When gang behavior is offered to show an individual's behavior, these cases raise many issues. Among them are:
1. Is a particular set of people a gang or not a gang? By what criteria (if any are explicit) is this classification or judgment made?
2. Is some gang at time T-1 the same or substantially the same as at time T-2? (How does the gang expert know?)
3. Is a branch of some gang in place 1 the same as the branch of the gang in place 2?
3A. Are there substantial variations in the behavior or characteristics of the chapters or subparts of this or that gang?4. Does a gang in place 1 belong to the same gang that a gang in place 2 does?
5. Is the behavior of members of some gang uniform or substantially uniform? If not, is the behavior of individuals who take the part of particular types of members of the gang -- e.g., the "gang clown" -- uniform or substantially uniform?
6. What evidentiary basis does a "gang expert" (typically a police officer) typically have for saying that such and such a group is a gang or that such-and-such a gang in place 1 and a gang in place 2 are part of the same gang or that members of such and such a gang -- or gangs in general? -- adhere to such and such rituals or have such and such attitudes or have such and such beliefs? Personal observation of gang behavior? Statements made by apparent gang members? Statements made by instructors in police academies? Statements made on the internet (which are or are not vetted for accuracy?)?
The "knowledge base" that is often available to "gang experts" seems to leave a lot of opportunities for urban myths to flourish there. This is not to say that police officers never know nuttin' useful about gang behavior in the neighborhoods in which the police officers prowl. It is to say that it is awfully hard to evaluate the typical gang expert's claim that he or she knows the behavior of such and such a gang and how some individual members of that gang behave, feel, or think.