Encouraging pro‐environmental behaviour (PEB) is an environmental and societal concern. Encouraging PEB focussing on how consumers see themselves (their identity) has blossomed. However, a theoretical assessment of this research is missing. Three main identity theories seem to best explain the research, specifically, and two‐fold, identity, and social identity theory (SIT), collectively known as the unified identity theory (UIT), and place identity theory (PIT). As these theories overlap more than differ in their understanding of identity, we argue that combining these theories is needed to avoid redundancies in identity theorizing, provide a universal approach to identity in terms of the processes and outcomes, and explain the PEB research most succinctly. Therefore, we understand identity similarly between the theories and offer a universal identity theory approach based on the theoretical definitions and assumptions. Finally, we demonstrate how the theory can be used to explain the research. Next, research was identified by conducting a systematic review using the Preferred Reporting Items for Systematic Reviews and Meta‐Analyses guidelines, where 62 studies were relevant. Multiple identities relevant for a given PEB are assumed and evidenced: 99. Identities are assumed to be either individually‐, group‐, and/or place‐focused, drawing on the specific subsets of the universal theory: identity theory, social identity theory, and place identity theory, respectively. Identities are assumed to relate to behaviour, where identity increased PEB with medium effect sizes. Finally, to move the field forward, we provide a theoretical framework of how to test identities in relation to other psychological variables relevant for PEB research.

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Prolific research suggests identity associates with pro-environmental behaviours (PEBs) that are individual and/or group focused. Individual PEB is personally driven, self-reliant, and are conducted on one's own (e.g., home recycling). Group focused PEB is other people-reliant and completed as part of a group (e.g., attending meetings of an environmental organisation). A wide range of identities have been related to PEBs. For example, a recent systematic qualitative review revealed 99 different types of identities studied in a PEB context. Most studies were correlational, few had an experimental design. However, the relationships between all these identities and PEBs have so far not been tested quantitatively with meta-analytical techniques. As such, a clear overview of this field is currently lacking. Due to the diverse nature of the field, a priori hypotheses were not possible and relatively broad definitions of identity had to be used to encompass all types of identities and the diverse meanings of identity that have been included in PEB research. What prior theory did allow for was to assess the distinction between two main types of identity, namely how people label, describe, and recognise oneself individually (individual identity), or as part of a group (group identity). Our overall goal was thus to assess the current state of knowledge on identities and PEBs. In 104 studies using a meta-regression following the preferred reporting items for systematic reviews and meta-analyses guidelines, our random-effects meta-analysis showed that the overall concept of identity associated with PEB with a medium Pearson's r (Aim 1). Furthermore, we found that individual identities associated more strongly with PEBs than group identities (Aim 2). The associations between individual and group identities were stronger when the identity and PEB were from the same category (e.g., when both were group-focused; Aim 3). Methodologically, the findings revealed that group identities and group PEBs were most strongly associated for self-reported rather than observed PEBs (Aim 4). Overall identity associated most strongly with group PEBs in the field rather than in the lab (Aim 5) and in student- rather than non-student samples (Aim 6). We discuss the theoretical and practical implications.

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