Defense lawyers have long questioned the reliability of some forensic science methods, but over the past decade, the debate has intensified. Two highly critical reports found that with the exception of DNA, many methods have not been tested rigorously enough to be considered scientifically valid.
A look at various forensic methods coming under fire and what the 2016 report from the President’s Council of Advisors on Science and Technology had to say about them:
Involves examining marks left on a victim or object, then comparing those with dental impressions taken from a suspect. Only a few empirical studies have been done to study the ability of examiners to accurately identify the source of a bite mark; those found false positive rates were so high that the method is “clearly scientifically unreliable at present.”
Two recent black-box studies found that latent fingerprint analysis is foundationally valid, but also found that false positive rates could be as high as one error in 306 cases in one study and one error in 18 cases in the other. “Additional black-box studies are needed to clarify the reliability of the method.”
In which examiners attempt to determine whether ammunition came from a specific gun based on marks produced by guns on the ammunition. The President’s Council said that there is now only one appropriately designed study to measure its scientific validity and to estimate its reliability, and that it needs additional studies.
In which examiners compare shoeprints found at a crime scene with specific shoes based on identifying marks. “Such conclusions are unsupported by any meaningful evidence or estimates of their accuracy and thus are not scientifically valid,” the report said, adding that it needs more studies.
Involves examiners comparing microscopic features of hair to determine whether a particular person may be the source. The FBI now acknowledges that microscopic hair analysis is inconclusive and uses it only in conjunction with DNA testing. The President’s Council found that studies cited in a Department of Justice report “do not provide a scientific basis for concluding that microscopic hair examination is a valid and reliable process.”
Complex DNA Mixtures
DNA analysis of complex mixtures of biological samples from multiple unknown people in unknown proportions; for example, from mixed blood stains. The President’s Council found that subjective analysis of complex DNA mixtures by examiners has not been established as scientifically valid, but said computer programs that use algorithms to interpret complex mixtures in an objective way are a major improvement.