In a gym I used to go to, there was a soda machine in the lobby. I'd been using it for months before I noticed, hidden away in a dark corridor, a drinking fountain. If their positions had been switched, would I have been more likely to drink water after training, rather than calorie-rich soda?

It's a fashionable idea at the moment that, on our own, we struggle to make healthy choices. It's not necessarily that we don't want to make the right choice, but if the worst choice is always the easiest, the deck is stacked against us. Maybe we need a nudge toward the healthy option.

New York City has been exploring this idea with two major health initiatives, designed to help the 2 million overweight New Yorkers. The first is banning trans fats in restaurants. The second is compulsory calorie counts on chain restaurant menus. The city faced some opposition to both plans. A voluntary proposition to phase out trans fats failed, despite 7,000 restaurant workers being trained in how to make the switch. A compulsory scheme finally came into force at the end of 2008. Restaurants also argued that being forced to display calorie counts violated their First Amendment rights, but the courts came down on the side of the city, and the 83 percent of polled adults who supported the initiative.

In the past, the state's legal powers have mostly been used to combat infectious diseases. We've seen an example recently, with schools closing to prevent the spread of flu. Can obesity really be seen as a health emergency that justifies interference in the same way?