Beware of forensic experts expressing certainty

Expert witnesses sometimes express certainty when there is room for doubt

It is generally understood that each of us has a unique fingerprint, and unique DNA. So, if a fingerprint or DNA sample is found at a crime scene, it should be possible to find a match that will assist in finding the perpetrator of the offence. But it is not always as simple as this. Forensic experts need to present this evidence in a manner that will not mislead the courts and risk miscarriages of justice.

The history of expert witnesses in miscarriages of justice is well known. The purpose of an expert is to assist the court in understanding specialist matters. If they present their evidence in such a way as to assist their own “side” of the case, they undermine their own position as experts.

For example, in the Dreyfus case in the 1890s, handwriting experts provided reports to suggest that Dreyfus had written a note containing military secrets that was sent to the German embassy in Paris, despite the fact that the handwriting more closely resembled that of another officer, Esterhazy (who later admitted to having written it).

In the case of the Birmingham Six in the 1970s, a chemist giving evidence for the prosecution said that he was “99.9 per cent” satisfied that two of the men had handled explosives, despite having used only a gateway test that produced no conclusive findings.


Even when experts carry out their work conscientiously, they may fall into the trap of believing that their results are more conclusive than is actually the case.

For example, after the 2004 Madrid train bombings, a fingerprint was found on a plastic bag containing detonating devices. It was entered into an international database, and was found to match the fingerprints of Brandon Mayfield, an attorney in the US state of Oregon. Mayfield had converted to Islam on his marriage to an Egyptian national, and the FBI kept his house and family under surveillance for months, before arresting him on suspicion of terrorist offences (despite the fact that he had never been to Spain). The Spanish police subsequently found that the fingerprints more closely matched another suspect, Algerian national Ouhnane Daoud. The FBI was very slow to admit its errors in the case.

In 2012, California businessman Raveesh Kumra died following a robbery in which he had been beaten up. DNA found under his fingernails was found to match that of Lukis Anderson, who was arrested for murder. Although Anderson had been in hospital at the time of the robbery, he spent five months in prison awaiting trial. It eventually transpired that Anderson had been brought to hospital by an ambulance crew about two hours before the robbery of Kumra’s house. The same paramedics had assisted both Anderson and Kumra, resulting in the accidental transfer of DNA, and the wrongful arrest.

Other cases have given rise to concern about the reporting of forensic evidence both in the US and in Europe. DNA results in the Italian case of Amanda Knox and Raffaele Sollecito case were found to have been incorrectly reported. In the Scottish case of police officer Shirley McKie, the taking of fingerprints by the police was heavily criticised following a lengthy government inquiry.

In the US in 2016, the President’s Council of Advisors on Science and Technology (PCAST) released a report entitled Forensic Science in Criminal Courts: Ensuring Scientific Validity of Feature-Comparison Methods, in which they wrote:

“Statements suggesting or implying greater certainty are not scientifically valid and should not be permitted. In particular, courts should never permit scientifically indefensible claims such as: “zero,” “vanishingly small,” “essentially zero,” “negligible,” “minimal,” or “microscopic” error rates; “100 per cent certainty” or proof “to a reasonable degree of scientific certainty;” identification “to the exclusion of all other sources;” or a chance of error so remote as to be a “practical impossibility.””

In Europe in 2015, the ENFSI (European Network of Forensic Science Institutes) published its Guideline for Evaluative Reporting in Forensic Science. This requires the interpretation of results to take place within the framework of the circumstances of the case. In particular, it requires that forensic scientists consider their findings in like of two competing propositions.

For example, fingerprint experts should consider the likelihood of the results they have found in light of, say, a prosecution assertion that the accused was at the scene of the offence and a defence assertion that the accused was elsewhere but that there are other explanations for the fingerprints. It would then be for the court to consider the results in the context of all the other evidence.

The ENFSI guidelines need to be adopted by the Irish courts when forensic experts are giving evidence. In the 2014 case of DPP v White, the Supreme Court dismissed an appeal in a case where a Garda fingerprint expert had said he had “no doubt” that fingerprints found at a crime scene matched those of the accused, even though three of the fingerprints fell below the relevant forensic standard.

The Supreme Court commented: “There is no necessarily correct way in which an expert ought to express the degree of confidence with which the expert holds the opinion of which evidence is given.” They went on to say: “Where an opinion is held with a high degree of confidence, it may be said that the holder is “highly confident” or is “certain” or has “no doubt” or, doubtless, a similar view can be expressed in other ways.”

Clearly, language expressing this level of certainty is at odds with the subsequent PCAST recommendations in the US, and the ENFSI guidelines. To have confidence in forensic results before the courts, they need to be presented in a balanced manner that is consistent with the primary duty of expert witnesses to tell the truth to the court.

The ENFSI guidelines arose from the work of Dr Sheila Willis, who was director general of Forensic Science Ireland for 15 years, and is now president of the Chartered Society of Forensic Sciences. She and her colleagues have for years had concerns about the language used by forensic experts in court, and the presentation of their evidence. Concerns about expert evidence have been expressed by the courts for years, and it is imperative that they take on board similar concerns expressed by the experts themselves. An accused person is entitled to a fair trial, in which expert evidence is presented in a truthful and balanced manner. The risk of wrongful conviction should be avoided at all costs.

Mark Tottenham is a barrister, and the author of The Reliable Expert Witness (Clarus Press, 2021), and A Guide to Expert Witness Evidence (Bloomsbury, 2019). For more information on the presentation of forensic findings, see the online course Challenging Forensic Science available from the University of Lausanne, via