Friday, October 21, 2016

Proving propensity

Propensity evidence is evidence about the defendant’s conduct, not directly connected to the presently alleged offending, but which shows a propensity to act in a way that is relevant to an issue now before the court. To what, if any, standard must propensity be proved before it can be used in the process of determining the facts on the present allegation?

The United Kingdom Supreme Court has held that propensity must be proved beyond reasonable doubt before it can be taken into account in fact-finding: R v Mitchell (Northern Ireland) [2016] UKSC 55 (19 October 2016).

The Court noted that the common law had not settled this question, and legislation only covered admissibility, not standard of proof.

New Zealand common law has taken the position, almost as a mere assumption, that no particular standard of proof of propensity is required, but that instead admissible evidence of propensity is just circumstantial evidence that can be considered with all the other evidence in determining guilt on the present charge. For example, see R v Guy (1996) 13 CRNZ 589 (CA). And if the defendant had been previously charged with offending that would be evidence of a relevant propensity, but had been acquitted, evidence of the facts supporting that earlier allegation can, notwithstanding the acquittal, be given to prove the propensity: R v Degnan [2001] 1 NZLR 280, (2000) 18 CRNZ 319 (CA).

Is there necessarily an inconsistency here with Mitchell? That case only requires the propensity, not the evidence of it, to be proved beyond reasonable doubt. Its effect is that the method for determining facts if propensity evidence is relied on is to consider all the admissible evidence of propensity, to assess it and to decide whether propensity is proved beyond reasonable doubt. If it is, then it may be taken into account with the evidence of the present allegation. If it is not proved to that standard it must be ignored. It is not mixed with the other evidence in the case when the fact-finder is deciding whether there is a propensity.

This is inconsistent with the New Zealand approach. See for example R v Holtz [2003] 1 NZLR 667, (2002) 20 CRNZ 14 (CA), where no standard of proof is required except for an ultimate conclusion of guilt. It is sufficient (at [39]) that the fact-finder “conclude” or be “satisfied” that the evidence of propensity establishes the relevant propensity. There the Court “rejected a general requirement that pattern or the like must be found beyond reasonable doubt before similar fact evidence may be used”.

If you wanted to spell out the difference in Bayesian terms, you could say that under the New Zealand approach all the likelihood ratios are combined in determining whether guilt has been proved beyond reasonable doubt, whereas the Supreme Court of the United Kingdom requires the likelihood ratios for the propensity to be assessed separately and only if they establish the propensity beyond reasonable doubt can they (with appropriate reformulations) be combined with the others. I say “with appropriate reformulations” because when considering propensity the ratios are: the probability of getting a particular item of evidence on the assumption that the propensity is proved, to the probability of getting that evidence on the assumption that the propensity is not proved. But when combined with the evidence directly relevant to the present allegation, the ratios are: the probability of getting the evidence on the assumption that the defendant is guilty, to the probability of getting the evidence on the assumption that the defendant is innocent. The difficulty with the Mitchell approach is that to establish a propensity beyond reasonable doubt it may be necessary to have evidence of numerous prior instances.

It is easy to think, without having sufficient evidence to support the opinion, that a type of behaviour shows a propensity. For example, suppose that D is charged with burglary. A witness could only say that the burglar carried a red bag. The defendant owns a red bag and on two previous occasions had used it while committing burglaries. This looks like a propensity. But what if further evidence showed that D had committed eight other burglaries while carrying a black bag. Is D’s propensity to burgle with a black bag or with a red bag? And is either proved beyond reasonable doubt to be a propensity?

Furthermore, using the same example, evidence may be more probative “of itself” than if it is used as propensity. Forget the other burglaries and ask how many of the possible present burglars have red bags? This might be a very small proportion. As the denominator it would make the Bayesian likelihood ratio large enough to give D’s possession of a red bag significant probative value. Perhaps this demonstrates that propensity, if treated in the Mitchell way, can be something of a red herring, detracting from the real value of the evidence that is directly relevant to the present allegation.