In my post yesterday I argued that if a precedent's age counts as a reason for adhering to it and its youth counts as a reason for retaining it, then the age (or youth) of the precedent isn't doing any real work. Crucial to the argument is the fact that, judged by various statements of the Supreme Court and individual Justices, the ranges of "old" and "new" precedents actually overlap. (Some of the comments provided additional examples. Thanks!) I certainly didn't mean to suggest that it's impossible for a proposition to be true at both ends of a spectrum but not in the middle.
Here are some familiar examples: If you have a bacterial infection, you want to take a "therapeutic dose" of antibiotic. If you take too little, you won't kill all of the infecting bacteria. If you take too much, you could harm the helpful bacteria that populate your gut. Indeed, we can make the point with ordinary nutrients: Too little food and you starve to death; the right amount and you thrive; too much and you become obese, with resulting ill health effects.
So what are we to make of the Fourth Amendment cases in which police cite as the reason for a traffic stop or subsequent investigatory activity---for which they must have at least "reasonable suspicion"---various opposite factors that seem to overlap? For example, the fact that a driver "didn't make eye contact" has been cited, as has the fact that the driver kept staring at the police officer, as has the fact that the driver's eyes darted around nervously. That appears to leave no pattern of ocular activity that is not suspicious. Likewise, drivers who drove fast and those who drove too slowly (thus giving the impression that they're trying to avoid police scrutiny) have had their speed invoked. There's either a judicial opinion or a law review article (which I couldn't find in 5 minutes on WestLaw---kudos to anyone who can locate it) that nicely lists the contradictory factors that courts have approved as bases for police investigation, much in the style of Karl Llewellyn's famous Vanderbilt article on the canons and their anti-canons.
It's tempting to say that in these circumstances, the police are merely using indicia like "the suspect was sweating" as a cover for a search or seizure that was either simply unjustified, or worse, based on an illicit criterion like the race of the suspect. And certainly that's true often enough to justify requiring "articulable suspicion," as the Supreme Court does. A hunch can just be a shield for prejudice. But it's also true that human beings can often detect suspicious patterns of behavior even when the conscious human mind can't articulate exactly what makes the behavior suspicious. That's part of the lesson of Malcolm Gladwell's Blink and Gavin de Becker's The Gift of Fear. So it's possible that much of what looks like inconsistent justifications for police activity arises out of a requirement that police articulate reasons for actions taken in response to cues that never rose to the level of consciousness.