Early in the 20th century the French scientist Edmond Locard captured the logic of forensic science in a single idea: when two objects come into contact, each takes something from the other and leaves something behind. Usually paraphrased as "every contact leaves a trace," it is the reason trace evidence exists.
Trace evidence in practice
The "traces" are the small, easily transferred materials that move between people, objects and scenes:
- fibres from clothing, carpets and upholstery,
- hairs,
- glass fragments from a broken window,
- paint flakes from a vehicle,
- soil and pollen,
- gunshot residue.
Individually tiny, together they can place a person in a location, link a suspect to a victim, or connect two scenes.
Transfer and persistence
Two questions govern how useful a trace is. Transfer — how readily material moves in the first place — and persistence — how long it stays before it's lost. A fibre may transfer easily but fall off within hours; glass embedded in a shoe sole may persist for days. Interpreting trace evidence means thinking about both, and about the possibility of secondary transfer, where material moves via an intermediate rather than by direct contact.
Corroboration, not proof
Trace evidence rarely identifies an individual the way DNA or a fingerprint can. Its power is corroborative: a bundle of consistent traces builds a picture that supports or undermines an account of events. Recovery is careful and non-destructive — tape-lifts, combings, and packaging that preserves where each trace came from — because, as Locard understood, the value is in the connection the trace reveals.