Forensic hair analysis has developed a bad reputation. The technique has relied on traits such as color, thickness, and curvature to link a suspect to a crime scene. But an ongoing reanalysis of old cases by the U.S. Justice Department found that analysts have often overstated their case in the courtroom; several people convicted based on a hair sample were later found to be innocent.
Now, sophisticated analytical techniques are giving hair a new role in forensics. The goal is no longer matching a suspect to a crime scene but using hair to infer physical characteristics or even the travel history of an unknown criminal or victim. Most hairs found at crime scenes don’t have enough DNA in them for analysis; “doing a chemical analysis and trying to determine some trait about the individual ... is really the only alternative,” says Glen Jackson, a forensic scientist at West Virginia University in Morgantown.
Keratin, the main component of human scalp hair, contains all 21 amino acids, but the ratios depend on the body’s biochemistry and differ from person to person. Hydrolyzing the amino acids and measuring their quantities yields a profile that, when compared with a database, gives an indication of a person’s sex, age, body mass index, and region of origin, Jackson says—although the accuracy varies by trait and more work is needed.
The ratios of isotopes—atoms of the same element that differ in the number of neutrons—in hair can also yield clues. The ratios of hydrogen and oxygen isotopes in drinking water vary from region to region and are captured in hair. As a result, isotopic analysis of hair can provide clues about where a person has been in the previous months—or years, if the hair is long enough. In 2008, a Utah company called Isoforensics discovered that “Saltair Sally,” an unidentified woman found dead in Utah in 2000, had repeatedly moved between the Pacific Northwest and the Salt Lake City area before she died—a clue that helped identify her in 2012. “People are coming to us and saying ‘Hey, I heard about this technique and I’ve got a cold case from 1976. Do you think it will help?’” says Isoforensics President Lesley Chesson.