Locard's exchange principle is the theoretical foundation for trace evidence collection. It states that:

• A Every crime leaves financial evidence traceable to the perpetrator
• B Biological evidence degrades at a predictable rate allowing PMI estimation
• C Fingerprints are unique and permanent, forming the basis of all forensic identification
• D Every contact between two objects results in a mutual transfer of material — the perpetrator both takes and leaves traces ✓

Explanation

Edmond Locard (1877–1966), the French criminologist, formulated the exchange principle: 'Every contact leaves a trace' — specifically that when any two objects come into contact, material is transferred bidirectionally. Applied to crime scenes, this means a perpetrator carries trace evidence away from the scene and deposits trace evidence at the scene. This underpins the forensic value of fibres, hair, paint, glass fragments, soil, GSR, DNA, and fingerprints. All other options are either unrelated or paraphrase different forensic principles.

Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.

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Written and medically reviewed by the StethoPrep medical team.