What is(n't) a friend? Dimensions of the friendship concept among adolescents.

Much research in network analysis of adolescent friendships assumes that friendships represent liking and social interaction, friendships are directed, and friendships are equivalent to one another. This study investigates the meaning of friendship for eight diverse cohorts of sixth graders. Analysis of focus group and survey data suggests that these adolescents construe friendship as a multidimensional role relation composed primarily of relational norms, expectations for mutual behavior. Their friendship definitions may also include mutual liking and interaction, and other structural expectations such as reciprocity, homophily, and transitivity. Lastly, boys and girls weight these dimensions differently in defining friendship.

Investigating the Temporal Dynamics of Interorganizational Exchange: Patient Transfers among Italian Hospitals.

Previous research on interaction behavior among organizations (resource exchange, collaboration, communication) has typically aggregated those behaviors over time as a network of organizational relationships. The authors instead study structural-temporal patterns in organizational exchange, focusing on the dynamics of reciprocation. Applying this lens to a community of Italian hospitals during 2003-7, the authors observe two mechanisms of interorganizational reciprocation: organizational embedding and resource dependence. The authors show how these two mechanisms operate on distinct time horizons: dependence applies to contemporaneous exchange structures, whereas embedding develops through longer-term historical patterns. They also show how these processes operate differently in competitive and non-competitive contexts, operationalized in terms of market differentiation and geographic space. In noncompetitive contexts, the authors observe both logics of reciprocation, dependence in the short term and embedding over the long term, developing into population-level generalized exchange. In competitive contexts, they find no reciprocation and instead observe the microfoundations of status hierarchies.

Greed and Fear in Network Reciprocity: Implications for Cooperation among Organizations.

Extensive interdisciplinary literatures have built on the seminal spatial dilemmas model, which depicts the evolution of cooperation on regular lattices, with strategies propagating locally by relative fitness. In this model agents may cooperate with neighbors, paying an individual cost to enhance their collective welfare, or they may exploit cooperative neighbors and diminish collective welfare. Recent research has extended the model in numerous ways, incorporating behavioral noise, implementing other network topologies or adaptive networks, and employing alternative dynamics of replication. Although the underlying dilemma arises from two distinct dimensions-the gains for exploiting cooperative partners (Greed) and the cost of cooperating with exploitative partners (Fear)-most work following from the spatial dilemmas model has argued or assumed that the dilemma can be represented with a single parameter: This research has typically examined Greed or Fear in isolation, or a composite such as the K-index of Cooperation or the ratio of the benefit to cost of cooperation. We challenge this claim on theoretical grounds-showing that embedding interaction in networks generally leads Greed and Fear to have divergent, interactive, and highly nonlinear effects on cooperation at the macro level, even when individuals respond identically to Greed and Fear. Using computational experiments, we characterize both dynamic local behavior and long run outcomes across regions of this space. We also simulate interventions to investigate changes of Greed and Fear over time, showing how model behavior changes asymmetrically as boundaries in payoff space are crossed, leading some interventions to have irreversible effects on cooperation. We then replicate our experiments on inter-organizational network data derived from links through shared directors among 2,400 large US corporations, thus demonstrating our findings for Greed and Fear on a naturally-occurring network. In closing, we discuss implications of our main findings regarding Greed and Fear for the problem of cooperation on inter-organizational networks.

Birds of a feather, or friend of a friend? Using exponential random graph models to investigate adolescent social networks.

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.