The Science of Forensics

Surprising facts about the real-life science of crime scene investigations

The legal profession has a term for the way juries regard forensic science--they call it the "CSI Effect." Juries expect to see nothing less than DNA matching for even the most minor infractions. If the forensic evidence isn't overwhelming, they will acquit, even in the face of reasonable doubt. Without question, the CSI and Law and Order franchises have reshaped the popular imagination by elevating science as the ultimate arbiter of truth. That, in and of itself, is good for science. What's dangerous about the proposition, however, are the standards and lengths to which the television shows hold the science they portray.

While it's undoubtedly important for people to know what's fact and what's fiction in crime scene investigation, here's a look into just what the present day facts of forensics science entail (we'll leave the fiction to the experts… in TV production, that is). In 2005, Congress tasked the National Academy of Sciences to survey the landscape of forensic science. The result, this past February, was a 255-page report. Here are a few of the surprising facts they found.

Error

"It's more likely that you will see testimony presented and results of analysis presented without quantification of certainty," says Dr. Constantine Gatsonis of Brown University, one of the committee co-chairs. That is a real problem for the forensic science community. "When you go into a hospital and doctors quote things to you, very often they don't quote error bars next to what they say, do they? [But] they have done lots and lots of studies so they know what the margins of error are. If it came to it, they could say, this is what we know about how accurate this test is." The problem with many of the subjective forensic science disciplines is that those studies have not been done in these fields. Without a recognized baseline with which to draw hard scientific conclusions, expert testimony will increasingly be called into question. In 2007, a Baltimore County Circuit judge refused to allow fingerprint analysis in a death penalty case because she found the analysis to be "a subjective, untested, unverifiable identification procedure that purports to be infallible."WikiCommons

Hair

Television detectives relish pulling a single hair from a crime scene, like the proverbial needle from a haystack. "If only the perp hadn't shed this one strand," they say, "he would've gotten away with it." In real life, though, a strand of hair is not the smoking gun CSI may have you believe. While hair evidence may be valuable in narrowing a field of suspects, matching it to one particular individual is not at all a straightforward task. Forensic scientists use a variety of indicators--color, length, thickness, whether hair has been dyed or curled--to assign a hair to an individual, but there is no threshold for how many features must be the same in order to declare a match. Even when the examiners determine a match under a microscope, the potential for error is high. A 2002 FBI study discovered microscopic matches to be wrong 12.5% of the time when subsequently checked with mtDNA analysis.iStockPhoto

Fibers

While fiber is examined with the same subjective criteria as hair, i.e., color and texture, fiber analysis can achieve a higher level of scientific veracity in the lab through analytical chemistry. These techniques reveal the makeup of a particular fiber's polymers at the molecular level. But even with that level of information, scientists can't individually match a fiber to singular source. They can determine whether it came from a particular manufacturing lot or if it came from a patch of carpet exposed to noonday sun, but they can't say whether it came from a specific sun-exposed patch or one from next door.Image courtesy of Moria via Flickr.com

Shoeprints and Tire tracks

Known collectively as impression evidence, shoes and tires leave identifiable marks on a crime scene which investigators can use to pare down a field of suspects and potentially link an individual to a time and place. The sole of a shoe can be worn down and accumulate bits of glass and rock in such a way as to make its feature set individual. But just as is the case with fingerprints, shoeprints and tire tracks come with a similar set of question marks concerning reliability and probability. The experts in the field argue that it is years of experience which informs their sense of the statistical probabilities. While this may in fact be the case, without peer-reviewed study, theirs is largely a closed system, susceptible to bias and without outside feedback.Tomas Fano via Flickr.com

The Forensics Coroner

We may tend to think of coroners and medical examiners as one in the same, but while their job descriptions usually match, their levels of qualification really don't. Whereas a medical examiner must be a physician, often specializing in forensic medicine, and is appointed to her post, a coroner is an elected official who needs no medical training. In many counties, coroner's must only pass a written exam to assume their posts. In 2007, an 18-year old girl in Portland, Indiana became a deputy coroner under her father. According to Dr. Gatsonis, "[this is] hardly the way to go about medical legal examinations. You need a lot of training to be able to do that sort of thing and to be able to direct it. Even if a coroner doesn't sit there and do the autopsy, they still need to be able to have enough expertise to be able to direct the analysis and the whole operation. That's a real problem."iStockPhoto

DNA in Forensics

Of all the forensic sciences, nuclear DNA typing is the gold standard. It's the one technique that has grown, over its 20-year lifespan, into a mature and reliable method for matching an individual to a biological sample. Barring contamination or mishandling, it has an exceptionally low rate of false positives. It has become so highly regarded that it's routinely applied to old, reopened cases in which thin forensic evidence was used to convict (before DNA typing had matured). How did DNA achieve this status? Through rigorous, peer-reviewed study in laboratory settings. And that is at the crux of the Committee's report--to find ways to bring the other sciences under this strictly defined umbrella of accepted standards and review. For a look at PopSci.com's favorite moments in forensics, click here!Image courtesy of GravityWave via Flickr.com