustiveness can hardly be a question because it can now be done through saliva swabbing. There are also the civil liberties of others being investigated and under suspicion who might be exonerated. And there are the civil liberties of victims.
The gathering of evidence to prove guilt or innocence of crime shoud be encouraged to make society safer, provided the means do not corrupt the ends – such as coercion, torture and the like. DNA evidence is just another tool in the prosecution or defence armoury, but it has some peculiar problems.
The DNA test itself has become very refined. DNA samples can be extracted from ever smaller samples of body tissue at crime scenes and a ever-wide range of body tissues – hair, skin, fingernails, saliva and so on. The testing, if the sample is big enough, is providing ever more conclusive results. Typically, scientists are saying that there is only one chance in between 100 million and 90 billion that tissue X (from the crime scene) came from some other randomly chosen person rather than the suspect whose DNA matches the DNA at the crime scene. That level of probability tantamounts to certainty. The probability varies according to the amount of the sample and the testing method.
Even so trial judges still direct juries in a way that – on its own — leaves far too much doubt in favour of the accused. He is an example. An Australian judge told a jury in the past year: “”Now if the profiles match it does not necessarily mean he must have been the assailant. If you accept the evidence of the scientist the chances of somebody else though unrelated to the accused is less than one in one hundred million. But nevertheless it is not an absolute certainty. So understand that. The scientist does not come in here and say “I’ve got this result therefore it must have been the accused who produced that spermatozoa”. He is not saying that. The scientist is saying, “in my opinion they match, the chances of it not being the accused’s spermatozoa is less than one in one hundred million so far as somebody unrelated to him being the source of it’. Okay so that is all he is saying.”
That judge was leaving it open to the jury to find that the crime-scene sample was not that of the accused. With such odds, such a finding would be untenable, when the burden of proof is beyond reasonable doubt. The test it not beyond ALL doubt. If judges are summing up like this, the accused should have no civil-liberties on the score of matching the two samples.
In fact, once the match has been made, as a step in the practical task of assessing guilt, you must conclude that the two samples come from the same human being.
But that is not the end of the question. Proving the two samples come from the same human being is only one step in the conclusion of guilt. And this is one of the problems with DNA evidence. The experts giving the evidence on the single question of whether the two samples come come the same human get som impressive with their statistical odds, that juries might leap to a conclusion of guilt based on those odds. They get dazzled. That would be dangerous.
In the early days of DNA testing the highest a prosecutor could put it was that one could not rule out the accused being the same person as the person from whom the crime-scene sample came. But the law has now moved more in line with the science and the adds are allowed to be put to juries.
In criminal law, prosecutors are bound to act fairly. Often when questions of admissibility of evidence arise, the jduge has to ask the question of whether putting the evidence to the jury would be more prejudicial than its probative value – its value in affoding proof of the accused guilt. So including evidence from an acquaintance that an accused always sneered at a victim would be more prejudicial than its probative value. Evidence that the accused hung around the victims house on many occasions might be prejudicial but might have enough probative value to get in.
In the early days of DNA evidence, defence counsel often argued that the prejudicial nature of the evidence outweighed its probative value. That argument has now (quite rightly) been thoroughly squashed by a case last year in the NSW Court of Appeal. The judges ruled that the probative value of DNA evidence is very high and that properly instructed juries can understand it, and juries can understand that the utterly conclusive nature of one piece of evidence in the prosecutor’s armoury does not mean that guilt is conclusive. Properly warned juries are not dazzled. The accused is not unfairly prejudiced.
Another case largely junked another defence attack which would have required every last laboratory assistant to give evidence of every last detail of the testing method. Evidence of the main supervising person and the read-outs will be enough, as it should be.
The high probative value of DNA evidence, however, must not relieve the prosecution from establishing guilt, which needs more than the DNA evidence that sample A and B come from the same human. The prosecutor must show that sample A did in fact come from a pertinent part of the crime scene or victim’s body and that there was no other plausible reason for it being there. A hair proved by DNA to be that of a husband found on the clothes on the body of a murdered wife, for example, does not prove very much. It would be surprising if some of his hair strands were not there. A hair proved by DNA to be that of an accused found on a victim who is otherwise a stranger, on the other hand, has more probative value. If it is a semen sample, it would be so probative as to be almost conclusive, especially if couple with some other non-DNA evidence.
The prosecution also has to show that the crime-scene sample was not planted. That is going to be an increasing problem, given the ease of planting a small tissue sample. The importance of independent forensic oversight of crime scenes will become critical.
Often, though, the DNA evidence is so good, that when presented with it, accused people plead guilty. In the case of prisoners, DNA testing is especially effective because criminologists’ research shows that nearly all the crime is done by a tiny proportion of people. A prison is a rich picking ground for cleaning up unsolved crime with the armoury of DNA testing. Further, given police know about a few people doing most of the crime, they tend to hound known criminals for unsolved crime – DNA can help them prove their innocence. It can also help those who have been set up and convicted.