Edinburgh Napier University

  We analyse the co-evolution of social networks and substance use behaviour of adolescents
and address the problem of separating the effects of homophily and assimilation. Adolescents
who prefer friends with the same substance-use behaviour exhibit the homophily principle.
Adolescents who adapt their substance use behaviour to match that of their friends display
the assimilation principle. We use the Siena software to illustrate the co-evolution of
friendship networks, smoking, cannabis use and drinking among sport-active teenagers.
Results indicate strong network selection effects occurring with a preference for same sex
reciprocated relationships in closed networks. Assimilation occurs among cannabis and
alcohol but not tobacco users. Homophily prevails among tobacco and alcohol users.
Cannabis use influences smoking behavior positively (i.e., increasing cannabis increases
smoking). Weaker effects include drinkers smoking more and cannabis users drinking more.
Homophily and assimilation are not significant mechanisms with regard to sporting activity
for any substance. There is, however, a significant reduction of sporting activity among
smokers. Also, girls engaged in less sport than boys. Some recommendations for health
promotion programmes are made.