Trace and transfer evidence

DEFINITIONS:Trace evidence is evidence that is present in very small amounts, requiring careful attention and often special techniques to detect. Transfer evidence is evidence that has moved from one person or object to another, such as from a crime victim to the perpetrator, or vice versa.

SIGNIFICANCE: Trace evidence and transfer evidence often take the form of fibers, hairs, soils, paint chips, and other tiny pieces of material that must be carefully collected from crime scenes and transported to the laboratory for analysis. The information gained from forensic analysis of such evidence can help law-enforcement investigators link suspects to crime scenes.

Although trace evidence and transfer evidence are conceptually distinct, the terms “trace” and “transfer” are frequently applied to the same materials: the debris (hairs, fibers, dust, glass and paint fragments, and other materials, natural and manufactured) that French forensic pioneer Edmond Locard characterized as containing the “mute witnesses” of criminal truth. Locard’s exchange principle, widely regarded as the fundamental idea behind forensic science, insists that “every contact leaves a trace.” This transfer—blood on a door handle, for example—need not be unperceived, but it is the difficult-to-perceive transfer (the “trace”) that is likely to survive attempts to scrub away evidence of mischief.

“Trace” Versus “Transfer”

When surfaces touch, transfer evidence is produced, although transfer can occur without direct contact. Surfaces pick up matter and leave matter behind, or disturb the material already there, or both. Stepping in the mud may leave behind a shoe print (possibly one with great individuality); it also may take away material from the mud and leave behind material from the shoe.

Transfer evidence thus can be viewed as including both “pattern transfer” evidence, such as imprints and impressions (of fingerprints, shoe prints, tire tracks, and so on), and “trace transfer” evidence, such as hairs, fibers, glass fragments, soil, and blood. The former is often referred to simply as “pattern evidence,” the latter as “trace evidence.” The phrase “trace, or transfer, evidence” (occasionally further abbreviated to just “trace evidence”) is sometimes used as a loose label for both.

As a practical matter, what falls into the category of trace evidence is influenced by bureaucratic factors such as crime lab organization, including spatial layout, equipment distribution, staffing contingencies, and funding requirements. “Trace” functions to some extent as a catchall category, and so the kinds of evidence that fall into that category can vary enormously. As one trace analyst has put it, “Trace analysis is the section of the crime lab where, if they don’t know where to send it, they send it to us.”

Problems with Trace Evidence

Some problems related to the use of hair, fiber, and other trace and transfer evidence in criminal cases have involved familiar concerns about proper quality control in the collection, preservation, testing, and documentation of evidence. Other problems have arisen owing to the professional or moral failings of individual “experts” who have overstated the significance of “matches.” Despite much loose talk about two or more items “matching” or “being similar” and about results “corroborating” or being “consistent with” a certain hypothesis, much trace evidence at best rules out certain possibilities without providing positive confirmation.

More decisive, positive implications depend heavily on judgments about probabilities. How likely is it that a fiber found on a victim came from the carpet in a vehicle belonging to the defendant? Given a glass fragment found on a that is similar to the glass in a crime victim’s home, when is it reasonable to conclude that it comes from that source? Forensic scientists devote much effort to trying to establish relevant numbers, and many statistical mistakes can be made in reasoning about these matters. Calculating the likelihood of variation among items of a given kind (such as hairs or glass fragments) is part of the problem and part of why it is important that forensic scientists conduct comparisons using control samples.

The emergence of DNA (deoxyribonucleic acid) analysis has played a role in decreasing the emphasis on trace and transfer evidence. It has also prompted recognition of serious flaws in both the scientific and the judicial utilization of results. In establishing the innocence of numerous individuals who have been convicted of crimes, DNA evidence has provided an of the evidence (often hairs, fibers, or other trace evidence) on the basis of which those individuals were originally convicted. This development has contributed to a decline in the prestige attached to the analysis of trace evidence.

