Finally, exercise habits benefit from social influence. A person is more likely to prioritize exercise when her family, friends, and coworkers value working out as well. “Lack of social support for exercise” is one of the most important barriers to physical activity, according to Active Living By Design, a health advocacy group. Having support from a friend makes exercise habits significantly more likely to stick.
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The contagiousness of healthy behavior is important, because the United States is heavily sorted by income. The rich cluster in high-income suburbs and metros. But not all sorting is by choice. Housing and transit policies force poor and even middle-class Americans away from the country’s richest, densest, and most productive places to live.
If health were contagious, one would see it in the data. Rich cities would become collectively obsessed with diet and exercise, while poor areas missed out on cultural waves of healthy behavior. As a result, the health gap between the rich and poor would grow.
This is exactly what’s happening. The richest 1 percent of men now live about 15 years longer than the poorest 1 percent, like the difference between being born in Germany and being born in Rwanda. The gap has grown significantly since the turn of the century. Geographical sorting is concentrating poverty, but it might also be creating neighborhoods that entrench bad health behaviors. There is already evidence that among poor and unhealthy neighborhoods, parents pass down bad habits to their children. By the time they are 5 or 6, children in less healthy areas drink more sweetened beverages, eat more fatty foods, watch more TV, and get less exercise.
The fact that some health behaviors might spread through networks is not an excuse to overlook the many structural barriers that prevent healthy eating and exercising. Junk food is cheap, and the density of convenience stores in poor neighborhoods makes it readily available. Long commutes, dangerous neighborhoods, and a lack of recreational facilities in poor areas make it difficult for poor parents to make time for exercise.
But the contagiousness of bad health behaviors could make these problems worse. By concentrating poverty in the U.S., policymakers have also concentrated deadly habits and raised the likelihood that young people in poor families will grow up in neighborhoods where bad health behaviors are the norm.
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What’s the fix?
When the government wants to change public habits, it tends to focus on supply: Ban the bad stuff, promote the good stuff. Local governments tax cigarettes. They ban trans-fats. They build grocery stores near low-income areas.
These are often good ideas. But what if governments thought about promoting healthy behaviors in a complementary way—by raising the demand for healthy habits by building more housing in or near the healthiest neighborhoods? When people join a group that exercises, they become more likely to exercise. When they join a group that doesn’t smoke, they have more social support for quitting. The same might happen at a city-wide level. When a city builds a park, it doesn’t just give families a place to exercise; it also creates a norm of public physical activity, which the neighborhood’s newest entrants might see and mimic. A city that bans smoking in restaurants not only makes it harder for smokers to find places to light up; but also, it creates a culture where quitters can give up cigarettes together.
Policymakers understand the economic benefits to dense housing in rich and productive cities, like San Francisco, New York, and San Jose. But there might there also be health benefits, too. Rich cities are America’s capitals of healthy lifestyles. If healthy behaviors are both local and contagious, there is a powerful moral case for building more housing in these healthy cities. It might be, literally, a life-saving policy.