Three attorneys in an Illinois public defender’s office took a test commonly used to qualify experts in fingerprint analysis for the Chicago Police Department. City prosecutors often cite the test in court as a reason to believe the testimony of police experts.

All three attorneys got a near-perfect score, despite having no training in fingerprint analysis.

The advocacy journalism website The Intercept summarized the attorneys’ research in a recent article. It explains some reasons to doubt fingerprint analysis and other tools the public generally thinks of as reliable and objective.

Studies call pattern-based police tools unreliable

A report from the National Academy of Sciences said that police use crime investigation methods and tools that are not based on science in any useful way.

As The Intercept put it, “the majority of the forensic disciplines lacked meaningful scientific underpinning.”

The Academy pointed to major reasons to doubt not only fingerprint analysis but handwriting and bite-mark evidence.

Except for DNA, the Academy found that all common methods of matching patterns to solve crimes are subjective, relying heavily on the person using them rather than the evidence itself.

Comparing fingerprints is not as easy as it looks

Almost everyone has heard that no two fingerprints are alike, but few know there is little evidence for the claim. However, there a lot of evidence that many fingerprints are extremely similar.

That fact comes from clear, clean and carefully collected fingerprint samples.

Almost all fingerprints found at crime scenes are messy. Police find them on uneven and grainy surfaces, caked in blood, smeared, incomplete and layered on top of one another.

These real similarities and noisy prints make it easier to say two prints are “potential” matches to a suspect already known to police.

When police run a print in an FBI database of millions of fingerprints, many so-called “close non-matches” are likely. A 2004 bombing in Madrid led to the FBI making an embarrassing and expensive arrest.

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