Check out William Saletan's skeptical take on "contagious shooting" as an explanation for recent police shootings:
It's natural to grope for a rational or mechanical explanation in cases like these. But it's not clear which kind of explanation this contagion is. If it's rational, it should be judged like any rational process, and cops should be culpable for it. If it's mechanical, it should be controlled like any mechanical process, starting with the guns supplied to police. We can't keep doing what we've been doing: giving cops high-round semiautomatic weapons because we trust them not to blast away like robots, then excusing them like robots when they blast away.
Supposedly, contagious shooting was coined four decades ago to explain copycat police fire during riots. Once you start describing a behavioral phenomenon as a predictable sequence of events—"post-traumatic stress disorder," for example—people start reading it as an excuse. Seven years ago, during the Diallo case, a lawyer for one of the accused officers pointed out that "contagious shooting" was in the New York Police Department patrol guide. "I suspect that this phenomenon may play an active role in this case for my client," he told reporters.
What makes contagious shooting a handy legal defense is its mechanical portrayal of behavior. You're not choosing to kill; you're catching a disease. In the Diallo era, the NYPD patrol guide explained that the first shot "sets off a chain reaction of shooting by other personnel." Officers "join in as a kind of contagion," said the Times. They "instinctively follow suit," said the Daily News, as one shot "sparks a volley from other officers." On Monday, the Times said contagious shooting "spreads like germs, like laughter." One former NYPD official called it the "fog of the moment." Another said "your reflexes take over." A third told CNN, "It's sort of like a Pavlovian response. It's automatic. It's not intentional."
This mess of metaphors is telling. Nothing can behave like germs, sparks, laughter, fog, instinct, and conditioning all at once. That's the first clue that "contagious" is being used not to clarify matters, but to confuse them. Another clue is that the same people who invoke it often point out that the number of shootings by police is low and has been falling. An urge that's so commonly resisted can't be irresistible.