Monday, June 08, 2009

A bit about David Bain’s retrial

The right tribunal

Last month was the fourteenth anniversary of the start of David Bain's first trial for murdering his parents, two sisters and brother. Last Friday he was acquitted on all charges. Judicial resistance to the idea that fresh evidence was sufficient to cast doubts on the guilty verdicts at his first trial was broken by the Privy Council on 10 May 2007.

It seems quite unlikely that without the availability of an appeal to the Privy Council the convictions would have been quashed. The Supreme Court of New Zealand, which now exercises that appellate jurisdiction, was created by promoting judges from the Court of Appeal. The Court of Appeal considered Mr Bain's case three times, and these are summarised by Lord Bingham ([2007] UKPC 33).

The faulty approach to determining whether a substantial miscarriage of justice had occurred because of the absence of the fresh evidence at the trial was so deeply ingrained in the way judges were thinking that even Sir Thomas Thorp, an advocate of protections against wrongful convictions, considered there had not been a miscarriage in Mr Bain's case.

I have noted the way the Privy Council corrected the Court of Appeal's approach (see second entry for 11 May 2007). In subsequent cases the Court tends to refer to the Privy Council decision – if at all – only to the extent that it approved the Court's statement of the law (a matter that was never in issue: it was the application of the law that was faulty). It cites its own judgment ([2004] 1 NZLR 638) for that statement of the law, without mentioning its error. See, for example, R v Morrice [2008] NZCA 261, R v Kingi [2008] NZCA 195, and R v F (CA300/08) [2009] NZCA 177. An obvious question for students is, has the Court's approach changed since the Privy Council's decision?

This is not to say that the Supreme Court is incapable of overturning any Court of Appeal decision in this case. In upholding an appeal against a pre-trial admissibility ruling, the Supreme Court has given itself the opportunity to clarify the law on s 8 Evidence Act 2006, which concerns the discretion to exclude evidence where its probative value is exceeded by the illegitimately prejudicial effect of admitting it. Judgment pending. [Update: see note for 12 June 2009]

Transposition of the conditional

Any sensible person will leave the decision on guilt or innocence to the jury. The only aspect of Mr Bain's recent trial that I mention here is an amusing example of a fallacy. A defence expert had given evidence about the death of David Bain's father, Robin Bain, who had died from a rifle shot to the left temple. Robin Bain was right-handed. The witness had said that this was not inconsistent with suicide. In cross-examination (according to the news media) the prosecutor had asserted that, as only about 3% of right-handed people who commit suicide by gunshot shoot themselves in the left temple, it was most unlikely that Robin Bain had committed suicide.

This error of logic is sometimes called the prosecutor's fallacy. But anyone can make it. It is a transposition of the conditional. The probability of suicide, given a wound to the left temple, is not the same as the probability of a wound to the left temple, given suicide. The former is about wounds to the left temple, and the latter is about suicides.

This is obvious from a simpler example (being a NZer, I take sheep ...): the probability of an animal having four legs, given that it is a sheep, is not the same as the probability of it being a sheep, given that it has four legs. The former is about sheep, the latter is about quadrupeds.

The prosecutor was using a statement about suicides as if it were a statement about left temple wounds. It was the latter that was relevant, not the former.

This is not to say that the evidence (3% of suicides by gunshot are to left temple) was not probative. Its value would be assessed in Bayesian terms by its likelihood ratio: the ratio of the probability of a left temple gunshot, given that this was murder, to the probability of a left temple gunshot, given that this was suicide. The missing information is the occurrence of murders by left temple gunshots.
A false start

According to reports, the judge started summing up the case by saying that the question was, "Was it Robin or was it David?"

That was the question for the police investigation, and for anyone playing detective, but the only question for the jury was, did David do it? Finding David not guilty was not the same as finding that Robin did it. But to find David guilty the jury would have had to exclude a reasonable possibility that Robin did it.

We do not know whether the jury acquitted David because they could not exclude a reasonable possibility that Robin did it, or because they thought it quite likely that Robin did it, or because they thought it more probable than not that Robin did it, or because they were sure Robin did it. Any of those is sufficient to acquit David.


David can receive compensation for around 13 years in prison if he can establish on the balance of probabilities that he is innocent.

Another tribunal (probably a QC or SC appointed by the government) will have to decide that. It seems a terrible inefficiency to discard the jury and not to ask for their decision on this.

It is not unusual to have juries grapple with two standards of proof. They do this in drug trials, where possession of a given quantity of drug gives rise to a presumption of purpose of supply, rebuttable on the balance of probabilities.

Having two standards of proof under consideration may help the jury understand the difference between the balance of probabilities and proof beyond reasonable doubt. The first question the jury asked after retiring to consider its verdicts was for clarification of the meaning of reasonable doubt. In accordance with R v Wanhalla – see 25 August 2006 – the judge gave minimal assistance on that.

A juror's experience

For an intelligent description of a juror's experience in this trial, see the report in the New Zealand Herald online, Sunday, June 7 2009, "Bain juror: we were hounded".

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