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Abstract
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Whenever a litigant needs to prove that a certain result was caused in a specific way, what could be more compelling than citing the infinitesimal probability of that result emanating from an alternative natural cause? Contrary to this intuitive position, in the present article, I argue that the contention that a result was due to a certain cause should remain unaffected by statistical evidence of the extremely low probability of an alternative cause. The only scenario in which the low probability of a natural cause would be relevant to the case at hand is if it were contrasted with another piece of statistical evidence: the frequency of the criminal activity among people who are similar to the accused. However, by connecting the use of probabilistic generalisations in legal fact-finding to the issue of free will, I hold that, in Criminal Law, contrasting frequencies in this manner is objectionable--as a matter of principle--regardless of how reliable the statistical analysis is. Consequently, if the low probability of a natural cause is probative only if contrasted with another piece of statistical evidence that is objectionable, then neither piece of evidence should be admitted in criminal trials.
When a litigant needs to prove that a certain result was caused in a specific way, what could make a stronger case than presenting the infinitesimal probability of that result emanating from an alternative cause? When Sally Clark was put on trial in England for the murder of her two babies, the prosecution called upon Professor Sir Roy Meadow, a leading British paediatrician of the time, who testified that the probability of two cases of Sudden Infant Death Syndrome (SIDS, or 'cot death') occurring in a family similar to Clark's was just 1 in 73 million.Footnote 1 The prosecution attempted to use this low probability to support the claim that the cause of death in both cases was their mother's conduct. The use of evidence of this sort is not unique to the English legal system. When John Veysey was tried in the United States for insurance fraud after he claimed, for the fourth time, that his house had burnt down, an actuary testified that the probability that four houses belonging to the same person would accidentally be burnt to the ground in succession was 1 in 1.773 trillion.Footnote 2 Statistical evidence is used not only in Common Law but also in Continental jurisdictions.Footnote 3 In the Netherlands, Lucia De Berk was accused of murdering seven babies and attempting to murder three more in the hospital department where she worked as a nurse. An expert witness for the prosecution testified that the probability of so many natural deaths coinciding with a particular nurse's shift pattern was 1 in 342 million.Footnote 4 It seems that adducing statistical evidence that establishes the infinitesimal probability of alternative explanations is an effective way for the prosecution to convince the fact-finder that the results in question (here, the infants' deaths or the house fires) were caused by the accused.
The use of such evidence has attracted criticism both in the academic literature and in public opinion, but these objections have mainly focused on the technical mistakes made when using this type of evidence, as well as the cognitive biases it often induces. For example, before Clark's conviction was quashed, the statistical calculation was described by Professor Philip Dawid, forensic statistician and expert witness on behalf of the defence, as 'extremely dubious'.Footnote 5 Following Clark's release, the wrongful conviction was predominantly attributed to this calculation,Footnote 6 both by expert statisticiansFootnote 7 and the media.Footnote 8 More generally, it has been argued that the use of statistical evidence by courts carries the risk of misusing statistics,Footnote 9 misrepresenting the information the evidence conveys,Footnote 10 and creating a false impression of accuracy...
Language: en |