In my opinion, the harm principle is often construed far too narrowly to adequately encompass the whole range of moral wrongs. I submit that the biggest common omission is self-corruption, and that this should cause us to somewhat re-evaluate liberal moral theory. Self-corruption, put simply, is acting so as to make oneself more likely to do harm in the future. Accepting that this is wrong can lead to potentially quite radical conclusions.
It is almost unnecessary to point out that there is widespread consensus that encouraging another to do wrong is itself wrong. Various incitement laws express our deep-seated belief that encouraging a crime is, morally speaking, committing the act itself only through an agent. Indeed, even if the event never occurs I am doing wrong in increasing the probability of harm. This need not be constrained to clear encouragement. By lying about a person to another I may encourage the latter to get angry and hurt the former without ever so much as mentioning the idea. From an ethical point of view and as long as there is the necessary guilty mindset, clearly this action is also wrong.
What I want to suggest is that there is no reason to constrain this to interactions with others. Our choices today can foreseeably alter our future actions and cause us to do real harm at a later date. Although our initial actions do not directly cause harm they increase the risk of it and, unless this can be justified (by weighing it against other factors), this must also be wrong.
But what do I mean by choices altering our future actions? An easy example would be a forgetful person choosing to throw away a note written to himself so that he will not remember to fulfil a promise. Failing to remember something does not look like a moral wrong, but acting earlier so as to cause this does. We can alter our future actions in a way which is wrongful right now.
However the central case of self-corruption is acting so as to change our character in some way. If doing so makes us more likely to cause harm in the future, then these early actions are themselves violations of the harm principle (even if harm does not in the end arise) unless they can be justified - they are prima facie wrong. To see what this means, I will first consider the example of promises.
It is sometimes suggested that unless there is a special meta-physical property to promises (in a 'thou shalt not lie' kind of way), there can be nothing wrong with violating them unless doing so also causes harm. While one might say that any breach of trust damages the sanctity of promises as a whole and so potentially society at large, this would only appear to be true where others might find out about the breach. Therefore a promise to a dying relative may often later be broken without appearing to damage anyone's trust in promises.
However self-corruption suggests a different conclusion. Every time we break a promise, we would appear to damage our own view of the inviolability of promises. Each time we break a promise, we make it more likely that we will do so again in the future, even when in these cases to do so would certainly cause harm and disappointment. We to some extent self-corrupt ourselves, altering our character in a negative way.
At this point I should point out that I am not arguing that upholding our promises is an absolute duty. Other considerations can well justify us not doing so, perhaps even making it immoral to do so. If I promise a dying relative to marry someone I do not wish to, it is probably most sensible to break this promise as to uphold it could cause unnecessary misery and harm. It may nevertheless still have been morally permissible to make the promise as a way of putting the dying relative's mind to rest. Moreover the situation may change after a promise so as to make performance gravely immoral. All I argue is that in all cases, self-corruption must be figured into considerations. Where there are no sufficiently weighty countervailing considerations, there is a duty not to self-corrupt. In fact, as long as self-corruption is constrained to cases where there are powerful reasons for it, the self-corruption will be less potent - less likely to cause us to act wrongfully when these reasons do not apply.
None of this, however, looks in the slightest bit radical. If it helps us see that there is always a prima facie obligation to uphold our promises then this does not seem to upset liberal moral theory. However what might do so is its implications for moral 'thought crimes'. Orthodox harm principle theory suggests that mere mental activity cannot generally be wrong. Only where it actually prepares for physical behaviour leading to harm does it violate the principle. I suggest this is misguided.
If thinking in a certain way or subjecting ourselves to certain stimuli changes our character so as to make us more likely to harm others then doing so is wrong. Imagine that I know that I become violent and am liable to hurt people after watching violent films. In this case it would seem that I am under a duty not to do so, at least not when I am likely to be around people afterwards. The situation is no different to drinking alcohol when I know that this makes me violent. In either case, it is wrong for me to risk other people's safety for no good reason.
What this means is that we should consider carefully the question of to what media we should expose ourselves. If violence really does make us more violent or pornography make us more likely to commit sexual offences, then unless there are suffiencient moral benefits to outweigh this, we should refrain from exposing ourselves to them. Now I am of the opinion that in most cases the benefits will outweigh this risk: Exposure to violenct media often allows us to vicariously release violent tendencies and exposure to pornography often allows us to similarly release potentially aggressive sexuality. However in certain cases, individuals may find that they respond negatively and take steps to avoid exposure.
As for the legal consequences of this argument, this is not a call for censorship. The result of the highly personalised account of the morality of violent and pornographic media set out above is that it must be judged on the individual level. However on the the moral level, my intention is to point out that we must be aware of the possibility that behaviour which looks purely private may well in fact mask violations of the harm principle through self-corruption. The idea of private activity should be carefully thought through before it is used as a general shield from criticism.