Fingerprints, Not Just For ID Anymore

The introduction to this Atlantic article is disturbingly shallow, ignoring the fact that the long-held belief that every person has unique fingerprints may not be the case, and more to the point, that the number of points required to make a match has been increased numerous times.

Fingerprints are the oldest and most widely used biometric marker. Artifacts unearthed from ancient Babylon, China, and Persia show that fingerprints were often used on clay tablets and seals for business transactions and official documents. The loops, whorls, and arches that emerge from the “friction ridges” that form on a fetus’s developing fingers become unique to each person, and it’s no surprise that fingerprint identification has also been the gold standard in law enforcement and forensics since about the early 1900s. More recently, fingerprint verification technology has become almost ubiquitous in our daily lives as an access key for everything from smartphones and computers to bank accounts, offices, and even health records.

So it’s unfair to expect much depth from someone who has no reason for knowing that he’s spewing “common wisdom” that’s kinda malarkey? Fair enough, although perpetuating myths isn’t really the sort of thing that someone should do if the point of their article is to raise a new problem in forensic science.

For all its utility, however, the image of this distinctive, swirling pattern has been the most information that you could extract from a fingerprint—though that’s starting to change. A raft of sensitive new fingerprint-analysis techniques is proving to be a potentially powerful, and in some cases worrying, new avenue for extracting intimate personal information—including what drugs a person has used.

Drugs? Yes, drugs.

The new methods use biometrics to analyze biochemical traces in sweat found along the ridges of a fingerprint. And those trace chemicals can quickly reveal whether you have ingested cocaine, opiates, marijuana, or other drugs. One novel, noninvasive forensic technique developed by researchers at the University of Surrey in the United Kingdom can detect cocaine and opiate use from a fingerprint in as little as 30 seconds.

The efficacy of these methods remains unknown, and will hopefully be subject to severe scrutiny before being put to use. It may suffer from the same science bias that has eluded courts forever, and allowed bad science to infiltrate the courtroom by officious experts explaining to judges how their conclusions are “the gold standard,” even though the gold standard has tarnished beyond recognition in every instance.

But assuming, arguendo, that this is a valid means of ascertaining whether a person has ingested illegal drugs, it raises an entirely different question. While the Fifth Amendment provides that no person can be compelled to be a witness against himself, that doesn’t preclude the police from obtaining a person’s fingerprints upon arrest, even though those prints may well prove to be the thing that causes the cell door to slam on them.

Fingerprints, for the purpose of identification, are not protected under the Fifth Amendment, which precludes compulsion of evidence as to one’s mental state but not one’s physical characteristics. Your prints are just a physical thing that reveal nothing about what’s going on in your head. So seizing your prints is permissible, and using them to match up with prints at the scene is allowed. Same with DNA for the purpose of identification.

But if prints can be used to detect drug use, they are no longer limited to identification based upon a physical characteristic. This will challenge the aphorism, “remember the rubric but forget the rationale.” Cops can take prints, as everybody knows, but that’s only because they’re merely a physical characteristic. It’s not about the inherent authority of police to seize your fingerprints, but about the use to which they’re put.

And the analogy to DNA is also obvious. Its use as an identifier is one thing, but since they’ve already got it in hand, perhaps under some bastardized plain view theory, why not milk it for whatever it tells about you beyond your identity?

You can’t blame science for continuing to rock and roll, but can law keep pace? Our sordid history with junk science, and fingerprints are very much a part of the law’s failure to put evidence through sufficient rigor before using it to cage people, continues to require scrutiny. We’ve had government commissions, twice now, calling out the failures of forensic science to do what it purports to do. In many instances, it’s total crap, a scam perpetrated on the courts and defendants for the sole purpose of assuring convictions when real evidence fails.

But that deals with the faux science side of forensics. The use to which fingerprints are put concerns the law side. Precedent holds that the police are authorized to seize people’s fingerprints upon arrest, as the Fifth Amendment does not apply to physical characteristics. But the rubric is “fingerprints can be seized” based on their limited utility as physical characteristics used for identification purposes.

If they should be used for entirely different purposes, for the ascertainment of whether a person ingested drugs, then the rationale allowing the seizure of prints under the Fifth Amendment no longer applies. It certainly won’t be in the cops’ best interests to draw this distinction, to limit their use of prints to the purpose for which they’re allowed and to demonstrate constitutional restraint by not exceeding that purpose.

Will the courts pick up the slack? Will judges remember the rationale, and not just the rubric? Will they revisit the application of the Fifth Amendment in light of this fundamentally different use? Or will they shrug it off in a simplistic “meh, prints are allowed. Move along”? History suggests that a lot of judges will shrug and go with the flow.

3 thoughts on “Fingerprints, Not Just For ID Anymore

  1. David

    Fingerprints is a red herring. According to the companies web page ( the technology is an assay of the sweat on your fingertip. Reading their material, one should be somewhat skeptical of their claimed abilities (remember Theranos). The technology does not have FDA approval and they claim to only be in early discussions, they don’t have a submission pending.

    Regardless, one would hope that a court could clearly see that this is a sampling of surface sweat and treat it as such even if they put the word “fingerprint” on the machine.

    I know off topic, but as someone that works in this field, the idea of taking an assay off a finger seems dubious even if it works. Our fingers touch a lot of stuff, if you just shook hands with a drug user, you may have swapped sweat.

    1. SHG Post author

      That the tech has issues is almost invariably the case, but the part you missed is the Daubert/Frye admissibility aspect. The key is “remember the rubric, forget the rationale.”

      For some judges:

      Fingerprints = Admissible

      And that’s that.

  2. Jyjon

    Well, here ya go.

    [Ed. Note: Link deleted. Some people just can’t figure out how this works.]

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