“With the exception of nuclear DNA analysis … no forensic method has been rigorously shown to have the capacity to consistently, and with a high degree of certainty, demonstrate a connection between evidence and a specific individual or source.”
That was the conclusion of a seminal 2009 report for Congress by the National Research Council’s Committee on Identifying the Needs of the Forensic Sciences Community.
The report was meant to gauge the state of the science in forensic analysis, and it was dismal. A host of common forensic disciplines came under attack as having no real scientific rigor behind them. Forensic techniques with insufficient science behind them included:
- Firearms examination
- Tool mark comparison
- Bite mark comparison
- Tire and footwear impressions
- Bloodstain pattern analysis
- Handwriting analysis
- Hair comparison
- Coatings (e.g. paint) analysis
- Chemical analysis, including drugs
- Materials (e.g. fiber) comparison
- Fluids analysis
- Fire and explosive analysis
The National Research Council’s report to Congress is not an outlier. In 2016, the President’s Council of Advisors on Science and Technology issued a report for the president, also finding that many common forensic techniques are not backed up by sufficient science. Moreover, it questioned the way that many forensic analysts testify about the surety of their methods, concluding that the statistical certainty of these results is routinely exaggerated.
Nevertheless, results of these forensic techniques are typically allowed as evidence in criminal cases. This is despite the U.S. Supreme Court’s direction in Daubert v. Merrell Dow Pharmaceuticals Inc., which requires courts to act as gatekeepers for scientific testimony.
Courts are only supposed to allow scientific and technical testimony when it is subject to peer review and the analyst’s methods are generally accepted in the scientific community. The National Research Council and the President’s Council concluded that most forensic techniques do not meet those standards.
A court declines ballistics testimony based on the poor state of the science
In September of last year, a D.C. judge ruled that the prosecution in one case could not bring forward evidence that the marks on a shell casing from a crime scene matched those of a gun the defendant had allegedly discarded. This type of evidence has been admitted in thousands of other criminal cases.
Ballistics is just the kind of “pattern matching” evidence that has little scientific basis. It is quite subjective whether the markings on a particular bullet casing match those made by a particular gun — and there are no statistics to back up the conclusion that they match.
The judge didn’t mean to say that ballistics evidence is useless. Instead, he meant to prevent the prosecution’s forensic witness from testifying that this bullet matched this gun. The best that can be concluded scientifically is that the gun cannot be excluded as a match to the bullet.
Most judges have little to no scientific background. Therefore, they tend to use a legal framework when evaluating potential evidence. That means looking to see if other judges have allowed the evidence. If they have, then the judge in question will generally be apt to allow the evidence to be presented, subject to cross-examination.
But all too often, juries see prosecution experts as reliable and defense experts as paid hacks. Letting faulty prosecution evidence into the case is essentially encouraging the jury to believe it.
After the two major forensic science reports were issued, some scientists have begun filling in the scientific underpinnings of their disciplines. But defendants are routinely being prosecuted using these unproven forensic techniques. They cannot wait for the science to catch up to the conclusions.