Defence Attorney's Fallacy
Definition
The incorrect argument that a large global or national population means many unrelated individuals could share the profile, while ignoring the geographic and circumstantial constraints that define the relevant suspect population.
- Type
- Statistical reasoning error in DNA evidence argument
- Core error
- Cites the total population size as the relevant comparison group
- What it ignores
- Geographic and circumstantial constraints on the suspect pool
- Counterpart
- Prosecutor's fallacy, the opposite misuse of match statistics
- Field
- Forensic statistics, random match probability interpretation
Common questions
How does the defence attorney's fallacy differ from a legitimate challenge to random match probability?+
A legitimate challenge questions the database, population reference, or laboratory error rate used to derive the statistic. The fallacy instead inflates the pool of plausible alternative sources by citing the whole national or global population, ignoring that only people with plausible access or connection to the crime are realistic alternative sources.
Why does this fallacy tend to work rhetorically even though it is statistically unsound?+
Large population figures sound persuasive and intuitive to a jury, since a small match probability multiplied by millions of people still yields a large expected number of matches nationwide. The flaw is that almost none of those hypothetical matches had any circumstantial connection to the crime, so the true relevant comparison pool is far smaller.
Related terms
- ENFSI DNA Working Group
- The European Network of Forensic Science Institutes DNA Working Group. Coordinates European forensic DNA standards, including the evaluative reporting guidelines adopted by...
- Hardy-Weinberg Equilibrium (HWE)
- The expected genotype frequency in a large, randomly mating population with no selection, mutation, or migration. Under HWE, the frequency of a...
- Likelihood Ratio (LR)
- The ratio of two conditional probabilities: the probability of the observed evidence given the prosecution's hypothesis (same source), divided by the probability...
- Prior Odds
- The ratio of the probability of the prosecution hypothesis to the probability of the defence hypothesis before the forensic evidence in question...
- Product Rule
- The rule that, under independence, the joint probability of a multi-locus genotype equals the product of the individual locus genotype frequencies. Used...
- 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...
- Random Match Probability (RMP)
- The probability that a randomly chosen unrelated person from the relevant population would match the evidence profile by chance. A very small...
- SWGDAM
- Scientific Working Group for DNA Analysis Methods; the US body that publishes interpretation guidelines and validation requirements for forensic DNA laboratories operating...
- Theta (FST) Correction
- A correction factor applied to allele frequency estimates to account for population sub-structure (the tendency for allele frequencies to differ between sub-populations)....