Police finally apprehended the so-called "barefoot bandit" on Sunday in the Bahamas. Authorities claim 19-year-old American Colton Harris-Moore had been on a two-year international crime spree, stealing cars, boats, and airplanes. In February, the suspect allegedly left chalk outlines of his feet on the floor of a grocery in Washington State. Could police use Harris-Moore's foot- or toe prints to link him to the crimes?
Absolutely. Your fingers, palms, toes, and foot soles are all covered with friction ridges arrayed in a unique and relatively permanent pattern, and U.S. courts will accept evidence from all four areas. While the rest of the body lacks these ridges, many scientists believe that all skin is unique. Researchers have argued that marks from lips, elbows, and ears could also be used to catch bad guys. Criminals don't often leave these prints, though, so there's been little practical need for studies showing that a lip is as special as a snowflak e, or that its surface features are preserved through years of sun exposure and abrasions. Crime scene investigators may attempt to lift toe and foot-sole prints from Harris-Moore's capers; they may also try to link him to the crimes based on the shape of his foot alone.
Despite the forensic importance of finger- and toe prints, scientists haven't quite figured out how they form. We know that the ridge patterns are set five months before you're born. It's clear that environmental factors influence their formation, since identical twins (as well as conjoined twins) have distinct sets. Beyond that, no one knows for sure exactly how you end up with your personal prints (PDF), or why friction ridges appear only on the hands and feet. They may be remnants of nerve formation, folded-over cells in the epidermis, or regions of skin that buckled in utero.
The why of fingerprints is just as slippery as the how. Friction ridges on your hands and feet are so named because scientists long thought they improved grip. But recent research suggests that the ridges, which cut down on the surface-to-surface contact between your fingers and an object by about one-third, actually reduce friction. The findings have given succor to contrarian scientists who believe that the primary role of skin ridges is to improve sensitivity or wick away sweat. Another possibility is that textured skin better matches nature's rough surfaces, helping to prevent blisters.
In theory, toe prints are just as unique and permanent as fingerprints. As a practical matter, the reliability of fingerprint analysis is itself the subject of some controversy, and no one knows whether toe-print matches are more or less likely to be accurate. In any case, fingerprints are far more useful to police: The FBI maintains a national database linking 66 million people to their fingerprints, and crime labs around the country can submit images remotely and receive a match within 10 minutes. Unfortunately, the system has no foot- or toe prints on record.
Occasionally, toe prints serve to complicate a case rather than solve it. Large, complete prints of fingers and toes are easy to differentiate. The former are elongated, while the latter—with the exception of the big toe—are quite round. But police often have to settle for fragments of a print. As a result, investigators sometimes unknowingly submit toe prints to a lab and come up with no match, because the system is looking through fingerprints.
While fingerprints have a longer history in criminal justice, foot- and toe prints have seen their share of gotcha moments. In June 1952, a burglar broke into a Scottish bakery, leaving a big toe print on the flour-dusted floor. The print was the only piece of evidence against William Gourley, and the jury took just 15 minutes to convict him. (The first criminal to be undone by fingerprints was filicidal Argentine Francisca Rojas in 1892.) Six months after Gourley's crime, James Walker Adams burgled a warehouse wearing socks with holes in them, leaving behind sole prints. The same attorney who represented Gourley defended Adams, yielding the same result.
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Explainer thanks Edward Robinson of George Washington University and Stephen G. Fischer of the FBI.