In the 14th paragraph of the “Introductory” chapter of On Liberty, John Stuart Mill has this to say about the temptation to run other people’s lives:

Though this doctrine [of suffering each other to live as seems good to themselves] is anything but new, and, to some persons, may have the air of a truism, there is no doctrine which stands more directly opposed to the general tendency of existing opinion and practice. Society has expended fully as much effort in the attempt (according to its lights) to compel people to conform to its notions of personal, as of social excellence. The ancient commonwealths thought themselves entitled to practise, and the ancient philosophers countenanced, the regulation of every part of private conduct by public authority, on the ground that the State had a deep interest in the whole bodily and mental discipline of every one of its citizens; a mode of thinking which may have been admissible in small republics surrounded by powerful enemies, in constant peril of being subverted by foreign attack or internal commotion, and to which even a short interval of relaxed energy and self-command might so easily be fatal, that they could not afford to wait for the salutary permanent effects of freedom. In the modern world, the greater size of political communities, and above all, the separation between spiritual and temporal authority (which placed the direction of men’s consciences in other hands than those which controlled their worldly affairs), prevented so great an interference by law in the details of private life; but the engines of moral repression have been wielded more strenuously against divergence from the reigning opinion in self-regarding, than even in social matters; religion, the most powerful of the elements which have entered into the formation of moral feeling, having almost always been governed either by the ambition of a hierarchy, seeking control over every department of human conduct, or by the spirit of Puritanism. And some of those modern reformers who have placed themselves in strongest opposition to the religions of the past, have been noway behind either churches or sects in their assertion of the right of spiritual domination: M. Comte, in particular, whose social system, as unfolded in his Systeme de Politique Positive, aims at establishing (though by moral more than by legal appliances) a despotism of society over the individual, surpassing anything contemplated in the political ideal of the most rigid disciplinarian among the ancient philosophers.

There are few of us who don’t feel the urge to run other people’s lives. Let me wrestle with a case where I feel this: the desire to intervene to reduce people’s consumption of sugary soft drinks in order to help restrain the rise of obesity–even to the extent of heavily taxing them. I actually think there are arguments on both sides for this. And since John Stuart Mill is a Utilitarian rather than a doctrinaire Libertarian, I think he would be willing to consider the arguments.

One of the most troubling arguments for taxing sugary soft drinks is that obesity puts a burden on the government budget because the government pays for a big fraction of people’s medical care. This is saying that since the government is intervening in one area, and people’s choices interact badly with that intervention, that the government should intervene in another way. That leads to a chain of argument that could justify more and more government intervention, until the sum of it all would look like a bad deal.

An interesting argument would be pointing to the research showing that people eat better and exercise more when the people around them are doing so. That means that there is an externality from good behavior, assuming that some people want to eat well and exercise more but are having troubling doing so because not all of them wants to eat well and exercise well–a part that would be less costly to defeat if others were eating well and exercising well. In a way, that argument questions whether there is a sharp boundary between one person and the next.

One of the best arguments for intervening to improve people’s health-related behaviors is to draw a boundary between different time slices of an individual, as I discussed in “Drug Legalization and Time Slices of People as Ethical Units.” If someone’s later self is treated as a separate individual, then harming that later self by eating badly and eschewing exercise could be akin to a crime.

One way of trying to get at this issue is by listening to people talk about the regrets that have about things they didn’t do when they were younger. But it is tricky. Some of the young may not care much about the welfare of their older selves; but it is also true that some of the old may not care much about the welfare of their earlier, younger selves. They may be wishing their younger self had saved more or eaten better, even at the expense of having an unpleasantly ascetic life. So regrets must be taken with a grain of salt, but listening to both regrets and what the young want to do despite the cost to their older selves can help bring a more balanced perspective.

I mentioned briefly above the possibility that people face an internal struggle to do the right thing. One way of modeling this is to imagine different selves battling it out within a person at the same time. From this point of view, taxing sugary soft drinks might be seen as taking sides in this civil war–with a certain amount of collateral damage on others who do not have an internal struggle. If this justification is not backed up by one of the others discussed in this post, it requires a non-obvious reason to favor one side of a person over the other side, as well as a full accounting of the collateral damage.

An important argument is to argue that people don’t understand, are confused and not thinking straight when they make the decision to drink sugary soft drinks. Educating people is actually quite expensive, while a tax on sugary soft drinks is mostly a transfer (admittedly a likely regressive tax taking money from the ignorant and some other groups and giving it to the government). This argument works best when (a) it has been demonstrated that an educational intervention based on the truth, undertaken on an experimental sample, causes people to shy away from sugary soft drinks and (b) the tax is tailored to affect primarily the subset in this category who would shy away from sugary soft drinks if only they knew the truth. That is, it is important to find signs of a genuine strong preference for sugary soft drinks that would make someone choose to drink them even knowing all of the consequences.

John Stuart Mill is surprisingly sympathetic to the argument for being paternalistic toward the ignorant. On that, see “John Stuart Mill on Benevolent Dictators.” But a minimal condition for making a legitimate intervention based on the ignorance argument is that someone should be allowed to opt out–in this case choosing to replace the sugary soft drink tax with a lump-sum tax equal to the soft-drink tax at the average level of consumption, say–upon passing a rigorous quiz demonstrating full knowledge of the consequences of sugary soft drinks.

One of the most powerful arguments against running other people’s lives is that it tends toward attempting to do things the one best way. But even if the one best way is identified correctly at a given point in time, standardizing everyone’s behavior will result in less experimentation. That is, the static benefit of getting everyone to conform to best practice has a dynamic cost of collectively learning less. (Of course, if everyone conforms to the same bad practice, relatively little would be learned from that as well–the problem arises when uniform best practice in a static sense replaces diversity.) This can be a very serious cost.

For comparison, the restriction of useful experimentation in systemic health care policy is one of my greatest fears about what Obamacare might do at some future time. (See for example Evan Soltas on Medical Reform Federalism–in Canada.) Those who want to keep sugary soft drinks free from taxes should try to spell out possible ways in which the consumption of sugary soft drinks might lead to the discovery of important new welfare-enhancing products or practices that are not currently seen clearly.

I won’t try to resolve this internal debate about whether or not to have a substantial tax on sugary soft drinks here; the main point I want to make is that anyone thinking of a paternalistic intervention should go through both an internal and hopefully external debate at least at this level of thoroughness.