Redefining membership in animal groups

Groups of animals—flocks, herds, shoals, and swarms—are often dynamical entities. Relative positions of group members change, and most groups divide and re-form on multiple timescales. Few studies, however, have attempted to define when an animal is or is not within a group. Most authors adopt arbitrary distance thresholds, such as the elective group size (EGS), which assume that animals closer than some threshold distance are in the same group. In the present article, we define a group-membership criterion derived from dynamical statistical considerations and based on detailed trajectories of all members of a moving group. We demonstrate the use of our criterion to track the comings and goings in shoals of zebrafish (Danio rerio) and the gradual dissolution of the shoal across multiple exposures to a testing tank. We present a novel measure of group cohesion based on our group membership criterion and demonstrate that excursions away from the shoal explain some previous observations of the dynamics of shoaling. Finally, we show that excursions away from a shoal are accompanied by an increase in swimming speed. Applying similar criteria to data from other species may clarify some of the common features of animal collective motion.     

From fault line to group fission: understanding membership changes in small groups

Group fissions occur when two or more members leave a parent group to either form a new group or join an existing group. This article investigates the interplay between two factors: the presence of an intragroup conflict and subgroup boundaries on the group fission process. It is hypothesized that subgroup divisions act as potential fault lines along which groups split after they experience conflict. The results of three experiments, one scenario study and two laboratory studies involving small task groups, support the group fault line hypothesis. The authors discuss the implications of these findings for theory and research on membership changes in small groups.     

Predicting the directionality of probability words from their membership functions

Teigen and Brun have suggested that distinct from their numerical implications, most probability phrases are either positive or negative, in that they encourage one to think of reasons why the target event will or will not occur. We report two experiments testing our hypotheses that (a) the direction of a phrase can be predicted from properties of its membership function, and (b) this relation is invariant across contexts, and (c) —originally formulated by Teigen and Brun (1999)—that strong modifiers intensify phrase directionality. For each phrase, participants encoded membership functions by judging the degree to which it described the numerical probabilities 0.0, 0.1, …, 1.0, and also completed sentences including the target phrase. The types of reasons given in the sentence completion task were used to determine the phrase's directionality. The results support our hypotheses (a) and (b) regarding the relation between directionality and the membership functions, but we found only limited support for hypothesis (c) regarding the effects of modifiers on directionality. A secondary goal, to validate an efficient method of encoding membership functions, was also achieved. Copyright © 2003 John Wiley & Sons, Ltd.     

Predicting eating disorder group membership: an examination and extension of the sociocultural model

The purpose of this research was to examine and extend portions of the sociocultural model of bulimia nervosa (Stice, E. (1994). Review of the evidence for a sociocultural model of bulimia nervosa and an exploration of the mechanisms of action. Clinical Psychology Review, 14, 633-661; Stice, E., & Agras, W. S. (1998). Predicting onset and cessation of bulimic behaviors during adolescence: A longitudinal grouping analysis. Behavior Therapy, 29, 257-276). Participants were women who reported engaging in binge eating at baseline and the 1-year follow-up (n = 26), women who began binge eating between these 2 points (n = 25), and women who did not report binge eating during the course of the study (n = 199). Results of the first discriminant function analysis provided support for the sociocultural model. However, the results of subsequent analyses suggest that additional variables, including stress, escape-avoidance coping, and interoceptive awareness, emerged as important. Implications of these findings for our understanding of the development and maintenance of binge eating are discussed.     

Pastoral care through small groups

Summary Pastoral care has for too long been conceived as the task of the ordained clergyman alone. The concept of shared leadership now offers us a unique tool to implement the pastoral care of the whole church—the laity as well as clergy. The small group, as a locus of care and concern and as a training tool, offers a unique way by which this ministry of the laity may become a reality, but training in small group process is vitally important to release this potential.     

Facilitating information exchange in small decision-making groups

The present study examined the effects of several conditions on the information flow during unstructured discussion in small groups. We build on the experimental work of Stasser and his colleagues on information sampling. The results of their research suggest that groups often fail to exchange information effectively. Three experiments with a 2×2 factorial design were set up to look for conditions that could facilitate the exchange of unshared information in particular. In a first study, we replicated the experiment of Stasser and Titus (1987). The main results of the original study were confirmed; additional discussion content analysis of video-recorded material made further differentiation possible. Apart from the mere mentioning of items during discussion, the reactions to and the repetition of the items were analysed. In the second experiment we introduced partially shared information and we made group members aware of the unique information that they can contribute. In the third experiment the nature of the task was manipulated and an attempt was made to prevent an information bias in favour of decision-supporting items. Conditions that significantly enhance the dissemination of unshared information were found. An empirical basis was gathered so that the guidelines for effective information sampling as they were laid down by Stasser (1992) could be reformulated.