Stanford researchers have just wrapped up the world’s biggest study into the link between income and life span. They analyzed US income tax and death records – over a billion of them. And guess what? The more you make, the longer you live.

On average, for men, the richest 1 percent live 15 years longer than the poorest 1 percent. But the study also shows that the way to fix the problem isn’t what you’d think. You’d think the answer would be to close the income gap somehow.

But in fact, when you look city-by-city, the effects of being poor matter much less in cities like San Francisco and New York even though income inequality is greater there. New York Times reporter Neil Irwin, who analyzed the study, says there a reason for that.

“The legislative efforts within New York City to deal with obesity, to deal with trans fats, to reduce smoking,” Irwin said.

That’s important because a big reason the poor die sooner is that they tend to smoke, eat too much and exercise less. But living in cities that are on a crusade against bad habits makes difference.

In fact for a low-income person, moving to a booming city like San Francisco or New York is more effective than medical care! Access to medical care had no effect on the average life spans of poor people.

“Living a long time is not just about what kind of healthcare you can get when you get sick. Living a long time is partly about healthy behaviors that happen throughout life over many decades,” Irwin said.

The researchers said that a poor person at age 40 who moves to a city where healthy living is part of the culture can add four and a half years to his life.

Assuming he can get a job there. Otherwise, he’s out living in a tent with thousands of other people who took that advice.