Pet-keeping is highly variable across cultures in both frequency and form. Cultural group selection offers a plausible explanation for the development and spread of this uniquely human phenomenon in that pet-keeping involves an inheritance system, socially transmitted norms and preferences, substantial between-group variation, and (albeit indirectly) intergroup competition.

We discuss cultural group selection under the view of the frozen plasticity theory and the different explanatory power and predictions of this framework. We present evidence that cultural adaptations and their influence on the degree of cooperation may be more complex than presented by Richerson et al., and conclude with the gene-environment-culture relationship and its impacts on cultural group selection.

As evidence that cultural group selection has occurred, Richerson et al. simply retrodict that humans use language, punish each other, and have religion. This is a meager empirical haul after 30 years. This contrasts sharply with the adaptationist approach to human behavior – evolutionary psychology – which has produced scores of novel, specific, and empirically confirmed predictions.

I outline key empirical evidence from my research and that of other scholars, testing the role of cultural group selection (CGS) in the evolution of human cooperation, which Richerson et al. failed to mention and which fails to support the CGS hypothesis.

We believe cultural group selection is an elegant theoretical framework to study the evolution of complex human behaviours, including large-scale cooperation. However, the empirical evidence on key theoretical issues – such as levels of within- and between-group variation and effects of intergroup competition – is so far patchy, with no clear case where all the relevant assumptions and predictions of cultural group selection are met, to the exclusion of other explanations.

I argue that demographic selection, migration, and cultural diffusion, three mechanisms of institutional change, have little in common. Two of these lack the key features associated with group selection: they do not present us with group-level selection pressures counteracting individual-level ones, need not produce behavioral altruism, and do not require competition between groups whose members cooperate preferentially with one another. Cultural norms vary, change, and influence cooperation; but that is not group selection.

Cultural group selection helps explain human cooperation, but social selection offers a complementary, more powerful explanation. Just as sexual selection shapes extreme traits that increase matings, social selection shapes extreme traits that make individuals preferred social partners. Self-interested partner choices create strong and possibly runaway selection for prosocial traits, without requiring group selection, kin selection, or reciprocity.

Richerson et al. propose cultural group selection (CGS) as the basis for understanding the evolution of cultural systems. Their proposal does not take into account the nature of cultural idea systems as being constituted at an organizational, rather than an individual level. The sealing partners of the Netsilik Inuit exemplify the problem with their account.

Richerson et al. argue that relatively large cultural FST values provide evidence for group structure and therefore scope for group selection. However, recent research on spatial patterns of cultural variation demonstrates that, as in the genetic case, apparent group structure can be a consequence of geographic clines, not group barriers. Such a pattern limits the scope for cultural group selection.

The reproduction of cultural systems in cases where cultural group selection may occur is typically incomplete, with only certain cultural traits being adopted by less successful cultural groups. Why a particular trait and not another is transmitted might not be explained by cultural group selection. We explore this issue through the case of religious syncretism.

The cultural group selection hypothesis is supported by considerations of short-term and long-term behavioral patterns of group members, and the short-term and long-term consequences of that behavior. The key to understanding cooperation lies in understanding that the effect of an extended behavioral pattern does not equal – and might even be opposite to – the added effects of its parts.

We agree that institutions and rules are crucial for explaining human sociality, but we question the claim of there not being “alternatives to CGS [that] can easily account for the institutionalized cooperation that characterizes human societies” (target article, sect. 7). Hypothesizing that self-interested individuals coercively and collaboratively create rules, we propose that agent-based hypotheses offer viable alternatives to cultural group selection (CGS).

Developmental research characterizes even the youngest learners as critical and selective, capable of preserving or culling cultural information on the bases of informant accuracy, reasoning, or coherence. We suggest that Richerson et al. adjust their account of social learning in cultural group selection (CGS) by taking into consideration the role of the selective learner in the cultural inheritance system.

The properties of individual carbon atoms allow them to chain into complex molecules of immense length. They are not limited to structures involving only a few atoms. The design features of our evolved neural adaptations appear similarly extensible. Individuals with forager brains can link themselves together into unprecedentedly large cooperative structures without the need for large group-beneficial modifications to evolved human design. Roles need only be intelligible to our social program logic, and judged better than alternatives.

The evidence compiled in the target article demonstrates that the assumptions of cultural group selection (CGS) theory are often met, and it is therefore a useful framework for generating plausible hypotheses. However, more can be said about how we can test the predictions of CGS hypotheses against competing explanations using historical, archaeological, and anthropological data.

Pain suffering has been naturally selected to be experienced and expressed within a wider social system. The communication of pain improves group coordination and decision-making about engaging in resource dependent and potentially risky endeavors. Recent findings warrant the development of a cohesive framework for understanding the reciprocal nature of pain expression and individual and group-level outcomes that can generate novel predictions on the heuristical expression of human suffering in naturalistic and clinical settings.

We raise and discuss two issues for clarification. First, over what timescale is cultural group selection proposed to have been active? Unprecedented cooperation is inferred to have arisen in hunter-gather life well before the recent period on which Richerson et al. focus. Second, what is the unit of selection? Groups (of human individuals)? Or cultural entities along the lines of “meme-complexes”?

The main objective of our target article was to sketch the empirical case for the importance of selection at the level of groups on cultural variation. Such variation is massive in humans, but modest or absent in other species. Group selection processes acting on this variation is a framework for developing explanations of the unusual level of cooperation between non-relatives found in our species. Our case for cultural group selection (CGS) followed Darwin's classic syllogism regarding natural selection: If variation exists at the level of groups, if this variation is heritable, and if it plays a role in the success or failure of competing groups, then selection will operate at the level of groups. We outlined the relevant domains where such evidence can be sought and characterized the main conclusions of work in those domains. Most commentators agree that CGS plays some role in human evolution, although some were considerably more skeptical. Some contributed additional empirical cases. Some raised issues of the scope of CGS explanations versus competing ones.

Memory is fleeting. New material rapidly obliterates previous material. How, then, can the brain deal successfully with the continual deluge of linguistic input? We argue that, to deal with this “Now-or-Never” bottleneck, the brain must compress and recode linguistic input as rapidly as possible. This observation has strong implications for the nature of language processing: (1) the language system must “eagerly” recode and compress linguistic input; (2) as the bottleneck recurs at each new representational level, the language system must build a multilevel linguistic representation; and (3) the language system must deploy all available information predictively to ensure that local linguistic ambiguities are dealt with “Right-First-Time”; once the original input is lost, there is no way for the language system to recover. This is “Chunk-and-Pass” processing. Similarly, language learning must also occur in the here and now, which implies that language acquisition is learning to process, rather than inducing, a grammar. Moreover, this perspective provides a cognitive foundation for grammaticalization and other aspects of language change. Chunk-and-Pass processing also helps explain a variety of core properties of language, including its multilevel representational structure and duality of patterning. This approach promises to create a direct relationship between psycholinguistics and linguistic theory. More generally, we outline a framework within which to integrate often disconnected inquiries into language processing, language acquisition, and language change and evolution.