DNA, Blue Bus, and phase changes
| Author | Edward K. Cheng,G. Alexander Nunn |
| Published date | 01 April 2016 |
| Date | 01 April 2016 |
| DOI | http://doi.org/10.1177/1365712715623556 |
| Subject Matter | Articles |
Article
DNA, Blue Bus, and phase changes
Edward K. Cheng
Vanderbilt University, USA
G. Alexander Nunn
Vanderbilt University, USA
Abstract
In ‘Exploring the Proof Paradoxes’, Mike Redmayne comprehensively surveyed the puzzles at
the intersection of law and statistics, the most famous of which is the Blue Bus problem, which
prohibits legal actors from ascribing liability purely on the basis of probabilistic evidence. DNA
evidence, however, is a longstanding exception to Blue Bus. Like Blue Bus, DNA presents
probabilistic evidence of identity. Unlike Blue Bus, DNA is widely accepted as legitimate, even
when it stands alone as so-called ‘naked’ statistical evidence. Observers often explain such
DNA exceptionalism in two ways: either that people break down in extreme cases, or
relatedly, that modern DNA testing generates effectively unique (as opposed to probabilistic)
identifications. While both explanations are understandable, they are unsatisfying in certain
ways. Breakdown theory seems unprincipled and falls victim to slippery slopes. Uniqueness
theory rests on a fiction and fails to delineate a threshold for when probabilities are sufficiently
small to be considered ‘unique’.
In this paper inspired by our reading of Professor Redmayne’s piece, we propose a quan-
titative explanation for DNA exceptionalism. Specifically, we argue that as random match
probabilities become smaller, the probability of error (i.e. mistaken identification) sharply
transitions from high to low. This sharp change in probability, which we label a ‘phase change’,
explains why legal actors can treat DNA as non-probabilistic evidence. The phase change
further avoids slippery slope problems and helps define when one can legitimately treat
DNA—or any similarly qualified forensic identification method for that matter—as a form of
direct evidence.
Keywords
Bayesian statistics, Blue Bus problem, DNA, evidence, probabilistic evidence,
proof paradoxes
Corresponding author:
Edward K. Cheng, Vanderbilt University, 131 21st Avenue South, Nashville, TN 37203, USA.
E-mail: edward.cheng@vanderbilt.edu
The International Journalof
Evidence & Proof
2016, Vol. 20(2) 112–120
ªThe Author(s) 2016
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DOI: 10.1177/1365712715623556
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