Crime Library: Criminal Minds and Methods

Bite Marks as Evidence to Convict

How It's Done

Experts identify patterns by examining either a whole or partial set of teeth. They first determine whether the bite is human or animal, and then record every detail about size, appearance, color, and location of the bite on the body, as well as the presence of other bites.

Photographs involve using two rulers, one laid against the length and one against the width. If rulers are not available, then something like a coin or matchbox is used for size comparisons (but photographs are taken without these items as well, to ensure that no one can say that evidence was hidden beneath them). Care is taken with the camera angle to make sure it is perpendicular to the bite mark, but other angles and light settings are photographed as well. Photographs can then be enhanced with computer technology. (Older techniques involved hand tracing on clear acetate.) Some methods involve infrared or filtered procedures, but ordinary film is employed as well.

If the bite mark shows a good indentation rather than being smooth, an impression can be lifted. This is done right away before the skin changes, and a special frame is made to support it. If the impression is left on food, care is taken to prevent dehydration or rot.

The area around the bite-mark is wiped with sterile cotton swabs moistened in distilled water to get any saliva left behind, and the cotton is then air-dried and placed in a sterile tube. Then a control sample is taken from another place on the victim's skin.

A dental impression or mold is then made of the upper and lower set of teeth of the suspect (always observing his or her constitutional rights). From this, the odontologist creates a transparency or uses computer imaging. Photographs are made of the maximum opening of the mouth and the way the suspect bites. The bone structure of the jaw is examined, along with the biting dynamics of the muscles and tongue. Sample bites are taken and master casts made. Saliva is also taken for DNA testing. If any of the teeth appear to be recently chipped, ground, or broken, the relative age of the condition is estimated. A continued dental history of the suspect should be charted, given the possibility that he or she may attempt to change their bite profile.

Properly scaled photographs are important to the legal process, as is the type of equipment used. Manually operated cameras with close focusing capability are best, with off-camera flash. The camera has to be rigidly supported, as with a tripod, because the details must be precisely recorded. Both color and black-and-white film is preferable to using only one or the other. Sometimes the bite-mark area has to be shaved to get a proper photo, and this has to be done with extreme care. When the wound is on a curved surface, photographs are made from several angles to capture the arch.

Digital cameras, which are getting increasingly better resolution, can be especially valuable in light of the computer software available for all kinds of enhancements. A poorly lighted photo, for example, can be brightened, and certain spots on a photo can be highlighted and enlarged for better examination. Digital photos also correct for angular distortions and digital software allows for electronic transmission to other specialists and for easy storage of image data.

Guidelines for court testimony are set forth by the American Board of Forensic Odontology, in terms of how an expert should offer an analysis of bite mark comparisons in court. The identification can be declared positive, probable, possible, not possible (the suspect is excluded) or insufficient as evidence to make a definitive statement. The highest standard is "reasonable medical certainty," wherein for all practical purposes, the perpetrator is confidently identified by the expert as the person who made the mark.

Forensic dental experts must be prepared for cross-examination that emphasizes the art of interpretation over the hard science of more precise identification processes. This is the way most attorneys attempt to persuade a jury that the interpretation is highly subjective and thus cannot be used to convict someone. If dental impression is the only evidence, the case may be weak at best. There certainly have been cases where a "probable" finding was contradicted by DNA evidence.

Not all dentists can testify in court. As with all experts, they have to go through a voir dire process of qualification, in which their credentials, expertise, and experience are examined for the court.

It must be noted that not all bite marks are of forensic value, especially since the analysis is generally vague. While there are some cases, such as Ted Bundy's, where the bite-mark analysis was pivotal, the "science" of dental impressions has relied more on anecdotal success than on a solid history of standardized testing. A bite mark has to be fairly clear, with several experts confirming the evidence independently, before a court should consider it as significant evidence.

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