5: Trace and Contact Evidence

Trace evidence is ubiquitous because it comes from the environment in which we interact as part of our daily lives. Although the environment in which we interact is populated by manufactured materials supplied at the transient whims of the global manufacturing economy, our behaviour as human beings remains highly individual. The strength of trace evidence—its value to the court—lies in being able to put the finding of these materials into the context of human behaviour. It is largely for this reason that a DNA-style statistic is not the appropriate means to express the strength of trace evidence in the court room. This chapter deals with the principles shared by all types of particulate trace evidence, focusing on the types most commonly examined in forensic science laboratories, typically fibres and glass. Laboratory processes are described, and some case studies are presented to illustrate particular points.