Everyone has a theory why, except for a gun-possession charge, a New York grand jury let Bernhard Goetz off: racism, reluctant witnesses, a soft DA. I say it is just another example, though a novel application, of the tipover phenomenon.

The tipover phenomenon occurs whenever so many people are breaking a law that it becomes unenforceable. It usually comes up in the context of private actions (like drinking, gambling or drug-taking) that society simply gives up trying to suppress. What makes the Goetz case unusual is that the law in question happens to be the prohibition against shooting people in subways.

That surprised me. Even in the New York subway, where there's a whole lot of mugging going on, I expected that the rules against shooting would still be considered in force. The grand jurors thought otherwise. They must have figured that any place where a felony and a half is committed every hour has achieved tipover. Having found the subway to be in a Hobbesian state of nature, they decided to suspend any pretense to law.

The result is a verdict of rare moral obtuseness. Goetz is now charged with illegal gun possession, when carrying the gun was the one logical and defensible thing he did: he meant it for self-defense, and sure enough he needed it just for that.

As for firing the gun and shooting two of his harassers in the back -- not guilty, not even charged. Yet that was a wholly gratuitous act of revenge. Vengeance is mine, saith the law: that is the basic idea of Western jurisprudence. But the West, it seems, stops at the subway turnstile. I suspect that is why Mayor Koch now wants to put judges in the subways: a little symbolism to convince people that the law does prvail below ground, despite the fact that the muggers, vigilantes and grand juries seem unanimous in saying otherwise.

It must be admitted that tipover is always hard to deal with. What do you do when breaking the law becomes routine? There is no single easy rule to follow. Sometimes you stand fast. Some social behaviors are so destructive that no matter how common the lawbreaking, you can't drop the law. Running red lights belongs to this category. (Not subway shootings, mind you, but we've gone over that.) If you don't gamely keep insisting on stop-on-red, no one will ever get across town.

Sometimes you give in. Some behaviors are so engrained that suppressing them with police power does more harm than good. Prohibition, for example. After 3,000 years, alcohol was not about to surrender to a constitutional amendment. We hung on to Prohibition for 13 years, though drinking as a social habit had reached tipover on the day it was enacted.

Sometimes you cash in. Consider what we've done about gambling. Not long ago state governments came to the conclusion that people are going to gamble anyway, so they legalized it. But this time they went one better. They claimed a piece of the action. What started out as a realistic concession to human weakness, turned into a comical exercise in official cynicism. I am now relentlessly advised by the Maryland lottery board that "You gotta play to win!" That's tipover with a venge- ance.

These are the fun cases. But tipover, which is the breakdown of old norms in an area of social life, can be serious business. To us Prohibition-era mayhem has the air of a Keystone Kops affair; but there is nothing whimsical about, say, today's illegal drug business. The really hard cases arise when society wants to discourage certain behaviors, but knows they will go on anyway and wants to avoid the corruption that accompanies unenforceable laws. What to do?

We don't know, so we improvise. We make drug-taking illegal, but enforcement selective: we go after pushers more than users, some drugs (heroin) more than others. We regulate pornography the opposite way. We make it legal but restricted: permitted in some places (combat zones, cable TV) but not others. The compromise is sloppy and not everyone is happy. Feminists and fundamentalists think the porn laws too lenient. Civil libertarians think the drug laws too punitive.

We are in for more hard cases and sloppy compromises. In an era of rapid social change, as the traditional non- legal means of social control erode, tipover becomes common. The authority of moral enforcers such as parents, teachers, and clergy has declined. The mass media, the principal substitute teacher, have adopted a position of studied neutrality. The law is left naked as the sole enforcer of a moral consensus that has largely vanished. One result is the rise of the "social issues," behind almost every one of which (abortion being the most prominent example) is an argument over whether the law should give in to the new moral consensus.

In static societies, the norms endure. In revolutionary societies, the norms all go at once. Ours is what the biologists like to call adaptive: we tip the norms over one at a time and finesse the question of how our laws are to acknowledge the change.

This modified limited hangout approach helps us not only to manage social change but to conceal it. Until, that is, a lottery jingle or a Goetz comes along to remind us how far we've come.