Group-level Traits Research Articles

Group-level traits are not units of selection.

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.

Group-level traits can be studied with standard evolutionary theory.

Smaldino's target article draws on and seeks to add to a literature that has partially rejected orthodox, gene-centric evolutionary theory. However, orthodox theory has much to say about group-level traits. The target article does not reference or refute these views, and provides no explicit arguments for this narrow approach. In this commentary we: (i) give two examples of topics that the target article might and probably should have discussed (cultural epidemiology and the psychology of individual differences); and (ii) argue that the orthodox approach has much more to say about the emergence of group-level traits than the target article recognises, or gives credit for.

The priority of the individual in cultural inheritance.

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.

Homogeneity of mind can yield heterogeneity in behavior producing emergent collaboration in groups.

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.

Group-level traits emerge.

Most commentators supported the thesis of the target article, though there were also those who were less fully persuaded. I will begin with a response to the most critical commentaries. First, I will justify an evolutionary perspective that includes group organization and nongenetic inheritance. Next, I will discuss the concept of emergence. Following that, I will transition to an exploration of ideas and concerns brought up by some of the more supportive commentators. This will include a discussion of different types of groups; the psychology of group-level traits; the uses and limitations of an institutional perspective; the link between transmission, adaptation, and selection; current and future methodologies; and the variety of fields that may benefit from a group-level traits perspective.

Why religion is better conceived as a complex system than a norm-enforcing institution.

Although religions, as Smaldino demonstrates, provide informative examples of culturally evolved group-level traits, they are more accurately analyzed as complex adaptive systems than as norm-enforcing institutions. An adaptive systems approach to religion not only avoids various shortcomings of institutional approaches, but also offers additional explanatory advantages regarding the cultural evolution of group-level traits that emerge from religion.

The primacy of scaffolding within groups for the evolution of group-level traits.

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.

Cultural evolution and emergent group-level traits through social heterosis.

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.

Strong group-level traits and selection-transmission thickets

I suggest that we should distinguish between weak and strong group-level traits. It is the latter that are especially problematic for developing models of cultural evolution. Using Campbell's notion of vicarious selection mechanisms, I argue that these mechanisms become intertwined with other group-level traits resulting in selection-transmission thickets that are obstacles to modeling cultural evolution.

Many important group-level traits are institutions

Smaldino makes a solid contribution to the literature on the evolution of human social organization by pointing out that group-level-traits (GLTs) often emerge from the interactions of group members in such a way that their effects are not easily partitioned into individual selection. However, we argue that he too readily dismisses institutional analysis as a tool for understanding these traits.

Culture as an aggregate of individual differences.

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 cultural evolution of emergent group-level traits.

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.

Studying the emergence of complicated group-level cultural traits requires a mathematical framework

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.

Group-level expression encoded in the individual.

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.

Language as an emergent group-level trait.

Following Smaldino's definition, we claim that language is also an emergent group-level trait, and propose two facets (human groups tend to organize in a way to efficiently trigger language and linguistic interactions can render formation of certain social organization) to verify this statement, both of which also provide a general framework to address the future work about group-level traits.

Individual-level psychology and group-level traits.

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.

Feedback, group-level processes, and systems approaches in human 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.

Tackling group-level traits by starting at the start.

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.

What is a group? Conceptual clarity can help integrate evolutionary and social scientific research on cooperation.

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?

Spatial pattern as an adaptive phenotype

Spatial patterns are common in nature across a broad range of scales, from body coloration patterns of animals to clustering of vegetation. The ultimate causes of these patterns are viewed very differently depending on whether they are traits of individuals or properties of aggregations. Traits of individuals are usually considered to be shaped directly by selection, while patterns of aggregation are typically viewed as incidental side effects of some other underlying processes or environmental heterogeneity. However, given the powerful influence that spatial structure can have on the susceptibility of a population to a dispersal-limited predator or pathogen, it may be useful to consider the possibility that spatial structure per se could serve as an anti-enemy adaptive phenotype. This group-level trait could evolve only if selection at the individual level does not overwhelm higher-level selection. To explore the plausibility of spatial structure as an adaptive phenotype, I consider the specific case of a spatially-explicit, evolutionary host–pathogen model. This model demonstrates the evolution of reproductive restraint, resulting in a low-density, poorly-connected landscape of host clusters that is resistant to the spread of the pathogen. Reimagining spatial structure as an adaptive phenotype may generate new insights of both theoretical and practical significance.