Baumeister and colleagues document how group homogeneity and differentiation affect group atmosphere, function, and performance. Their discussion focuses primarily on small face-to-face interactive task-oriented groups. The message is that these groups perform poorly when they are excessively homogeneous and undifferentiated, but well when there is role differentiation and individual members feel they have unique distinctiveness within the group.

Here, I draw on social identity theory (Turner et al. Reference Turner, Hogg, Oakes, Reicher and Wetherell1987; see Abrams & Hogg Reference Abrams, Hogg, Dovidio, Hewstone, Glick and Esses2010; Hogg Reference Hogg and Burke2006) to argue that Baumeister and colleagues' analysis becomes more textured when the group is theorized cognitively as an identity-defining social category. This perspective, which does not fundamentally distinguish small interactive groups from large sociodemographic categories, argues that all social groups provide their members with a shared social identity that defines one's attributes as a group member.

People cognitively represent groups they are in (in-groups) and those that they are not in (out-groups) as prototypes; fuzzy sets of attributes that capture key similarities within the group and differences from relevant outgroups (cf., Cantor & Mischel Reference Cantor, Mischel and Berkowitz1979; Wittgenstein Reference Wittgenstein1953). Because group prototypes are fuzzy, groups are by definition always internally differentiated; into those individuals or subgroups that more closely match the prototype and those that less closely match the prototype.

The implications of intragroup differentiation based on relative prototypicality are very significant for group life, particularly when people identify strongly with a group they consider an important and central part of their self-concept and identity. Under these circumstances, people are highly attentive to prototypicality (Haslam et al. Reference Haslam, Oakes, McGarty, Turner and Onorato1995; Hogg Reference Hogg, Williams, Forgas and von Hippel2005a). The prototype defines the group and thus one's identity as a group member; so prototypicality becomes a critical perceptual and evaluative standard. People are highly vigilant for and attentive to reliable information about the prototype; they need to know what the prototype is, how prototypical they are, and how prototypical other members are. There are at least five corollaries of group differentiation based on prototypicality.

Corollary 1. Prototypical members have most influence in the group (e.g., Abrams & Hogg Reference Abrams and Hogg1990; Reicher et al. Reference Reicher, Spears and Postmes1995). People pay particular attention to highly prototypical members as reliable sources of information about the prototype and thus about their identity and how they should conduct themselves. Because prototypes are typically widely shared within a group, prototypical members have widespread and disproportionate influence across the group as a whole. Prototype-based differentiation creates an influence gradient.

Corollary 2. Prototypical members can more effectively lead their groups (Hogg & Van Knippenberg Reference Hogg, Van Knippenberg and Zanna2003; Hogg et al. Reference Hogg, Van Knippenberg and Rast2012). Because prototypical members better embody the group's attributes and identity it is assumed that their fate is linked to the group and they can be trusted to do no harm to the group. The group follows their lead, allowing them to be normatively innovative identity entrepreneurs (e.g., Abrams et al. Reference Abrams, Randsley de Moura, Marques and Hutchison2008). Innovation and identity entrepreneurship are key aspects of effective leadership. Prototype-based differentiation creates a leadership effectiveness gradient.

Corollary 3. Prototypically peripheral members not only have relatively little voice, but also they can be marginalized and treated as deviants by the group. This is particularly the case if as individuals they are viewed as having unlikable attributes that reflect poorly on the group's prestige (e.g., Marques & Páez Reference Marques and Páez1994), or viewed as having attributes that blur the in-group–out-group boundary and compromise in-group entitativity and identity clarity (e.g., Marques et al. Reference Marques, Abrams, Páez, Hogg, Hogg and Tindale2001). Two of the most powerful motives underpinning social identity dynamics are the pursuit of an evaluatively positive identity (e.g., Abrams & Hogg Reference Abrams and Hogg1988) and having a clearly defined identity that reduces feelings of self and identity uncertainty (Hogg Reference Hogg and Zanna2007; Reference Hogg, Van Lange, Kruglanski and Higgins2012). Prototype-based differentiation can marginalize nonprototypical members.

Corollary 4. Group members who feel they are, or are perceived as, prototypically peripheral can overconform to group norms and engage in extreme intergroup behaviors (Goldman & Hogg Reference Goldman and Hogg2016; Hogg Reference Hogg2014). If a group and its associated identity are important and central to self-definition, feeling prototypically marginal elevates identity uncertainty and motivates behavior aimed at securing recognition, trust and membership centrality in the group. Prototype-based differentiation can cause prototypically marginal members to become zealots.

Corollary 5. Group members who constitute a minority subgroup occupying a prototypically peripheral position in the larger group not only experience membership uncertainty and marginalization as individuals, but also a threat to their subgroup identity – particularly when the overarching group subverts diversity by denying subgroup identity distinctiveness and imposing its own identity (Hogg Reference Hogg2015; Hogg & Wagoner Reference Hogg, Wagoner, Williams and Nida2017). Under these circumstances, prototype-based differentiation at the subgroup level can cause marginal subgroups to seek autonomy within group, exit the group, or exercise minority influence to convert the larger group.

Baumeister et al. build a case for how group heterogeneity can improve the functioning and performance of primarily small task-oriented groups. In this commentary, I have focused on the social identity function of groups to argue that all groups are internally differentiated in terms of the prototypicality of their constituent members and subgroups, and that prototype-based differentiation has far-reaching consequences.

From the perspective of group functioning prototypical individuals and subgroups have disproportionate influence that casts them in a consensually recognized leadership role. This can facilitate group functioning, but it can also place emphasis on group-based “popularity” rather than effective leadership attributes. In contrast, nonprototypical individuals can feel excluded and marginalized, causing them to overconform and engage in extreme intergroup behaviors to earn acceptance by the group. This loyal and zealous behavior can sometimes benefit the group, but can also create hostile intergroup conflict. Finally, nonprototypical minority subgroups can exit the group, restructure the group, or try to convert the group. This behavior of subgroups can compromise or enhance group functioning – for example, the group can cease to exist or be restructured to perform better.

The ubiquitous situation where members and subgroups are perceived, evaluated, and reacted to in terms of how well they fit the group's identity can have some positive consequences, but can also have negative consequences for social identity and group functioning.