When society seeks to overrule the individual’s judgment, Mill wrote, it does so on the basis of “general presumptions,” and these “may be altogether wrong, and even if right, are as likely as not to be misapplied to individual cases.” If the goal is to ensure that people’s lives go well, Mill contends that the best solution is for public officials to allow people to find their own path. Here, then, is an enduring argument, instrumental in character, on behalf of free markets and free choice in countless situations, including those in which human beings choose to run risks that may not turn out so well.

Mill’s claim has a great deal of intuitive appeal. But is it right? That is largely an empirical question, and it cannot be adequately answered by introspection and intuition. In recent decades, some of the most important research in social science, coming from psychologists and behavioral economists, has been trying to answer it. That research is having a significant influence on public officials throughout the world. Many believe that behavioral findings are cutting away at some of the foundations of Mill’s harm principle, because they show that people make a lot of mistakes, and that those mistakes can prove extremely damaging. …

To Mill’s claim that individuals are uniquely well situated to know what is best for them, Conly objects that Mill failed to make a critical distinction between means and ends. True, people may know what their ends are, but sometimes they go wrong when they choose how to get them. Most people want to be healthy and to live long lives. If people are gaining a lot of weight, and hence jeopardizing their health, Conly supports paternalism—for example, she favors reducing portion size for many popular foods, on the theory that large, fattening servings can undermine people’s own goals. In her words, paternalism is justified when