Exchange Principle
Definition
Locard's core idea that physical contact between two surfaces moves material in both directions, so a perpetrator both deposits and removes trace material at a scene.
- Originator
- Edmond Locard
- Core claim
- Contact between two surfaces transfers material both ways
- Application
- Perpetrator deposits and removes trace material at a scene
- Field
- Trace evidence transfer analysis
Common questions
Does the exchange principle guarantee that transferred material will always be found?+
No. It predicts that transfer occurs, not that the transferred material will survive, be detectable, or be found. Amount transferred, persistence, and the thoroughness of the search all affect whether trace evidence is actually recovered.
How is the exchange principle applied differently from simple trace evidence collection?+
It is a theoretical foundation rather than a technique: it tells investigators to look for two-way transfer, meaning both what the perpetrator left behind and what they may have picked up from the scene, rather than searching in only one direction.
Related terms
- Persistence
- The property that friction ridge configuration remains stable from birth through death and into decomposition, as long as the papillary dermis survives....
- Primary Transfer
- The direct movement of biological material from an animal to a surface it contacts. When a dog sheds hair onto its owner's...
- Secondary Transfer
- The movement of a glass fragment from a primary surface (which contacted the broken glass) to a secondary surface (which contacted the...
- TPR Framework
- Transfer, persistence, recovery: the three sequential filters that every trace passes through, and the lens analysts use to weigh what a positive...
- Trace Evidence
- Small quantities of material transferred between people, objects, and locations during contact. Forensic geology focuses on the geological subset of trace evidence:...