Person-to-person spread of smoking cessation appears to have been a factor in the population-level decline in smoking seen in recent decades. Moreover, there appear to have been local smoking cessation cascades since whole connected clusters within the social network quit roughly in concert. This suggests that decisions to quit smoking are not made solely by isolated individuals, but rather reflect choices made by groups of individuals connected to each other both directly and indirectly up to three degrees away. Individuals appear to act under collective pressures within niches in the network. As a further reflection of this phenomenon, individuals who remained smokers were observed to move to the periphery of the network,[
xxvii] and the network became progressively more polarized with respect to smokers and nonsmokers over the period 1971-2003, with relatively fewer social ties between these groups.
We also found that the educational background of connected individuals matters. The higher their educational level, the more likely friends were to emulate each other with respect to smoking. In this regard, the diffusion of smoking cessation is in keeping with prior studies of diffusion of diverse “innovations.”[
xxviii,
xxix] This finding is also consistent with the idea that local social niches may arise within the network and that this may in turn contribute to the well known socioeconomic gradient in smoking. That is, groups of individuals may develop self-reinforcing norms that augment individual decisions to quit and this may lead to macro-socioeconomic patterns, similar to the adoption and spread of fashions first among high-status individuals in society.[
28]
While connected persons might share an exposure to common environmental factors (e.g., cigarette taxes), the experience of simultaneous events (e.g., workplace smoking cessation campaigns), or other common features (such as genes or socio-demographic attributes) that cause them to start or quit smoking simultaneously, our observations nevertheless suggest an important role for a process involving group dynamics and person-to-person spread. The group-level cessation of smoking is not solely due to people in the same household or workplace quitting together since even contacts who were geographically separated evinced inter-personal effects and since workplace effects seem to depend on workplace size in a way that suggests that actual interpersonal contact is important. Moreover, the fact that immediate neighbors do not affect subjects helps exclude joint exposure to local environmental factors (such as tobacco marketing, local taxes, or cigarette availability) as an explanation for the observations. Our models control for a subject’s prior smoking status, which helps to account for sources of confounding that are stable over time (such as childhood exposures, personality, or genetic endowment). Finally, our models also control for contacts’ prior smoking status, thus helping to account for a possible tendency of smokers to form ties among themselves.
Based both on social theory and on our previous work on obesity,[
11] we expected that people would be more likely to emulate the behavior of people they nominated rather than people who nominated them as friends. The results suggest such a pattern. Moreover, such a pattern provides suggestive evidence against the role of confounding since any confounding factor would not respect the directionality of social ties.
Smoking in contacts might influence smoking in subjects by diverse biopsychosocial means, including (1) changing subject’s norms about the acceptability of smoking, (2) more directly influencing subject’s behaviors (e.g., a contact asking the subject not to smoke, or, conversely, a contact sharing cigarettes), or even (3) fostering dependence through the inhalation of second-hand smoke. Our data are not capable of distinguishing these. Yet, the lack of geographic variation in the impact of contact behavior on subjects suggest that social norms may be an important factor -- since they may spread more easily over geographic distance than behaviors.
It is possible that a change in the smoking behavior of more than one contact may be required for a subject to change, and there may be additive or even threshold effects whereby a subject’s probability of quitting depends on having not one contact, but two or more, quit.[
xxx] This may be especially likely in the case of smoking which is very often deemed an explicitly social -- and hence shared -- behavior. Consequently, when a smoker runs out of easily available contacts with whom he or she can smoke, he or she may be increasingly likely to quit. This possibility is also consistent with the group-level quitting we observed.
Network phenomena might be exploited to spread positive health behaviors.[
xxxi,
xxxii,
xxxiii,
xxxiv] Indeed, smoking and alcohol cessation programs that provide peer support – that is, that modify the social network of the target – are more successful.[
32,
34] People are connected, and so their health is connected.[
xxxv,
xxxvi] Collective interventions may be more effective than individual interventions. Moreover, medical and public health interventions to get people to quit smoking might be more cost-effective than initially supposed since health improvements in one person might spread to others.[
35,
xxxvii,
xxxviii] Finally, the isolation of smokers within social networks suggests that blanket policy approaches (e.g., advertising, taxation) may be usefully supplemented by interventions targeting small groups. In the case of smoking cessation in the last three decades, there is evidence of a cascade of salubrious behavior, and cessation of smoking in one person appears to be highly relevant to the smoking behavior of others nearby in the social network.
The publisher's final edited version of this article is available at 
