Often groups of people behave in consistent ways for a long time because they share a belief that the consistent way everyone is behaving will cause things to be bad for any individual who deviates from it.
For instance, there is a line in the sandwich shop. From a perspective so naive to our ubiquitous norms that it is hard to imagine, you might wonder why the person standing at the back does so, when the shopkeeper is much more likely to get sandwiches for people at the front. The reason of course is that if he were to position himself in the ample physical space between the person at the front and the shopkeeper, there would be some kind of uproar. Not only would the person at the front be angry, but everyone in the line would back them up, and the shopkeeper probably wouldn’t even grant a sandwich to the line-jumper.
So key to our ubiquitous tendency to stand peacefully in line is the fact that our common behavior is ‘stand in line and get angry with anyone who jumps it’ not just ‘stand in line’ which would be immediately exploited until the norm upholder gave up or died of starvation.
An interesting thing about this extra clause is that it is about our hypothetical behavior in circumstances that rarely happen. If our norms work well enough, we might go for years all peacefully standing in line, without anyone ever trying to push in at the front, because why would they?
An upshot is that if serious norm violations are rare, people might become pragmatically ill-equipped to respond to them. They might forget how to, or they might stop having the right resources to do so, physical or institutional. Or if generations are passing with no violations, the new generation might just fail to ever learn that they are meant to respond to violations, or learn what that would look like, since they never observe it. And maybe nobody notices any of this until norms are being violated and they find they have no response.
For instance, suppose that occasionally people sort of wander toward the front of the line in ambiguous circumstances, hoping to evade punishment by feigning innocent confusion. And those in the line always loudly point out the ‘error’ and the room scowls and the person is virtually always scared into getting in line. But one day someone just blatantly walks up to the front of the line. People point out the ‘error’ but the person says it is not an error: they are skipping the line.
The people in the line have never seen this. They only have experience quietly mentioning that they observe a possible norm violation, because that has always been plenty threatening. Everyone has become so used to believing that there is terrifying weaponry ready to be pulled out if there really were a real norm violation, that nobody has any experience pulling it out.
And perhaps it has been so long since anyone did pull it out that the specific weapons they stashed away for this wouldn’t even work any more. Maybe the threat used to be that everyone watching would gossip to others in the town about how bad you were. But now in a modern sandwich shop in a large city, that isn’t even a threat.
The world is full of sufficiently different people that in the real world, maybe someone would just punch you in the face. But it seems easy to imagine a case where nobody does anything. Where they haven’t been in this situation for so long, they can’t remember whether there is another clause in their shared behavior pattern that says if you punch someone because they got in line in front of you at the sandwich shop that you should be punished too.
Does this sort of erosion of unexercised threats actually happen? I am not sure. I think of it if for instance a politician behaves badly in ways that nobody had even thought of because they are so obviously not what you do; and then get away with it while onlookers are like ‘wait, what?! You can’t do that!’ But I don’t know enough details of such cases to judge whether they are cases of threat erosion.
Another case where I guess people might experience this is in bringing up children, because threats of punishment are often made, and the details of the relationship is changing as the children change, so if you don’t exercise the threats they can cease to be relevant without you noticing.
I probably saw something close to this in looking after my brothers. My brothers were into fighting for fairness and justice, where ‘fairness’ is about one’s right to have stuff someone else has, and ‘justice’ is about meeting all slights with tireless bloodthirsty revenge. So my main task was dealing with fights, and threats were relevant. When my brothers were small, I was physically in control of what they could have or eat or punch, so could uphold threats. Later they were really big and I couldn’t have enforced any threats if they decided to make it a physical conflict. This hadn’t been strongly tested by the time it dawned on me, and I continued to bluff. But there was perhaps an earlier period when I didn’t realize I was now bluffing, where if they had realized first, I would have been left with no response. This isn’t quite a case, because noticing that my threats were decaying didn’t let me strengthen them. But had I been an adult in charge of money and the car and such, things may have been different.
I’m not sure if such a situation would last for long in important cases where a replacement threat is feasible. If violations are still clearly bad for a set of people who have general resources to invest in threats, I’d often expect a new and compelling response to be invented in time.
Meteuphoric 
There is a related phenomenon where an entire community figures out that X is really bad and cannot be allowed, and people who do X are shunned or ostracized. Think of X as, for example, being rude to your coworkers. After persistent rudeness leads to a horrible work environment, everyone starts shunning and ostracizing those whose behave rudely. This works, the offenders either quiet down or leave and everyone lives happily ever after. Except then the knowledge somehow gets forgotten. Perhaps the composition of the community changes. A few people start engaging in X again. It isn’t so much that people don’t know how to reply — they do — but they don’t see X as a terribly big deal, having forgotten just how bad things get when violations of X were commonplace.
