A lot of forensic evidence is surprisingly flawed

In 2009, the National Research Council (NRC) issued a Congressionally mandated report for the Justice Department on the state of the forensic science being used in U.S. criminal cases. It was intended to highlight best practices in the field and make sure that federal forensic analysts were doing their best science so our society could limit wrongful convictions.

However, the NRC ultimately concluded that most forensic evidence is deeply flawed. Its performance lacks any mandatory standardization, certification or accreditation. Forensic evidence can easily be misinterpreted. And, forensic analysts often exaggerate the degree of scientific surety they can provide.

“With the exception of nuclear DNA analysis, however, 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,” reads the report.

In 2015, the FBI issued a blockbuster report of its own. It admitted that its own examiners’ testimony was exaggerated or erroneous in 90% of cases reviewed that involved microscopic hair comparison analysis. To this day, cases are being challenged and it is unclear how many convictions based on FBI hair comparison examiners were ultimately wrongful.

In 2016, the President’s Council of Advisors on Science and Technology (PCAST) issued its own report on the scientific validity of a group of forensic procedures known as “feature comparison methods.” These include, for example:

  • Bite mark comparison
  • Latent fingerprint analysis
  • Ballistics/firearm mark comparison
  • Footwear/footprint matching
  • Tire track matching
  • Hair and fiber comparison
  • Handwriting analysis

Feature comparison methods attempt to tie a sample from a crime scene with a sample from another source based on the presence of similar patterns, impressions or other features.

After reviewing thousands of academic papers and other materials, the PCAST ultimately found that feature comparison methods are not scientifically valid. At best, some of the methods are scientifically valid only under limited circumstances. Substantially more evidence is needed before these methods can be shown to be valid. And, the testimony that analysts give is highly problematic and prone to exaggeration.

Many false convictions may be based on faulty forensic testimony

It’s difficult to say whether forensic science is or is not fundamentally flawed. What we do know is that, especially when it comes to matching patterns, analysts often make mistakes. They may be subject to confirmation bias or contextual bias when performing their examinations. In other words, the tests may be subjective. Furthermore, analysts tend to exaggerate the scientific certainty of their conclusion that the crime scene and defendant samples are from the same source.

Real life is not like “CSI: Crime Scene Investigation.” At a crime scene, high-quality samples may be difficult to obtain. They can be degraded or contaminated. The samples can be mislabeled, swapped, or incorrectly processed in the lab. Analysts can be poorly trained, motivated to help the prosecution, or simply wrong.

Once invalid forensic evidence is admitted at trial, however, it is highly likely to be believed. Defendants need thorough, aggressive defense counsel to protect them from bad forensic evidence being introduced and to challenge it when it is.