Download PDF - Robson Hall Faculty of Law

126 MANITOBA LAW JOURNAL |VOLUME 35 NUMBER 1
Even once the controversy over how DNA match statistics should be
computed was resolved, these probabilistic statements themselves were seen as
problematic by many observers, leading to a significant push to bar the discussion
of match statistics in front of juries. 73 One of the worries associated with
presenting probabilistic evidence is the danger of committing logical fallacies, that
is of the trier of fact reaching conclusions that appear to make sense but are
actually logical errors. In the case of DNA evidence, the best-known example of
this sort of problem is known as the “prosecutor’s fallacy” where the probability of
a random DNA match is conflated with the probability of innocence of the
accused. 74 While the prosecutor’s fallacy is a mistake that can be made by any trial
actor, jury behavior has come under particular scrutiny with the idea that juries
tend to overweight evidence presented numerically. Mock-jury studies have to at
least some extent supported this idea and even suggested that the precise wording
used to present such evidence may significantly influence jury deliberation. 75
DNA evidence seems to place significant demands on all legal actors, including
juries, that they may or may not be able to meet.
Defence lawyers also face challenges when DNA evidence is presented. One
of the biggest problems noted has been the tendency of defence counsel to not
adequately challenge DNA evidence 76 and make inappropriate or unnecessary plea
agreements. 77 While this “blackboxing” 78 occurs with other forms of forensic
evidence, 79 it is particularly notable with forensic DNA. This may occur as much
because of a lack of the resources required to retain qualified experts to evaluate
the quality of the forensic DNA evidence provided. This is particularly a problem
for public defenders in the United States as their workload has increased, and at
least one office has adapted to this problem by developing in-house expertise
among their own legal staff to be able to perform at least limited evaluations of
such evidence. 80 Any lack of adequate defence review is troubling since errors in
73
74
75
76
77
78
79
80
See, for example, Harmon, supra note 71, at 185.
See Vincenzo Rondinelli, “Three Card Monty: Presenting DNA Statistical Evidence To Juries”
(2002) 3 CR (6th) 52 for a summary of the logical problems possible when DNA evidence is
used.
See Koehler, supra note 69, for an example of this sort of study.
William C Thompson et al, “Part 1: Evaluating Forensic DNA Evidence: Essential Elements of a
Competent Defense Review” (2003) 27 Champion 16 online: National Association of Criminal
Defense Lawyers .
William C Thompson et al., “Part 2: Evaluating Forensic DNA Evidence: Essential Elememts of
a Competent Defense Review: Breaking Open the Black Box: How to Review the Electronic
Data” 26 Champion 24 online: National Association of Criminal Defense Lawyers
.
“Blackboxing” here refers to a situation where the internal elements of a test become invisible
and the focus is only on the result.
See Janine Arvizu, “Shattering the Myth: Forensic Laboratories” (2000) 24 Champion 18.
Michele Nethercott, “Faulty Forensic Evidence” (2003) 27 Champion 61 online: National
Association of Criminal Defense Lawyers .