Someone who worked at NASA once told me a similar dynamic could explain why the program undergoes periodic disasters (e.g., explosions of the space shuttles Challenger and Columbia). In the wake of each disaster, everyone is gung-ho about safety. But then a decade or two passes and a new generations of middle managers rise in NASA. The new people have never had a major disaster on their hands. For decades, they witnessed successful launch after successful launch. It isn’t that they ignore safety regulations; they don’t. But they just aren’t as paranoid about safety as the old crowd. Feynman’s writings about the Challenger explosion point out that a number of concerns were raised by NASA scientists pre-launch, but all were not taken as seriously by middle managers.
For pretty much this reason, advice like “the best response to a mugger is to just give them what they want” has always seemed to me questionably moral, insofar as the risk-reward assessment doesn’t seem to consider what’s best for society, but instead to take the rate of muggings as a given.
This sort of thing is one reason why I’m heavily in favor of gossip and heavily against norms against gossip. Gossip is a pretty multi-purpose protocol for punishing bad behavior in communities: ideally, if you do bad stuff people will talk, everyone will know, your friends will start avoiding you, etc.
One thing that could happen when people lose the nerve for informally enforcing social norms: They quietly seethe as the line jumper gets served first, and then begin agitate for some *authority* to sort this out such outrages in the future. You know, some written rules, along with deputized enforcers who may/must initiate the fitting punishment procedure.
This pattern is an important story of our time. We are genuinely losing social skills for navigating public space. Once upon a time, dealing with line jumpers [and noisy neighbors, annoying random kids, crude suitors, detractors, people defending uncomfortable ideas, etc.] was effortless and not especially distressing; now similar interventions are awkward and uncomfortable. We feel unequipped to deal with that kind of thing, so we want to outsource the job to some kind of administrator.
There was a Terry Pratchett quote in which the Watch was trying to intimidate a crowd into not rioting – they know they’re basically bluffing, but they still have to put on a show for the crowd anyway…
This happens a lot when cultures collide. An insular community in an unstable (but sometimes internally preferable) equilibrium comes into contact with a more cosmopolitan community that has inevitably slid into a more stable higher entropy equilibrium, the result of which is the spread of the more stable higher entropy equilibrium. I think this is why the culture of all diverse cosmopolitan cities are basically the same, not only across space but across time, while insulated communities vary greatly.
For example, I once saw a white guy cross a street at the wrong time in Tokyo because no cars were coming. (Japanese people seem not to do this for some reason.) A short Japanese cop blew his whistle repeatedly, as loud as he could, and stood somewhat in the way of the jaywalker, while the other pedestrians made quiet sounds of disapproval (kshhhh). But the jaywalker just kept going, as if nothing was happening, to the bewilderment of everyone. Finally, when he’d reached the other side, the cop, though visibly frustrated, was compelled to return to his station at the intersection, so that was the end of it.
I talked to the jaywalker and apparently he does this all the time. Basically he thinks lots of rules in Tokyo have no purpose nor consequence, so he’s actually doing them a favor by making this apparent. (Under or over rated: cultural chauvinism?)
Here’s someone else’s description of Japanese police, whose detective work consists mostly of politely asking suspects if they did it or not:
“The idea that cunning, stubbornness, and lies were to be expected from a criminal — that it was in order to deal with such people that the police existed — didn’t seem obvious to them at all. In their minds, they weren’t incompetent or unimaginative or lazy. They were the unlucky victims of that rare thing in the world’s most law-abiding country: the dishonest criminal.”
— http://www.nytimes.com/2012/06/25/opinion/japans-inept-guardians.html
It seems obvious to me that a new community seeded with half Tokyo immigrants and half NYC immigrants, chosen uniformly at random, would more closely resemble NYC than Tokyo, at least with respect to policing and crime, and probably many other things as well. NYC is the higher entropy, stabler equilibrium. It’s memes have already survived, or rather are already the outcome of, repeated intermixing with foreign memes.
I’m reminded of false vs true vacuum states in cosmology, where tunneling to the true vacuum is unlikely, yet irreversible — once reached somewhere, it dominates everywhere. Or instances in which a land bridge connects a small island to a large continent — inevitably the species of the island are wiped out with little impact upon the mainland. Or instances in which the germs of a high-population-density people wipe out a low-population-density people. Threat erosion is the weak immune system of the noble/insular community.
It seems to me, actually, that a number of cryptographic protocols and their applications could be seen as aimed at addressing this problem. Consider old high-trust-society/low-trust-society distinction. Low-trust society sucks because you have no reason to believe that others won’t screw you over. You try to guard against this (such as with threats of punishment), but oftentimes the only real way to guard yourself (or the only viable punishment) is to just not do business, so things don’t get done and therefore things suck. High-trust society, you can trust other people, things get done, it’s great — but the downside is that often when something goes wrong, the threats of punishment are too eroded and never get invoked. (And yet people are trustworthy enough that this doesn’t result in instant decay to the low-trust equilibrium.) If you consider what sort of society crypto enthusiasts seem to be trying to build, it’s one of low trust-in-the-crypto-sense, but high effective-trust-in-terms-of-ability-to-get-things-done-and-uneroded-threats.