Your whole world smiles with you

Fowler and Christakis thought they could document the spread of happiness more convincingly by studying the copious records of participants in the Framingham study, a massive effort launched by the National Heart, Lung and Blood Institute in 1948 to find common causes of cardiovascular disease. Participants gave researchers the names of their parents, spouses, siblings, children and close friends, including many who were also study volunteers. That allowed the researchers to track multiple relationships for each participant out to several degrees of separation.

Fowler and Christakis focused on 4,739 people who were part of the second-generation cohort that joined the study in 1971, in part because many of them had parents and children in other cohorts. The researchers rounded out their networks by using home addresses to locate neighbors and employment information to identify co-workers. Altogether, they constructed a social network that included 12,067 study volunteers who were linked to each other through 53,228 ties.

In earlier studies of the network, Fowler and Christakis showed that obesity and smoking spread among groups of friends and relatives.

To assess happiness, the researchers relied on how much the volunteers said they agreed with four statements like "I was happy" and "I enjoyed life." The questions were asked three times between 1983 and 2003.

The results were striking:

A happy friend who lives within a half-mile makes you 42% more likely to be happy yourself. If that same friend lives two miles away, his impact drops to 22%. Happy friends who are more distant have no discernible impact, according to the study.

Similarly, happy siblings make you 14% more likely to be happy yourself, but only if they live within one mile. Happy spouses provide an 8% boost -- if they live under the same roof. Next-door neighbors who are happy make you 34% more likely to be happy too, but no other neighbors have an effect, even if they live on the same block.

"We suspect emotions spread through frequency of contact," Fowler said. As a result, he said, people who live too far away to be seen on a regular basis don't have much effect.

The one exception was co-workers, perhaps because something in the work environment prevented their happiness from spreading, the study found.

The research was funded by the National Institute on Aging and the Robert Wood Johnson Foundation.