In this article, we use newly developed statistical methods to examine the generative processes that give rise to widespread patterns in friendship networks. The methods incorporate both traditional demographic measures on individuals (age, sex, and race) and network measures for structural processes operating on individual, dyadic, and triadic levels. We apply the methods to adolescent friendship networks in 59 U.S. schools from the National Longitudinal Survey of Adolescent Health (Add Health). We model friendship formation as a selection process constrained by individuals' sociality (propensity to make friends), selective mixing in dyads (friendships within race, grade, or sex categories are differentially likely relative to cross-category friendships), and closure in triads (a friend's friends are more likely to become friends), given local population composition. Blacks are generally the most cohesive racial category, although when whites are in the minority, they display stronger selective mixing than do blacks when blacks are in the minority. Hispanics exhibit disassortative selective mixing under certain circumstances; in other cases, they exhibit assortative mixing but lack the higher-order cohesion common in other groups. Grade levels are always highly cohesive, while females form triangles more than males. We conclude with a discussion of how network analysis may contribute to our understanding of sociodemographic structure and the processes that create it.

Recent research has focused on the role of social networks in facilitating participation in protest and social movement organizations. This paper elaborates three currents of microstructural explanation, based on information, identity, and exchange. In assessing these perspectives, it compares their treatment of multivalence, the tendency for social ties to inhibit as well as promote participation. Considering two dimensions of multivalence—the value of the social tie and the direction of social pressure—this paper discusses problems of measurement and interpretation in network analysis of movement participation. A critical review suggests some directions for future research.

No abstract

Centralized sanctions (selective incentives) and informal norms have been advanced as distinct solutions to collective action problems. This article investigates their interaction, modeling the emergence of norms in the presence of incentives to contribute to collective goods. Computational experiments show how collective action depends on a three-way interaction among the value of incentives, the rivalness of incentives (ranging from independence to zero-sum competition), and group cohesiveness (effectiveness of peer influence). This investigation shows a broad range of conditions in which social norms promote the collective good and thus peer influence complements a centralized regime of selective incentives. It also shows conditions in which the two systems clash because incentives lead to antisocial norms that discourage contributions to collective goods. In these conditions, social scientists must reconsider the widely predicted relationships of collective action to selective incentives, group cohesiveness, and second-order free riding.

This paper explores the role of social networks in channeling individuals’involvement in local activism. A case study of a grassroots environmental group examines variation in members’levels of involvement, using three levels of explanation: individual attributes, strong and weak ties between members, and memberships in other organizations. After demonstrating that high‐ and low‐level members are very similar in personal attributes, it focuses on social ties and organizational affiliations. As expected, the data suggest that an individual's level of involvement is increased by strong ties to other members, structural similarity to other high‐level members, and fewer ties to nonmembers. Extramovement organizational affiliations are often assumed to diminish actors’structural availability, though empirical research in differential recruitment has generally revealed a positive effect on participation in social movements. This study addresses a microstructural explanation for the variation between competition and mutualism in a local multiorganizational field, as it shows how organizational goals condition the effect of outside affiliations on level of participation. Paradoxically, ties to other organizations with irrelevant goals appear to facilitate participation in this group, while ties to groups with congruent goals diminish participation.