Last week dawned clear and bright in the nanny state of New York City. Monday's paper brought word that the city's new health commissioner was working on ways to get residents to exercise more. That same morning, Michael Bloomberg announced his own latest assault on unhealthy behavior. By 2012, the mayor hopes "to lower the proportion of adults who drink one or more sugar-sweetened beverages each day by 20 percent." Tuesday's news was about plans to forbid smoking at parks and beaches.

If the past pattern holds, initial gasps of outrage at such bureaucratic interference will sputter into acceptance. When Bloomberg extinguished smoking in restaurants, bars, and workplaces in 2002, the New York Post denounced him as the "Mommy Mayor" and his approval rating plummeted. Since then, similar bans have spread to Dublin pubs and Paris cafés and the muttering has gone away. Last week the mayor of London, Boris Johnson, shared a stage with Bloomberg at Columbia University and mocked him for banning margarine—i.e., trans fat—which eateries in the five boroughs can no longer use. But restaurants adapted to that rule without much fuss, as they did to the city ordinance requiring them to post calorie counts on their menus. Most people prefer living in a healthier town.

The uptick in paternalism is not merely a New York phenomenon. Outdoor smoking bans have been spreading around the country. President Obama recently named Thomas Frieden, the man behind New York's smoking and eating directives, to head the Centers for Disease Control. Last week, Obama told the magazine Men's Health that he likes the idea of funding health-care reform with a tax on soft drinks. Sen. Max Baucus's health-care bill relies on a requirement that individuals buy health insurance—and fines them if they don't.

One can see why politicians find the paternalistic option so compelling. Behavior modification through public policy works. With help from free nicotine patches and taxes that bring the cost of a pack of cigarettes to nearly $10, New York's smoking rate has dropped from 21.5 to 15.6 percent. That's tens of thousands of lives saved. Seat-belt laws have saved hundreds of thousands more lives nationally with no fiscal impact. Paternalism is the method of government activism most amenable to an impoverished public sector.

The problem is justification. Standard liberal theory holds such a role for government to be abhorrent. In On Liberty, John Stuart Mill makes the classic case against laws restricting purely private behavior. Because there's no evidence that outdoor secondhand smoke does harm, New York's latest proposal stands in stark violation of this principle.

Legal scholar Cass Sunstein, Obama's regulatory czar, has tried to find a way around this conundrum. Sunstein's book, Nudge: Improving Decisions About Health, Wealth, and Happiness, grounds paternalism in behavioral economics. If, as evidence suggests, people aren't always rational economic actors, then it's reasonable to steer them toward better choices. Sunstein's "soft" paternalism argues for policies that work by changing the default setting, such as by requiring workers to opt out of rather than into 401(k) programs.

Yet soft paternalism can't provide a justification for all or even most of the instances in which society already accepts infringements on Mill's principle, such as motorcycle-helmet laws, restrictions on discrimination in private clubs, or laws against drugs and prostitution. In these cases, the justification is that some choices are simply unacceptable. The underlying left-right divide is not about whether government should promote virtue, but rather about what kind of virtue it should promote. Republican paternalism tends to uphold morality and social order. Democrats, by contrast, often deploy it for health and safety reasons. Both sides take pleasure in mildly persecuting those who fail to meet their civic ideals.

Because Democrats hold power at the moment, they face the greater peril of paternalistic overreaching. It would be wise for them to observe Sunstein's line. To exhort, nag, nudge, tax, and regulate people for the sake of diminishing purely self-destructive behavior is defensible. But to take choices away on the grounds that people should know better is infantilizing—and likely to hurt those who bear the burden of favoring more intrusive government. Liberals should show restraint, lest the freedom to be stupid go up in smoke.

Jacob Weisberg is also the author of The Bush Tragedy and In an Uncertain World: Tough Choices from Wall Street to Washington .

© 2009