Why Is Bad Science Allowed in the Courtroom?

If you watch CSI or other crime-and-justice-type shows, you’re familiar with the trope of a detective having a breakthrough based on a small piece of evidence like a bite mark. On TV, forensic evidence is often the difference between catching the perpetrator and not. It can also make the difference between a guilty or not guilty verdict in real life. The problem, as a newly released report outlines, is that one of these forensic techniques is not reliable and fails key scientific tests.

A recent review of bite mark analysis shows it is unscientific.

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Bite mark analysis

At its core, bite mark analysis involves an individual making a subjective judgment about whether or not a bite mark matches the teeth of a suspect. The bite mark analyst might take measurements from photos to help them do this. But ultimately the accuracy of the technique can be evaluated using the same criteria we use to evaluate the scientific status of psychology research’s claims. Can basic assumptions of the technique be verified? Can the findings be reliably repeated?

A new report from the National Institute of Standards and Technology (NIST) concludes that “forensic bite mark analysis lacks a sufficient scientific foundation.” The report identifies three key assumptions of bite mark analysis that aren’t supported by the data:

1. Patterns of teeth marks left after a bite are not unique to an individual, the way that DNA or fingerprints are. In other words, you can’t identify specific people based on bite marks.
2. Bites don’t leave consistent patterns on human skin. Someone biting another person’s skin isn’t going to leave a mark that is reliable. As the report notes, bites from the same mouth can look different because the angle of the mouth can differ, because the biter and person being bitten can be moving during the bite, and simply because skin (unlike, say, an apple or chocolate) is stretchy.
3. No research shows a reliable way to accurately measure bite marks. There isn’t a “gold standard” technique for the field that experts can use to analyze a bite mark, the way that there is in measuring and analyzing the whorls in fingerprints.

A new report finds that people cannot be identified by their bite marks.

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The report notes that experts and researchers working on bite mark analysis have called for more rigorous research since 1960. The current evidence, however, is pretty bleak. After reviewing the evidence, a 2019 workshop on human bite marks concluded that the first question—whether bite marks specific to an individual, like fingerprints—was no longer even relevant because it was “highly unlikely” teeth had characteristics that could be used to identify individuals. This directly contradicts the claims, made by some people working in the area, that bite marks can be compared to fingerprints or DNA. They can’t, and researchers have concluded that we should stop trying. There isn’t enough detail available from people’s teeth.

More problematic, several studies have shown that bite mark analysts can’t even agree with each other on what the bite mark evidence shows. In one 2015 study, 39 experts were asked to review 100 case photos. They were asked to answer basic questions, like whether a bite mark was human or not, and whether there was enough evidence in the photo to determine what (human or animal) or who (which person) made the bite. In only 8% of the cases did the experts agree at a high rate (90%). In other words, for almost every case photo given, experts couldn’t even agree on the basic facts: was the photo of a human bite mark, and did it have enough information to be useful.

A 2016 study found similar results: experts couldn’t identify whether a bite mark was human or animal, or whether it was adult or child. Further, experts didn’t even agree with themselves when looking at the same case eight weeks later. Similar results were found in 1975, 1998, and 2001. This indicates that experts aren’t using a reliable or consistent technique to analyze bite marks.

It may be possible, through future research, to identify some useful bits of information from a bite mark. This is not likely to look like fingerprint evidence, where there’s a match with a specific individual. Instead, it is likely to involve the development of a standardized process for examining and measuring bite marks that may be able to draw more limited conclusions (e.g., whether a bite came from a person or a dog). But the field isn’t there yet.

Why do we believe?

Many people have a natural deference to authority. They believe that institutions, like the courts, and experts, like the people who give testimony on forensic bite marks, should be trusted. Having some level of respect for authority is healthy, and helps people understand and navigate their daily lives. There are also many people working in well-established institutions, like the courts, who are earnestly trying to make sure the institutions are fair and honest. At the same time, a natural deference to authority needs to be tempered with a reasonable willingness to update your beliefs as new evidence is presented.

Need for Closure is a personality trait that may explain a reluctance to accept new ideas.

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A personality trait may help explain why bite mark analysis is still accepted—and seen as persuasive—in some courtrooms. Need for closure reflects a preference for a definite answer (any answer!) to questions over ambiguity or doubt. Research has shown that people high in need for closure have more resistance to a newcomer’s innovative ideas. Research on legal decision-making found that law students, on average, scored above the midpoint on need for closure. Higher scores on need for closure also predicted greater self-serving biases when evaluating settlements of a hypothetical personal injury case. People high in a need for closure therefore may be less likely to accept new evidence suggesting that a forensic technique—like bite mark analysis—is flawed.

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Yet, the evidence on bite mark analysis is in. As it is currently practiced, it is not a reliable or scientific discipline. Bite mark analysts can’t tell a human from an animal, and they certainly can’t tell individual people apart. Their testimony is just a way of laundering uncertainty: improperly turning hunches and speculation in for something that sounds more definite and scientific. Prosecutors who use bite mark analysis as evidence are trading on the perception of scientific rigor for short-term gain—a greater likelihood that juries will find in their favor—while giving away long-term credibility. The question is whether lawyers and judges can overcome the human tendency to want definite answers for the reality of ambiguity. Will courts favor the messy truth over the tidy fiction and stop accepting bite mark analysis? The jury is still out.