Meadow's law
|
|
Meadow's law is a precept used until recently by social workers and police officers investigating cases of multiple apparent cot death within a single family. The "law" states that since cot deaths are so rare, "One is a tragedy, two is suspicious and three is murder." The name comes from controversial British paediatrician Sir Roy Meadow, who claims that many unexplained infant deaths are in fact caused by mothers suffering from Munchausen Syndrome by Proxy. In the 1990's, Meadow's ideas gained much currency in the British legal system, and many parents have been convicted of murder on the basis of his expert opinion. In 2003 however, a number of high profile acquittals have cast much doubt on the validity of Meadow's law, and many convictions previously thought "safe" are now under review.
Criticisms of Meadow's law
Critics of Meadow's law state that it is based on a fundamental misunderstanding of statistics, particularly relating to probability, likelihood, and statistical independence.
At the trial in 1999 of solicitor Sally Clark, accused of murdering her two sons Meadow testified that the odds against two such deaths happening naturally was 73,000,000:1, a figure which he obtained by squaring the observed ratio of births to cot-deaths in affluent non-smoking families (approximately 8,500:1).
This caused an uproar amongst professional statisticians, whose criticisms were twofold:
The prosecutor's fallacy
Firstly, Meadow was accused of espousing the so-called prosecutor's fallacy in which the probability of "cause given effect" (i.e. the true likelihood of a suspect's innocence) is confused with that of "effect given cause" (the likelihood that innocence will result in the observed double-cot-death). In reality, these quantities can only be equated when the likelihood of the alternate hypothesis, in this case murder, is close to certainty. Since murder (and especially double murder) is itself a rare event, the probability of Clark's innocence was certainly far greater than Meadow's figure suggested.
Statistical independence
The second criticism was that Meadow's calculation had assumed that cot deaths within a single family were statistically independent events, governed by a probability common to the entire affluent-non-smoking population. No account had been taken of conditions specific to individual families (such as a hypothesised "cot death gene") which might make some more vulnerable than others. The occurrence of one cot-death makes it likely that such conditions exist, and the probability of subsequent deaths is therefore greater than the group average (estimates are mostly in the region of 1:100).
Some mathematicians have estimated that taking both these factors into account, the true odds may have been in the region of 2:1 in favour of Clark's innocence, a far cry from Meadow's 1:73,000,000 against.