Defender's Fallacy
Definition
Dismissing match evidence by citing the large absolute number of people who share the characteristic, ignoring all other evidence that narrows the suspect population.
- Type of error
- Statistical reasoning fallacy
- Used by
- Defence counsel challenging a match statistic
- Related error
- Prosecutor's fallacy (opposite misuse)
Common questions
How does the defender's fallacy differ from the prosecutor's fallacy?+
The prosecutor's fallacy inflates guilt by treating the match probability as the probability of innocence; the defender's fallacy does the opposite, treating the raw count of matching people in a large population as proof the match is meaningless, while ignoring the case-specific evidence that narrows that population.
How should an expert respond to the defender's fallacy in testimony?+
By explaining that a match statistic is only one strand of evidence and must be weighed alongside opportunity, motive, and other physical evidence that together narrow the pool of possible sources, rather than treating the population figure in isolation.
Related terms
- Activity-Level Proposition
- A proposition about what happened. Example: 'The suspect handled the knife' versus 'The suspect never handled the knife.' Activity-level propositions require the...
- Daubert Standard
- The US federal evidentiary standard (Daubert v. Merrell Dow Pharmaceuticals, 1993) requiring that expert testimony be based on scientifically valid methods with...
- Frye Standard
- The US legal test for admissibility of scientific evidence, originating from Frye v. United States (1923), which required that a technique be...
- Prosecutor's Fallacy
- The error of treating the RMP (or its reciprocal) as the probability that the defendant is innocent, or as the probability that...
- Source Attribution
- The inferential step of linking a trace to its donor. Serology identifies blood as human and potentially as a specific genetic type;...