Many of the most important properties of human groups – including properties that may give one group an evolutionary advantage over another – are properly defined only at the level of group organization. Yet at present, most work on the evolution of culture has focused solely on the transmission of individual-level traits. I propose a conceptual extension of the theory of cultural evolution, particularly related to the evolutionary competition between cultural groups. The key concept in this extension is the emergent group-level trait. This type of trait is characterized by the structured organization of differentiated individuals and constitutes a unit of selection that is qualitatively different from selection on groups as defined by traditional multilevel selection (MLS) theory. As a corollary, I argue that the traditional focus on cooperation as the defining feature of human societies has missed an essential feature of cooperative groups. Traditional models of cooperation assume that interacting with one cooperator is equivalent to interacting with any other. However, human groups involve differential roles, meaning that receiving aid from one individual is often preferred to receiving aid from another. In this target article, I discuss the emergence and evolution of group-level traits and the implications for the theory of cultural evolution, including ramifications for the evolution of human cooperation, technology, and cultural institutions, and for the equivalency of multilevel selection and inclusive fitness approaches.

Smaldino suggests that patterns that give rise to group-level cultural traits can also increase individual-level cultural diversity. I distinguish social roles and related social network structures and discuss ways in which each might maintain diversity. I suggest that cognitive analogs of “cohesion,” a property of networks that helps maintenance of diversity, might mediate the effects of social roles on diversity.

Although both a “simple dyad or a population of thousands” are groups, these are, respectively, face-to-face embodied groups and collective symbolic groups. We applaud Smaldino for recognizing and describing the concept of the group-level trait. As an expansion, we propose an evolutionary-developmental model of face-to-face groups that scaffolds theorizing the evolution of cultural group-level traits.

We agree that emergent group-level properties are important; however, we disagree that current approaches, especially culture-gene coevolutionary (CGC) approaches, have neglected them. We explain how CGC helps demystify the tumult of humans' group-level complexity by “starting at the start,” and why (a) assuming undifferentiated individuals and (b) focusing on cooperation are actually powerful tools to this end.

Smaldino's proposed extension of the theory of cultural evolution embraces emergent group-level traits. We argue, instead, that group-level traits reduce to the traits of individuals, particularly when it comes to the question of how group-level traits are inherited or transmitted, and that this metaphysical fact is integral to the theory of cultural evolution.

Understanding the cultural evolution of complicated group-level traits requires the mathematical formulation of a dynamical system with birth and death events at multiple levels, that is, at the level of individual humans and at the level of groups of humans. Both levels are characterized by cultural traits that have complicated transmission, innovation, and inheritance mechanisms and that can undergo a form of Lamarckian evolution.

An explicit role for emergent group-level traits, social role diversification, and coordination makes good sense when thinking about human evolution. To most effectively understand these and other facets, we need to move toward an integrative model that assumes niche construction and encompasses positive feedback systems at individual, subgroup and larger group levels, demographic processes, and local ecologies.

Smaldino argues that evolutionary theories of social behavior do not adequately explain the emergence of group-level traits, including differentiation of roles and organized interactions among individuals. We find Smaldino's account to be commendable but incomplete. Our commentary focuses on a simple question that has not been adequately addressed: What is a group?

Smaldino proposes a conceptual extension to the theory of cultural evolution to include emergent group-level traits as a unit of selection. It is important to recognize the role that group-level traits have played in the evolution of human culture. However, the emergent group-level trait of division of labor provides an illustrative example that is implementable within the existing framework of cMLS theory and may not even need a departure from the standard model.

We question Smaldino's argument that culture plays the active role of maintaining and transmitting social organizations of differentiated individuals. Culture is an aggregate of individual differences in psychological variables within and between groups; it was not designed by group-level selection to maintain the structured organization of individuals. We conclude that Smaldino fails to present the crucial mechanism by which group-level traits are maintained, transmitted, and evolve.

The idea that structured organization of differentiated individuals influences group behavior is not new to evolutionary approaches. An adequate theory of groups needs to incorporate explicit processing, which is central to means-end reasoning involved in leadership and to the construction of ideologies that rationalize group structure. Explicit processing is also central to knowledge of others' reputations, thus enabling altruistic cooperation.

The theoretical concepts developed in the target article, in which the author proposes a new paradigm of cultural evolution based not on the individuals' characteristics, but rather on more global collective properties described as “group-traits” (which emerge when a group of individuals exhibit both differentiation and organization), may have a broader scientific impact that transcend the boundaries of social and evolutionary psychology, paving the way for the emergence of macro-neuroeconomics and social evolutionary game theory.

This commentary suggests (1) that there are precedents for Smaldino's “collaboration” in the history of economic thought before 1900 and (2) that the distinction of collaboration from what is thought of as cooperation in game theory is less clear than Smaldino suggests.

Psychological research on social influence illuminates many mechanisms through which role differentiation and collaborative interdependence may affect cultural evolution. We focus here on psychological processes that produce specific patterns of asymmetric influence, which in turn can have predictable consequences for the emergence and transmission of group-level traits.

Smaldino proposes emergent properties of human groups, arising when individuals display both differentiation and organization, constitute a novel unit of cultural selection not addressed by current evolutionary theory. We propose existing theoretical frameworks for maintenance of genetic diversity – social heterosis and social genomes – can similarly explain the appearance and maintenance of human cultural diversity (i.e., group-level traits) and collaborative interdependence.

The evolved psychological process for producing social norms is both needed to facilitate emergent group-level traits and capable of delivering such a process. I discuss how this process can work to generate group-level traits and how specific mechanisms established to buttress social norms similarly can explain how group-level traits are supported.

All group traits, “emergent” or otherwise, are ultimately dependent on the traits and behaviours of the individuals that constitute the group. Unless a process of “group reproduction” is envisaged, this means that the evolution of group traits can in principle be studied in an individualistic way, by studying the dynamics of the underlying individual traits on which they depend.

Smaldino is right to argue that we need a richer theory of group-level traits. He is wrong, however, in limiting group-level traits to units of cultural selection, which require explanations based on group selection. Traits are best understood when explanations focus on both process (i.e., selection) and product (i.e., adaptation). This approach can distinguish group-level traits that arise through within-group processes from those that arise through between-group processes.

Models of cultural evolution need to address not only the organizational aspects of human societies, but also the complexity and structure of cultural idea systems that frame their systems of organization. These cultural idea systems determine a framework within which behaviors take place and provide mutually understood meanings for behavior from the perspective of both agent and recipient that are critical for the coherence of human systems of social organization.

We argue that Smaldino has not established that group-level traits constitute a unit of selection distinct from selection on individuals, as group-level traits are neither replicators nor interactors. Moreover, we argue that Wimsatt's analysis of emergence and aggregativity supports an understanding of group-level and other emergent traits as explanatorily reducible to the individual level.