By the time you finish reading this, you will know everything you need to know in order to testify, in court, as a gang cop.
Don’t let me mislead you: you won’t know everything you need to know to be a gang expert. That takes years of training in sociology, anthropology and/or psychology, plus a lot of field work. There are actually very few gang experts around. But you will know everything you need to know to testify, under oath, in court, about gangs and why the jury should convict any gang member about whom you testify.
The post goes on to detail the ways in which the officer who has arrested your client will be testifying as an "expert"as to why your client's actions meet the definition of the crime charged (fill in the blank).
For a great 2008 2nd Circuit Court of Appeals case addressing this issue look to United States v. Meja. A short quote from the Eleventh Circuit shows the courts growing concerns with a police officer as a percipient witness:
If the officer expert strays beyond the bounds of appropriately “expert” matters, that officer becomes, rather than a sociologist describing the inner workings of a closed community, a chronicler of the recent past whose pronouncements on elements of the charged offense serve as shortcuts to proving guilt. As the officer's purported expertise narrows from “organized crime” to “this particular gang,” from the meaning of “capo” to the criminality of the defendant, the officer's testimony becomes more central to the case, more corroborative of the fact witnesses, and thus more like a summary of the facts than an aide in understanding them. The officer expert transforms into the hub of the case, displacing the jury by connecting and combining all other testimony and physical evidence into a coherent, discernible, internally consistent picture of the defendant's guilt.
In such instances, it is a little too convenient that the Government has found an individual who is expert on precisely those facts that the Government must prove to secure a guilty verdict-even more so when that expert happens to be one of the Government's own investigators.
United States v. Meja, 545 F.3d 179, 191 (2nd Cir. 2008).