Why do judges keep letting bad science into criminal trials?
By Rhonda Wheate - Posted on 22 July 2021
Relying on past decisions made by other courts – called "precedents" – is a handy shortcut for the UK legal systems and similar systems in the USA, Canada, Australia and New Zealand. It saves running the same arguments over and over again, it helps keep court decisions consistent over time, it reduces trial running-times, and helps lawyers advise their clients about how their case might be decided by a court. Precedent means that cases already decided by higher courts ‘bind’ any subsequent cases heard in lower courts, or courts at the same level. So a decision of the Supreme Court must be adhered to in all similar subsequent cases in that system, unless the decision has been modified by parliament, or the lower court can successfully ‘distinguish’ its current case from the decision in the precedent.
Unfortunately, this means that where a court allows poor quality evidence into a trial, such evidence is likely to be used again and again in subsequent cases. There is a further way to ‘game the system’ of precedent though, and some prosecutors use this for emerging or controversial forensic science techniques that are very convincing for juries but might have unproven or highly contentious scientific credentials. Here the prosecution is able to wait for a case where the defence is poor (underprepared, under-resourced, or incompetent) and will present the sub-standard forensic science evidence to the court in that case. Because the defence is underprepared, under-resourced or incompetent, the poor quality forensic science is not challenged; it is accepted by the court as evidence against the accused.
This means that in subsequent cases, even if the defence is stronger and starts asking good questions about the validity and reliability of that type of forensic evidence, the prosecution can say – hand on heart – ‘well this evidence has already been accepted in X number of cases, your Honour’…
This is a very persuasive argument for allowing the evidence into the current case, because… it is precedent. So even though the forensic evidence might be unreliable, invalid, poorly executed, or totally without scientific merit, the defence will be hard-pressed to get it excluded from the case against their client. Judges are far more likely to follow the decisions of other judges in other cases and admit the evidence.
Sound far-fetched? You might be surprised to learn that several important scientific bodies have repeatedly found that much of the ‘forensic science’ we take for granted in not actually scientific. For instance in 2009 the National Academy of Sciences (USA) found:
“[M]uch forensic evidence—including, for example, bitemarks and firearm and toolmark identifications—is introduced in criminal trials without any meaningful scientific validation, determination of error rates, or reliability testing to explain the limits of the discipline.” (at page 108)
And more recently the President’s Council of Advisors on Science and Technology (PCAST) found:
“…expert witnesses have often overstated the probative value of their evidence, going far beyond what the relevant science can justify. Examiners have sometimes testified, for example, that their conclusions are “100 percent certain;” or have “zero,” “essentially zero,” or “negligible,” error rates. … such statements are not scientifically defensible” (at page 3)
This has become a problem even with what judges and juries might consider to be ‘gold standard’ forensic science such as DNA profiling evidence.
How can the prosecution get away with this? They have the advantage of being repeat players in the game of criminal law, whereas your average criminal has nowhere near the same collective experience and resources. Even with big budget cuts to the prosecution, police, and forensic laboratories, the prosecution remains THE repeat player, with an ever-replaceable cast of lawyers, police, laboratories, and expert witnesses on hand. In contrast, each accused person is little more than a one-shot player, unaware of and unable to control the behaviour of the prosecution over millions of cases and hundreds of years. The final piece of the puzzle is that judges are bound by precedent, with one judge explaining:
“The problem for the defense is that every single court...has admitted this testimony…I reluctantly [admit the evidence] because of my confidence that any other decision will be rejected by the appellate courts, in light of precedents across the country, regardless of the findings I have made” (at page 108 of the 2009 American report)
Cherry-picking cases to insert unreliable forensic science into the criminal law is an abuse of the system. So what is the solution? It isn’t to ban the use of scientific evidence in criminal trials, or even to ban the use of new, controversial or untested scientific evidence. The fact is, forensic science experts know things that we don’t know, and that’s makes them extremely useful in criminal trials because victims deserve justice, and the liberty of an accused person hangs in the balance. Nor is the problem solved by getting rid of our system of precedent, which has served us well for centuries. Rather, we need to make sure that the defence is properly equipped to hold the prosecution to account in every case, so that judges are able to make sound decisions, and poor quality expert evidence is no longer allowed to slip through the cracks and into the criminal law.
Further analysis of this and other issues can be found in a co-authored article by Professor Donald Nicolson and Dr Wheate in the Juridical Review: ‘The Young grandfather: a new approach to the reliability of expert evidence in Scottish criminal cases or business as usual?’ 2020 Jur. Rev. 111.