Gang culture expertise - proving motive or murderous intent by Peter Doyle QC

We are as practitioners familiar with defending in alleged gangland tit for tat fatal shooting cases. What room is there in such cases for police expert evidence as to gang culture and what relevance attaches to such evidence?
Recently (20 October 2015 Myers v The Queen, Cox v Same  and Brangman v Same) the Privy Council (on appeal from the Court of Appeal of Bermuda) considered the admissibility of expert police evidence on gang culture in three cases where the appellants had been convicted of murder and attempted murder.
In support of other evidence that in the first two cases  they had been the gunman, the prosecution was permitted to call a police officer assigned to a unit which targeted criminal gangs as being an expert on such gangs in Bermuda.
He had identified the gunmen and victims in each case as members of rival gangs and gave evidence about the activities of such gangs and the culture in which they operated.
That culture embraced the shooting of a random member of a rival gang in response to a perceived insult or attack by that gang on one of its own members.
The prosecution had adduced evidence that shortly before the shootings a member of the victim's gang had insulted or harmed a member of the defendant's gang as well as ballistic evidence to show that the gun in question had been used in other shooting incidents inferred to be gang related.
In the third case gang evidence was ruled admissible by the trial judge to rebut a defence claim during cross examination of the survivor of the shooting, who had identified the defendant as the gunman, that he was mistaken and that the latter had no motive to shoot him.
The Board in dismissing the appeals identified the circumstances where such evidence would go beyond mere propensity and be relevant and admissible.
Thus, it would be relevant if it showed that a defendant had a motive to kill the victim that went to support the case that it was him rather than someone else and or that he did the act with murderous intent rather than accidentally or without intent to do at least grievous bodily harm.
Evidence that one defendant was a member of one gang likely to have felt aggrieved by some action of the deceased's gang and moreover likely to have reacted by targeting  the deceased on grounds of his membership of the opposing gang, was capable of showing that he had a motive to kill the victim.
Such evidence, as distinct from evidence that was of background or contextual interest, and which furthermore could be said (i) to truly add something beyond mere propensity and (ii) which could assist the jury to resolve one or more issues in the case, was relevant.
The test as to the ambit of gang evidence would depend in any particular case on what legitimate role it might have in helping the jury to resolve one or more issues in the case.
However, the police officer giving such evidence had to have made a sufficient study of gangs whether by formal training or through practical experience in order to assemble what could properly be regarded as a balanced body of specialised knowledge which would not be available to the tribunal of fact.
It had to be understood by such an officer that once he was tendered as an expert he was not simply a part of the prosecution team but had a separate duty to the court to give independent evidence whichever side it favoured.
This decision will no doubt provide a breeding ground for adducing what in essence is bad character evidence of the gang's philosophy.
Thus beware any defendant who signs up to a gang that practices what it is said to preach. In an appropriate case, even if a defendant only joined the gang on the very day of the shooting in question, he may find that the prosecution can seek to prove that he was signed up to its culture whatever the length of his membership.

Peter Doyle QC