Chapter 3 — Key Takeaways: Physical Evidence
A one-page card. If you remember nothing else from this chapter, remember this.
The core claims
- Locard's exchange principle — every contact leaves (and takes) a trace — is the foundation of the whole field. It guarantees a trace exists. It does not guarantee anyone will (1) detect it, (2) find it persisted, (3) tell it apart from its background, or (4) get from it the answer the case needs.
- Category determines handling and meaning. Physical evidence sorts into transient (vanishes — document, don't collect), conditional (state-dependent — destroyed when conditions change), pattern (geometry is the evidence — photograph with a scale), transfer (moved by contact — collect physically, guard against contamination), and associative (links a person/object/place — a built-in reminder that associate ≠ identify ≠ prove).
- Three samples, kept rigorously distinct: the questioned sample (source unknown), the known/reference sample (source certain — what you compare against), and the control sample (substrate, reagent, positive, elimination — proves the result is attributable to the evidence, not the surface, the chemicals, or the collector). A result reported without its controls is unverifiable.
- Most cases are won or lost in the handling, not at the bench. Degradation (decay), contamination (foreign material added — can fabricate a false association), and bad packaging destroy evidence before any instrument runs.
- Reconstruction names a sequence, not a suspect. Physical evidence is excellent at order (what happened in what order); the actor, the clock-time, and the intent are inferences it cannot supply alone.
The packaging rule (do not get it backward)
| Evidence type | Threat | Package |
|---|---|---|
| Biological (blood, swabs, clothing) | Trapped moisture → bacteria/mold → DNA decay | Air-dry, then breathable paper |
| Fire debris (suspected accelerant) | Evaporation of volatiles | Airtight metal can / nylon bag |
| Liquid samples | Spillage/loss | Sealed, leakproof |
What physical evidence CAN vs. CANNOT establish
| CAN establish | CANNOT establish |
|---|---|
| That a crime occurred (or didn't) | The identity of the actor (only the source of the material) |
| Association — or, more cleanly, exclusion | Intent or state of mind |
| A sequence of events (order) | The time of contact (traces carry no timestamp) |
| Presence of a substance/condition (with valid instruments) | Guilt (a legal conclusion, never an instrument's finding) |
The verb ladder (climb only as far as the evidence carries you)
excludes → cannot exclude / consistent with → strongly supports → ~~proves~~
Exclusion is forensic science's strongest, cleanest voice. A single robust mismatch can eliminate a suspect; an agreement only narrows the field. Reserve "proves" for almost nothing.
Method-validity verdict for this chapter
Physical evidence is the raw material, not a method — so it has no single validity rating. But the chapter sets the yardstick the rest of the book uses: confirming that a substance is present (instrumental) sits high; reasoning from a pattern to an event sits lower, because the inference is softer and more examiner-dependent. The category you are holding (and which question you ask of it) decides how far up the validity spectrum (NAS 2009 / PCAST 2016) it can honestly go. Position on the spectrum is a ceiling on reliability, never a guarantee — a top-spectrum method (DNA) is still ruined by contamination, a swapped label, or a missing control.
Themes advanced here
- Exclusion over proof: physical evidence's surest power is association and, above all, exclusion — not identification of an actor.
- Cognitive bias is the chief threat: Locard's optimism, missing elimination samples, and mislabeled packages each let expectation contaminate interpretation; controls and blind handling are the locks.
What you can honestly say on the stand
"This evidence is consistent with the questioned item having a common source with the reference sample, and I can state how it was collected, packaged, and controlled to support that. It associates the two items. It does not, by itself, establish who performed any act, when the material was deposited, or any person's intent — and the appropriate controls were [run / not run], which bears on how much weight the result can carry."
And its mirror — the strongest sentence the chapter gives you:
"On the evidence, I can exclude that source: it does not share the features it would have to share if it were the origin."