Similar Fact Evidence

R. v. P.E.L., 2013 BCCA 542:

[20]        Similar act evidence exemplifies a type of propensity reasoning.  It could be colloquially expressed by the aphorism that a leopard does not change its spots.  Of course, this sort of reasoning collides squarely with the long established common law rule that no one should be convicted simply by reason of rascality or a disreputable history – see Harrison's Trial(1692), 12 How. St. Tr. 833 (Old Bailey (London)), referred to by Binnie J. inR. v. Handy, 2002 SCC 56 (CanLII), 2002 SCC 56, [2002] 2 S.C.R. 908 at para. 32.  He said at p. 926, para. 37:

The policy basis for the exclusion is that while in some cases propensity inferred from similar facts may be relevant, it may also capture the attention of the trier of fact to an unwarranted degree. Its potential for prejudice, distraction and time consumption is very great and these disadvantages will almost always outweigh its probative value. It ought, in general, to form no part of the case which the accused is called on to answer. It is excluded notwithstanding the general rule that all relevant evidence is admissible: Arp,supra, at para. 38; Robertson,supra, at p. 941; Morris,supra, at pp. 201-2;R. v. Seaboyer, 1991 CanLII 76 (SCC), [1991] 2 S.C.R. 577, at p. 613.

[21]        It is always a question of weighing probative value against prejudicial effect.  Ironically, the more prejudicial (in many cases), because of oft-repeated conduct, the more likely such contested evidence may survive the high hurdle of admissibility because of its great cogency and high probative value.