Saturday, December 15, 2012

Myth of Fingerprints

“I’ve seen them all and, man, they’re all the same”. OK, perhaps not quite all the same as Paul Simon says, but there are enough similarities to bring doubt on the surety of “a match”. TV and movies make it all seem so simple. Throw the latent print found at the scene or on the murder weapon into a computer and out pops the name (and often a picture) of the guilty party.

This was brought up to me a few years ago when a juror asked if we had fingerprinted the weapon that caused the death in a death by suicide to prove that the individual had indeed shot himself. That and some recent reading brought out a couple of tidbits that I thought I would share. 

Fingerprint identification happens when the arches, whorls, and loops present in a fingerprint are compared to some prints that are on record in a database somewhere or compared to a set taken from a specific individual. The comparison process finds parts that seem distinctive in the found print and those bits are compared to the set of possible fingerprints looking for similarities. So the second limiting factor in print ID is that you have to have a print of a known individual to compare with. That does not always exist, although it always seems to on TV. 

The first limiting factor is getting that latent print, latent print means one left behind to be found. The thing to know here is that the average latent print is only about 20% of a fingerprint, severely limiting the amount of the print information available for comparison. In the case of a gun trigger, the maximum amount of a print that could be found (not even considering that gunmetal is a poor surface to recover a print from) would be well less than that 20%. Definitely not enough data points for any real comparison.

Even if you have a full print, is this comparison thing an exact science as we are led to believe? Consider that a study done a few years ago in seasoned fingerprint examiners (I have lost the exact reference) showed as much as a 1 in 5 misidentification rate. Many times there are similarities enough to fool the most seasoned of investigators into making an incorrect finding, let alone the initial fingerprint screen by computer comparing thousands upon thousands of prints. It is not an exact science. The best that can be truthfully said is that there is a certain probability of certainty that this print matches a given individual’s. There is always the possibility of at least very similar prints being present in multiple individuals.

Do we really know that people with identical fingerprints don’t exist? In one episode of the Sopranos (don’t you miss the Sopranos?), Christopher waxed poetic on just this issue. His point was that without actually comparing everyone’s fingerprints in the entire world, alive and dead, you can never know for sure that there are no identical fingerprints. Keep in mind that Nancy Knight of the National Center of Atmospheric Research found 2 identical snowflakes during a snowstorm in Wisconsin a few years ago. If the myth of no identical snowflakes existing has been busted, who are we to say that the same is not true of fingerprints?

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