depicts part of the network in 1971 and in 2000. There was a substantial change in the prevalence of smoking and in the social life of smokers during the period from 1971 to 2000. In 1971, there were many more smokers as compared with 2000, and the smokers occupied the center of their circles of friends and family to the same extent that nonsmokers did. However, by 2000, most people had stopped smoking, and those who still smoked were more likely to be at the periphery of the network. Moreover, there was an increased tendency for smokers to be connected primarily to other smokers and for there to be relatively separate clusters of smokers and nonsmokers. Additional quantitative evidence is available in the Supplementary Appendix.