Law enforcement officials are trying to ascertain the owner of a jawbone that washed up on a beach in Aruba last November. They've ruled out missing Alabama teenager Natalee Holloway. Now they're considering Amy Bradley, who disappeared 12 years ago on a Caribbean cruise. The DNA is too damaged to be useful, but investigators say the bone likely belonged to a C aucasian woman. In the absence of DNA, can you really determine race from a jawbone?
Probably not. Forensic anthropologists try to infer the ancestry, gender, and age of human remains by measuring their dimensions and observing their features with the naked eye. The jawbone is one of the more useful bones in the body, as researchers have compiled a number of mandibular traits (PDF) that they think differ slightly between races. For example, if you place an Asian jawbone on a table, the bottom of it will likely maintain contact with the tabletop all the way around. African and Caucasian mandibles, in contrast, tend to undulate, or rise and fall along the lower border. The gonia—that's the area beneath your ears where the jawbone turns upward—generally curve more sharply in an Asian jaw. People of African descent often have slightly curled surfaces on the rear edges of their jaws, whereas European jaws are more likely to have a flatter edge. (Take a look at this picture.) But all of these traits are imprecise indicators. It's impossible to identify a person's ancestry definitively from a single bone.
Investigators can also take bone measurements using calipers, then input the data into a University of Tennessee database containing a reference library of measurements from more than 1,800 bones of known ancestry, age, and gender. The program will determine whether the mystery bone falls within the typical ranges for various racial groups. It's pretty hard to hazard a reliable guess with one or two measurements, since the ranges overlap. But if a series of measurements for a whole skull, or an entire skeleton, tend to fall more comfortably within one range than another, the forensic anthropologist can make a determination.
Racial classification is an inexact science, if that's even the right word for it. Forensic anthropologists never make definitive ancestry pronouncements. They say a bone is "consistent with" European ancestry or "likely" of Asian ancestry. And practitioners say it takes years of experience to achieve mastery, since you have to see piles and piles of disembodied mandibles to be able to recognize the sometimes subtle differences among them. (Although one study (PDF) has suggested that the grizzled veterans of forensic anthropology are no better at surmising race than their bright-eyed protégés.)
The practice of inferring race from bones is also somewhat controversial. While today's forensic anthropologists don't like to talk about it, the discipline has its roots in the pseudoscientific 19th-century practice of using skull measurements to prove Caucasian intellectual superiority. The methodology has improved since then. When researchers develop a hypothesis about racial variation, they conduct blind tests on hundreds or thousands of skulls of known ancestry to test its reliability. They also test their own consistency, looking at the same skulls several times in different orders to make sure they usually make the same call on its structures and shapes.
There are people who think even the modern techniques are bunk. They argue that more physical variability exists between individuals of the same race than between races and point out that less than 15 percent of physical variation can be attributed to race. In addition, marriage between people of differing ancestries has become so common that forensic racial determinations can actually hinder an investigation. If someone had his mother's African jaw and his father's light skin, investigators would be sent out looking for the wrong person, since his neighbors might have considered him white.
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Explainer thanks Ann Bunch of the College at Brockport and John Williams of Western Carolina University.