The state of the natural environment is a topic of increasing concern, with climate change, loss of biodiversity, and diminishing natural resources all posing eminent threats to the well-being of the planet and its inhabitants. Much of this environmental degradation is caused by human behaviour that can be changed. Psychologists have realised their role in understanding and influencing pro-environmental behaviours to help (see Chapter 1). Most psychological research of environmental behaviour has focused on the individual person as the unit of analysis. While this has been helpful, less attention has been given to how group memberships, and the social influences these create, affect environmental behaviour. Because environmental behaviour often occurs within a social context, understanding the social element may be critically important to promoting environmentally sustainable behaviour (see Chapter 2). Using the social identity approach, this research investigates how various aspects of social group membership interact with individual attributes to influence environmental behaviour. Three related strands of research explore this issue (see Chapter 3 for an overview). In Chapter 4, two studies (Studies 1 and 2) examined how group feedback in the form of social comparisons affect individual behaviour. Based on social identity theory, it was predicted that positive social comparisons would lead to more positive behaviour, and less positive comparisons to less positive behaviour, especially among individuals who identified strongly with the target ingroup. Results from both studies found some support for these hypotheses on certain (but not all) behavioural dependent measures, both at the time of manipulation and one week later. This supports the notion that individual social identification strength can moderate behavioural response to group-level feedback on environmental topics. In Chapter 5, Study 3 considered how interaction within groups via discussion might induce group norms about environmental behaviour that over-ride the effects of intergroup comparisons. A design similar to Study 1 was used, with the addition of a small-group discussion following the feedback manipulation. Discussion content was hypothesised to predict environmental behaviour, with the feedback manipulation having less impact than in Study 1. Results found that the more participants discussed environmental behaviours, the more they engaged in them one week later. This effect was independent of pre-existing environmental values, suggesting that the effects of group interaction were not merely a reflection of existing individual orientations. Following the discussion, values were also found to be very strong predictors of behaviour, a result not found in Study 1, suggesting that group interaction not only shapes individual behaviour but also reduces the classic value-action gap. Together, these findings point to the powerful role that intra-group interaction can play in forming norms of environmental behaviour and shaping individual responses. In Chapter 6, two studies (Studies 4 and 5) explored how comparisons within a group over time (i.e., intra-group comparisons) may function differently to comparisons between groups (i.e., inter-group comparisons), which were explored in Chapter 4. Based on the findings in Chapter 4, positive intergroup comparisons were predicted to result in more positive individual intentions, whereas negative intergroup comparisons were expected to result in reduced intentions. With respect to intra-group comparisons, however, the opposite pattern of effects was predicted. The results of Study 4 did not support these hypotheses. However, feedback from participants suggested that the experimental design may have produced reactance. To address this, Study 5 made use of a revised design, and the results of this study indicated support for the hypotheses. Importantly, in addition to negative and positive comparisons having opposing effects depending on whether these were intra- or inter-group, the processes behind these effects also differed. The effects of intra-group comparisons were mediated by shared responsibility whereas the effects of intergroup comparisons were mediated by environmental value centrality. These results are integrated and discussed in Chapter 7. The recurring theme of these results is that group-level feedback can interact with individual-level variables in subtle but powerful ways, leading to differing outcomes of environmental behaviour. These findings highlight the socially imbedded nature of individual environmental actions, and suggest new avenues for theoretical and practical work in the environmental domain. In particular, on the basis of the studies included in this thesis it is recommended that psychologists who are interested in understanding and changing individual environmental behaviour should incorporate an understanding of intra- and inter-group processes into their theorising and future research.

Dans le cadre des analyses des données de survie, la comparaison de plusieurs groupes d’individus, où l'événement d'intérêt est censuré par intervalle, représente un défi méthodologique. Lorsque le suivi des patients au cours de l'étude n'est pas continu, l'événement d'intérêt pourra survenir entre deux dates d'observation; il est dit censuré par intervalle. Des tests de comparaisons des distributions des temps de survie pour plusieurs groupes, adaptés à la censure par intervalle, ont été développés (tests du score, tests de pseudo log-rank pondérés, tests des rangs). C’est dans ce contexte que nous avons proposé deux nouveaux tests de comparaisons de groupes adaptés à des situations particulières de censure par intervalle. Le premier test concerne une situation où l’hypothèse alternative considère que les fonctions de risque instantané se croisent. Le second test concerne une situation où la population étudiée comporte une fraction non à risque pour l’événement d’intérêt. Ces deux tests ont fait l'objet d'une application sur des données réelles d'immunologie clinique. / In the context of analysis of survival data, the comparison of several groups of individuals, where the event of interest is interval censored, represents a methodological challenge. When the monitoring of patients during the study is not continuous, the event of interest may occur between two observation dates; it is said "interval censored". Tests of comparisons of survival time distributions for several groups, adapted for interval censoring, have been developed (score tests, weighted pseudo log-rank tests, rank tests). In this context, we have developped two new group comparison tests adapted to the particular situations of interval censoring. The first test apply to a situation where the alternative hypothesis considers that the hazard functions cross. The second test concerns a situation where the study population has a fraction not at risk for the event of interest. Both of these tests have been applied to real clinical immunology dataset.