Criminal justice

Criminal justice

Can forensic scientists match—with certainty—a bullet, fingerprint, or strand of hair to a crime suspect?

Despite what we see on TV and in the movies, no, they can’t. In fact, the only way to connect a piece of evidence to a specific person with near-certainty is through nuclear DNA analysis of biological material.

What’s the problem with these types of forensic evidence?

In courtroom statements, forensic scientists sometimes testify that there is a “match” between a piece of evidence and a suspect. Or they assert that there is “reasonable scientific certainty” of a match. But methods like fingerprint analysis and bullet (or other “toolmark” evidence) analysis haven’t been sufficiently tested by scientists to determine how accurate they are.

When DNA analysis became widely used in the 1990s, it revealed cases in which “matches” made by fingerprint, hair, bite mark, or bullet analysis were wrong. Studies are under way to figure out the limits of what many traditional forensic methods can reliably tell us—and how accurate they truly are.

Is it possible forensic science isn’t as scientific as we might think? We sat down with a man who spent 23 years in prison for a crime he didn’t commit to learn about what put him there.

Why the uncertainty?

For one thing, we don’t know how common—or rare—specific characteristics are across people and across firearms. There is some evidence that fingerprints, for example, are unique to each person. But science doesn’t yet know how much variety and similarity there are in people’s fingerprints. It’s possible that two people’s prints might be similar enough that they could be confused, especially because fingerprints from a crime scene are often partial, smudged, or less distinctive than prints taken in a laboratory or police station.

Law enforcement officers locate latent fingerprints on the side of a van. Credit

Any other factors leading to uncertainty?

Yes. Subjectivity. The judgment and expertise of the forensic examiner play a big role in bullet, fingerprint, and other kinds of pattern analysis (so called because it focuses on matching patterns between pieces of evidence). Conclusions about a match may differ among examiners.

In one study, some examiners did not agree with their own previous conclusions about a set of fingerprints when they later reexamined the prints in a different context.

As with other kinds of pattern analysis, matching the distinctive marks on bullets and cartridges to a particular firearm relies largely on the judgment and expertise of the forensic examiner. Credit

Well, how do we know DNA evidence is reliable?

Rigorous experiments. Research has shown how much genetic similarity and difference there are across the population. That research also identified a distinctive set of “loci”—segments of the human chromosome—that are highly specific to each person. If the loci on a piece of evidence match the loci on a suspect’s DNA, it’s a strong indicator that the two DNA samples came from the same person.

The probability of a mistaken match is small, and scientists can often quantify it. We know the degree of genetic variation among people, so it’s possible to say things like “there is a 1 in 10 million chance that this match is incorrect.” That’s important, because investigators and juries need to decide how much significance to attach to each piece of evidence.

So far, the ability to quantify the risk of a mistake is unique to DNA analysis among all of the forensic disciplines.

Aren’t there different types of DNA?

Yes. Nuclear DNA is found in the nucleus of nearly every cell in a person’s body and is inherited from both the mother and father. It’s the most reliable source for connecting evidence (such as blood or saliva) to a suspect. The possibility of a false positive is quantifiable and small.

If there isn’t sufficient nuclear DNA in the evidence sample, then mitochondrial DNA (mtDNA) might be analyzed. It’s often more plentiful in a sample because there are large numbers of mitochrondria in most cells. It’s especially useful in things like hair analysis because hair samples contain little nuclear DNA, unless the hair root is attached. This testing can show whether a suspect could or couldn’t be the source of the evidence, but it can’t connect the evidence with a specific person with the same reliability as nuclear DNA. That’s because mtDNA is inherited from only the mother, and all people with a common female ancestor (within the past few generations) share a common profile.


Is DNA analysis foolproof?

No, even DNA analysis isn’t perfect. As with all types of forensic analysis, errors can occur—mostly human errors. Laboratories may mislabel or lose samples, or data may be misinterpreted. Mixed DNA evidence (that is, DNA that appears to have come from more than one person) at crime scenes adds to the challenge of linking evidence to a specific suspect.

And all of this is important because…?

Scores of talented and dedicated people serve the forensic science community. They perform vitally important work in the prosecution and conviction of criminals—and in the exoneration of innocent people. However, new scientific tools have revealed serious flaws in the system, and these flaws must be addressed. We don’t want innocent people being punished for crimes they didn’t commit. Even once is too often.

Take a Deep Dive

Ready to learn all of the details? Check out the report: Strengthening Forensic Science in the United States: A Path Forward.

Know it all? Prove it.