The goal of much of Sunstein’s work is to identify ways to counteract the impulsive influences of System 1 when they lead us astray. If people are inclined to overlook the consequences of certain behaviors, for example, he suggests that the government require warnings and disclosures—such as fuel-economy labels on cars and calorie information on restaurant menus. Because irrational tendencies to focus on the short term and to procrastinate dissuade us from saving for retirement, Sunstein advocates default rules, such as automatically enrolling workers in retirement plans, leaving them the choice of opting out. He calls such measures “soft paternalism,” to be distinguished from the “hard paternalism” of flat bans or mandates, which do not leave people free to decide for themselves.

Sunstein has laid out these arguments before, most prominently in two books, Nudge and Simpler. In Why Nudge?, he covers much of the same terrain—repeating some passages from his earlier books almost verbatim. But here he deploys behavioralism to take on John Stuart Mill’s famous case against paternalism. In On Liberty, Mill argued that paternalism was unwarranted in large part because individuals know better than others do what will serve their interests. For that reason, unless an individual’s actions harm someone else, Mill maintained, the state should stay out of her business. This “harm principle” is one of the foundations of modern liberalism.

Mill’s case against paternalism is undermined, Sunstein says, by man’s propensity to err and sabotage his own interests. If we know that people make predictable mistakes, then paternalistic interventions designed to mitigate those mistakes may increase people’s welfare overall. Even when our actions harm no one else, government intervention may therefore be justified. And because the way that choices are presented to us inevitably influences our decision making (for example, people are more likely to choose the first item on a list over subsequent items), Sunstein advises the government to design “choice architecture” in ways that nudge us toward choices that better serve our ultimate ends.

The behavioral findings that Sunstein invokes provide a much-needed corrective to legal and public-policy reasoning based on erroneous assumptions about rational decision makers. But not all of his ideas about implementing these scientific insights are equally persuasive. For example, he notes that when people are asked to make decisions in a foreign language “that they speak, but in which they are not entirely comfortable,” System 2 takes over, and common impulsive errors are significantly reduced. Sunstein contends that the same advantages might be obtained by engaging in cost-benefit analysis—on the grounds that such analysis is like a foreign language and will eliminate cognitive biases. Yet if human beings fail to support climate-change regulations because they’re wired to downplay long-term risks, overreact to immediate dangers, and resist change in general, won’t those same tendencies infect their cost-benefit analysis of the regulations?