Absence of Evidence Is Not Evidence of Absence

One of the most frequently contested subjects in court proceedings is the significance of not finding fingerprints on weapons, particularly firearms. Portrayals on television and in movies often suggest that fingerprints would have to be left on a gun or a knife if the person handled it. And defense attorneys, almost without exception, will use the absence of fingerprints on items to try to show that their client could not have committed the crime in question.

Yet, it is a mistake to attach any real significance to the absence of fingerprints. The statement "Absence of evidence is not evidence of absence," coined by bloodstain pattern analyst Herb McDonnell of Corning, New York, sums up this concept. "A positive statement cannot be made from a negative finding" is another way to say the same thing. Not finding a fingerprint means no more than just that: No fingerprints were found.

While juries are often persuaded to believe that fingerprints would have to be left on a gun or a knife if a person handled it, nothing could be further from the truth. Consider, for example, weapon design. The surfaces that are typically handled on weapons are by design textured so as not to be slippery. While it is certainly possible to leave a fingerprint on a weapon, in many instances it almost requires an overt effort to do so. Likewise, if a person had just washed and dried his or her hands prior to handling a weapon, it would be improbable that any fingerprints would be deposited. While it is still important always to test a weapon for fingerprints and not simply assume none will be found, it is more likely than not that the weapon will bear no fingerprints.

There are all sorts of explanations for why evidence might not be found, such as the following:

■ It was never there to begin with.

■ It was present but was removed prior to testing.

■ The testing was improperly carried out.

The real point, again, is that negative evidence of any sort has no real significance beyond the fact that it was not found.

the print and get much greater adhesion of the powder. Another reason for using superglue development before powdering is that it allows the fingerprint examiner to view dark detail against a light background, which most people find easier. The fuming process takes place in a closed chamber, with the item that is to be superglue-fumed placed on a rack near the top of the chamber. The superglue is vaporized using a hot place inside the chamber, with a container of hot water also placed inside to increase the humidity. Large items, such as rifles, can be fumed inside superglue closets or large chambers specifically designed for this purpose. A typical small chamber is shown in the photograph.

Superglue fumes can be applied to the entire interior of a vehicle without requiring any special equipment if atmospheric conditions are appropriate. During hot summer months in areas with high humidity, all that is required is to put a small dish of superglue inside the closed vehicle and stand by for a while as Mother Nature does her thing. The entire interior surface of the vehicle will be coated with superglue fumes, developing latent prints in the process. The down side of this, of course, is that the vehicle interior and windows will then be covered with the residue, and it will be very difficult to remove. This method, therefore, is reserved for vehicles that are not likely to be returned to their owners and driven again.

Super glue chamber (Courtesy of the author)

Superglue wands that allow isolated fuming of objects are also available. These wands use butane (cigarette lighter fuel) to heat up and vaporize disks that are impregnated with superglue. The fumes so created emanate from the end of a tube, allowing the investigator to direct them onto the surface of interest. Superglue may thus be used at crime scenes without the need for a fume hood or other special equipment. This technique is also particularly useful when large, heavy objects, such as floor safes, need to be checked for latent prints.

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