depicts the largest connected sub-component of the social network of friends, spouses, and siblings in the year 2000. Clusters of drinking and abstaining are visible in the network.
Drinking in the Framingham Social Network in 2000
shows the correlation between principals and contacts with regards to their drinking behavior (numerical results for this and the other figures can be found in the online appendix
). The results indicate that principals are 50% (95% C.I. 40% to 62%) more likely to drink heavily if a person they are directly connected to (at one degree of separation) drinks heavily. The size of the effect for people at two degrees of separation (e.g., the friend of a friend) is 36% (95% C.I. 25% to 48%) and for people at three degrees of separation (e.g., the friend of a friend of a friend) is 15% (95% C.I. 8% to 25%). At four degrees of separation, the effect disappears (4%, 95% C.I. –2% to 10%), in keeping with the “three degrees of influence” rule of social network contagion that has been exhibited for obesity, smoking, happiness, depression, loneliness, word-of-mouth advertising, and the spread of ideas among inventors (10
). Analyses of the full network also show that subjects are 29% (95% C.I. 23% to 36%) more likely to abstain if a person they are directly connected to (at one degree of separation) abstains. The size of the effect for people at two degrees of separation (the friend of a friend) is 21% (95% C.I. 16% to 27%) and for people at three degrees of separation (the friend of a friend of a friend) is 5% (95% C.I. 1% to 10%). Again, at four degrees of separation the effect disappears (2%, 95% C.I. –1% to 6%).
Relationship of Social and Geographic Distance to Heavy Drinking and Abstaining in Connected Persons
It is notable that in the decline in the effect size with social distance contrasts to a lack of decline in the effect size as people become more geographically distant from one another. This result was confirmed by testing an interaction between distance and the effect size. These results suggest a friend or family member who lives hundreds of miles away is associated with as big an effect as one who lives next door.
The actual alcohol consumption behavior in individuals’ social contacts affects individuals’ alcohol consumption behavior. shows the smoothed bivariate relationship between the fraction of a principal's friends and family who drink heavily and abstain at one exam, and the average number of drinks per day at the following exam. Being surrounded by heavy drinkers increases by about 70% (95% CI: 35% to 142%) the amount of alcohol a person drinks at the next exam compared to those who are not connected to any heavy drinkers. Conversely, being surrounded by abstainers cuts in half the amount of alcohol a person drinks at the next exam.
Effect of Heavy Drinking and Abstaining Contacts on Principals in the Framingham Social Network
When the principal's future alcohol consumption behavior, controlling for age, gender, education, and exam was regressed on the number of heavy drinking, moderate drinking, and abstaining contacts, each additional heavy drinking contact was found to significantly increase the likelihood that a principal drinks heavily by 18% (95% CI: 11% to 25%, p<0.001) and decreases the likelihood principal abstains by 7% (95% CI: 2% to 12%, p=0.009), but has no effect on moderate alcohol consumption behavior (CI: –8% to 1%, p=0.113). Conversely, each additional abstaining contact significantly reduces the likelihood of heavy drinking by 10% (95% CI: 4% to 15%, p=0.001), increases the likelihood of abstaining by 22% (95% CI: 17% to 28%), and also decreases the likelihood of moderate drinking by 11% (95% CI: 8% to 14%). Finally, each additional contact that drinks moderately has no significant effect on heavy drinking (95% CI: –2% to 7%, p=0.214) but it significantly decreases the probability of abstaining by 5% (95% CI: 2% to 9%) and increases the likelihood of moderate drinking by 6% (95% CI: 2% to 9%).
We next evaluated the extent of dyadic, inter-personal association in alcohol consumption behavior. As discussed in the methods section, these models account for homophily by including a time-lagged measure of a contact's alcohol consumption behavior. We evaluated the possible role of unobserved contemporaneous events by separately analyzing models on subsets of the data involving various principal/contact pairings.
summarizes the associations from the models (numerical results can be found in the online appendix
). With respect to friends, we found significant gender differences in the spread of heavy alcohol consumption behavior. If a principal's female friend started drinking heavily, then the principal's chances of drinking heavily increased by 154% (95% CI: 30% to 354%). In contrast, a male friend's heavy alcohol consumption behavior appears to have no significant effect on the principal. The type of friendship also appeared to be important: a female who thinks of the principal as a friend but not vice versa (a contact-perceived friend) does not appear to have a significant effect, but the overlapping confidence intervals indicate that the difference in the effect size is not significant. Gender also played a role among spouses. Heavy drinking by a wife increased the likelihood that the husband drank heavily by 196% (95% CI:91% to 329%), whereas heavy drinking by a husband increased the likelihood that a wife drank heavily by 126% (95% CI:67% to 202%). Among siblings the effect was significantly smaller and did not differ whether the contact was a sister (37%, 95% CI:0% to 85%) or a brother (34%, 95% CI:8% to 66%). Immediate neighbors and coworkers did not exhibit any significant effects on a principal's drinking behavior.
Contact Type and Drinking in the Framingham Social Network
These analyses were repeated for abstention behavior and showed broadly similar results. The effect of female friends abstaining (42%, 95% CI: 9% to 84%) was about the same size as the effect of male friends abstaining (44%, 95% CI: -3% to 106%), though the latter was barely insignificant. Abstaining wives, on the other hand, did appear to exhibit somewhat more influence (74%, 95% CI: 40% to 115%) than husbands (56%, 32% to 82%), but the effect of a sister (28%, 95% CI: 13% to 45%) was actually somewhat weaker than the effect of a brother (39%, 95% CI: 19% to 60%). Once again, immediate neighbors and coworkers had no effect on a principal's drinking behavior with respect to abstention.