“Law is man’s attempt to civilize society. Science is man’s attempt to reveal truth. Forensic science, then, is the intersection of civilization and truth.” —Sean McKinney: “A Trace of Gold”
A lofty sentiment from my fictional forensic scientist, but the reality in both fiction and life is closer to the philosophy of Heraclitus (or Patti Smith) who said the only constant is change. Forensic science in the late nineteenth century saw the introduction of techniques like Bertillon’s anthropometry—identifying a person through a series of physical measurements—an inexact method that resulted in numerous misidentifications. Anthropometry was soon replaced by fingerprint comparison. Over the years a number of analytical techniques have come and gone, some replaced with more accurate tests, and some discredited altogether.
Forensic analysis of physical evidence is primarily based on the comparison of a known to an unknown. We look for patterns that are similar, allowing us to make “reasonable” assumptions about the role the evidence played in a crime. Pattern comparison standards have evolved over time with technologies that are more discriminative.
Types of evidence include serological (blood, saliva, semen), trace evidence (hairs, fibers, glass, paint, gunshot residue), visual comparisons (fingerprints, foot and tire prints, tool marks, fired evidence, fracture matches), etc. Many of these disciplines have changed over the years and new ones have been added, such as the analysis of digital evidence like photo, video, and audio forgeries. Most use some sort of pattern comparison, whether it’s a visual comparison of the striations on a fired bullet or DNA profiles from two samples of body fluids.
DNA Comes In
I can’t overstate the impact the introduction of DNA analysis in the 1980s/90s has made on the field. Previous serological comparisons like blood type or secretor status are now obsolete. DNA evidence has been instrumental in exonerating persons who were wrongly convicted of crimes they didn’t commit. But its statistical models have upset the apple cart for a whole host of techniques.
In 2009, the National Academy of Sciences released a report calling into question analytical techniques without the kind of specificity that’s attributed to DNA analysis. The effect of that report was to send a shockwave through the legal and scientific communities. Some of the changes it produced were long overdue; for example, the elimination of visual comparisons of hair evidence. But it also paved the way for defense attorneys to challenge the accuracy of other types of analysis, even fingerprint comparisons.
The problem with relying on DNA analysis to the exclusion of other techniques is that it ignores their use as investigative tools. The presence of gunshot residue on a person doesn’t tell us whether or not a suspect fired a gun. Its value in the courtroom is often overstated by both the prosecution and the defense. But it can tell investigators that a person either “discharged a firearm, was in the vicinity when a firearm was discharged, or came in contact with a surface on which there was gunshot residue.” This information is often sufficient to place someone at the scene of a crime, giving detectives a reason to look at that person more closely. Gunshot residue analysis itself has evolved significantly. The dermal nitrate or paraffin test was replaced with atomic absorption analysis, which has mostly given way to the scanning electron microscope coupled with an X-ray spectrometer.
Two of my favorite television fictional detectives are Homer Jackson on “Ripper Street” and the modern version of Sherlock Holmes on “Elementary.” Homer Jackson is a drunk, an opium smoker, and a doctor who aids the Whitechapel police as they investigate crime in 1890s London. The writers do a good job of giving him analytical expertise that could (maybe) have been possible at the time. Jonny Lee Miller’s modern-day Sherlock uses all the investigative techniques we’d expect from Holmes and adds twenty-first century technology to the mix.
Forensic scientists don’t solve crimes. They associate evidence with persons or events. Due to advancements in technologies and continued research, the analytical techniques they use are always subject to change. Forensic science in fiction reflects the period the characters “live” in, but those fictional characters can also evolve, reflecting advancements in the science. Nevertheless, I believe this statement by the original Sherlock Holmes will endure: “You know my method. It is founded on the observation of trifles.”