Materials that are transferred from one person or place to another person or place. The evaluation of transfer evidence is the heart of traditional forensic science and often is conducted using microscopy. Edmond Locard (1877-1966) is credited with developing what has become known as Locardâ€™s exchange principle, paraphrased as every contact leaves a trace. Locard was particularly interested in everyday materials such as dust that are among the most common forms of transfer evidence. Other ubiquitous types of materials found as transfer evidence are hairs, fibers, glass, soil, and paint chips. To illustrate, consider a physical struggle between two people. Hairs from both may be exchanged, as may fibers of clothing. Once the two separate, each may bear trace evidence of their contact. However, the transfer evidence is not permanent. By definition, transfer evidence is easily exchanged, and, within a few hours, the original hairs and fibers transferred will likely be lost to secondary transfers. The longer the time since the initial contact, the more evidence is lost.
Traditionally, transfer evidence is thought of as microscopic evidence, but the category is much broader than that. Large fragments of glass can be transferred from a car window to the victim of a hit and run. Back spatter from gunshot wounds can be transferred to the gun and the hand that holds it. By writing on a pad of paper, the tracing of the letters can be transferred to the pages below it, creating indented writing. As
in many forensic analyses, the key tests in the analysis of transfer evidence often involve comparing samples to determine if they had a common source. For example, if a distinctive dust was found on the clothing of a suspected burglar, it would make sense to see if that dust could have come from the site of the burglary under investigation. In many ways, forensic science still revolves around Locardâ€™s ideas of transfer evidence, even if the scope has become much larger than that which he envisioned more than a century ago.