The Future of Trace Evidence

In announcing a symposium on trace evidence sponsored by the Federal Bureau of Investigation (FBI) and the National Institute of Justice (NIJ) in 2007, Sandra Koch of the FBI Laboratory’s Trace Evidence Unit noted that “collection, preservation, analysis, and eventual use in court [of trace/transfer evidence] have declined in recent years.” Half a decade earlier, retired forensic scientist Larry Ragle had lamented that “it has become more difficult for the lab personnel to justify spending the time it takes to characterize trace evidence, hairs, and fibers, or even to train new scientists in the techniques,” and that in some jurisdictions “crime scene investigators no longer spend the time to search for and collect the standards necessary for comparison should trace evidence be important as the investigation progresses.”

Trace evidence stands in the shadow of the rapid advance of DNA analysis, but it might nonetheless have a brighter future. The progress of nanotechnology (the science of ultrasmall—molecular-scale—particles and processes) may be expected to lead to the identification of yet subtler class characteristics of materials, as well as an increased ability to detect individualizing features of wear and idiosyncrasies of manufacture in what were formerly indistinguishable mass-produced objects. (Whether such developments will have practical application within the context of underfunded and over-tasked crime labs is another question.)

Moreover, trace evidence and transfer evidence are important for their role in the maintenance of an open-minded, holistic approach to forensics. As one crime lab director has observed:

It used to be that when our firearms unit received a bullet, the first thing they’d do would be they’d wash it off so they could see all their little grooves and markings so they could do their comparison. . . . [They now realize that] there might be blood on there that’s important. . . . Directors of crime labs have to . . . not lose the ability of people to recognize evidence beyond their one little specialty.

The value of trace evidence to law enforcement and forensic scientists often depends on the quality of the equipment available to forensic analysts. Due to consistent advancements in the field, modern, well-maintained analytical machines can provide far more detail than older or poorly-maintained machines, allowing analysts to draw more information from trace evidence. Though some agencies are able to replace their trace analysys equipment every ten years, many law enforcement agencies and forensics laboratories lack the funding to purchase new equipment. To compensate for this, experts recommend that analysts cooperate with other labs, sending important trace evidence to labs with high-quality, specialized equipment.

Bibliography

Aitken, Colin G. G., and Franco Taroni. Statistics and the Evaluation of Evidence for Forensic Scientists. 2d ed. Hoboken, N.J.: John Wiley & Sons, 2004.

Fisher, Jim. Forensics under Fire: Are Bad Science and Dueling Experts Corrupting Criminal Justice?New Brunswick, N.J.: Rutgers University Press, 2008.

Fletcher, Connie. Every Contact Leaves a Trace: Crime Scene Experts Talk about Their Work from Discovery through Verdict. New York: St. Martin’s Press, 2006.

Houck, Max M., ed. Mute Witnesses: Trace Evidence Analysis. San Diego, Calif.: Academic Press, 2001.

Houck, Max M., ed. Trace Evidence Analysis: More Cases in Mute Witnesses. Burlington, Mass.: Elsevier Academic Press, 2004.

Kelly, John F., and Phillip K. Wearne. Tainting Evidence: Inside the Scandals at the FBI Crime Lab. New York: Free Press, 1998.

Lee, Henry C., Timothy Palmbach, and Marilyn T. Miller. Henry Lee’s Crime Scene Handbook. San Diego, Calif.: Academic Press, 2001.

Pollock, Edward. "Improving the Analysis and Collection of Trace Evidence Samples." National Institute of Justice, 2 Nov. 2020, nij.ojp.gov/topics/articles/improving-analysis-and-collection-trace-evidence-samples. Accessed 18 Aug. 2024.

Ragle, Larry. Crime Scene. Rev. ed. New York: Avon Books, 2002.

"Trace Evidence: The Role in Forensic Science." UF Health, 14 Oct. 2022, forensicscience.ufl.edu/2022/10/14/trace-evidence-the-role-in-forensic-science/. Accessed 18 Aug. 2024.