A revolution in the science of social networks began with
a stash of old papers found in a storeroom in Framingham, Massachusetts, USA. Personal records of 5,124 male and female subjects from a Heart Study started in 1948. The ongoing project has revealed many of the risk factors associated with cardiovascular disease, including smoking and hypertension.

Smoking: Together We Quit, Divided We Fail
In the early '70s, 65% of Framingham  residents ages 40 to 49 smoked regularly. By 2001, only 22% consumed one or more cigarettes daily. But the smoke didn't clear at random: Friends and family had a decisive influence. "People quit together," Fowler says, "or they didn't quit at all."....
When smokers quit their friends were 36% more likey to
follow suit......see article