How does one draw the line between the sort of morality a legal system should enforce, and the sort of morality the legal system should leave to its own devices? It seems that there are some cases where the law should clearly enforce morality (special laws against child abuse, for example), yet there are clearly other cases where the law doesn't and shouldn't have much to say (for example, the law does not systematically punish people who lie to their spouses, and most would probably argue that it shouldn't). But what is the distinction between the two sides of the boundary?

Whether the law should regulate immorality, and not just harmful behavior, is the issue about whether legal moralism is defensible. A classical liberal view (sometimes associated with John Stuart Mill) is that liberty may be restricted to prevent harm, but not to enforce morality as such. In evaluating legal moralism, one important question is what would count as harmless immorality. Legal moralism used to be debated about whether the state should prohibit pornography or homosexuality. But, of course, it is questionable whether pornography or homosexuality is per se immoral. You mention the case of infidelity. It’s an interesting question whether infidelity causes harm. One might think so, in which case it presents questions of harm prevention, as well as legal moralism. Most would agree that it is immoral, but many liberals would deny that the state should regulate. A clearer case of harmless wrongdoing might be a case in which a minor promise is broken but with no ill effect or someone borrows a friend’s property without his permission. Here too many would think that the law has no business intervening. But we might want to distinguish two different explanations of this fact. On one view, there simply is no reason to regulate immorality as such. On another view, there is reason to regulate immorality (to improve the moral quality of the world), but there may be even stronger reason not to regulate if legal intervention is an inefficient tool and causes more harm than good. The first view rules out legal moralism, but the second does not. If there are some cases where there is sufficient reason, on balance, to regulate harmless wrongdoing, then that provides some support for the second view. Some might think that punishing unsuccessful criminal attempts (cases in which it was only luck that prevented the completion of the criminal attempt) was justified, even though they caused no harm. Though harmless, someone might insist that criminal attempts risk harm and that harm prevention allows us to regulate the risky as well as the harmful. Other cases of harmless immorality that many would think we should legally regulate include desecration of the dead and bestiality. A consistent rejection of legal moralism would presumably have to abstain from regulating such harmless wrongs. If we find this kind of abstention hard to accept, we may be attracted to a form of legal moralism that says that there is always some reason to regulate immorality but that whether regulation is on balance best depends upon various other considerations, such as the gravity of the immorality, the prospects for successful prevention of immorality, and the costs of various kinds in regulation.

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