The evolution of the social brain


Today I come back on my last post on the paper by Alós-Ferrer that is an excellent overview of what we know about the social brain. In a nutshell, the social brain hypothesis means that one cannot causally reduce human behaviour to internal neurophysiological determinants, but always needs to refer to external causal factors. However, I think that this characterization would still be incomplete. To clarify the argument, one needs to detail the underlying mechanistic approach. I argue that the mechanistic approach needs to be put in an evolutionary framework to fully understand the social brain.

The social brain hypothesis is evolutionary in origin. That means, we approach the brain as a result of human evolution in the biological sense and start out from the idea that the most general function of the brain is to enable human sociality and social interaction in groups. This perspective is very productive, for example, in explaining certain alleged dysfunctions of behaviour in the context of modern societies. For example, we might argue that the brain is specialized on handling probabilities and risk in the context of intensive small group interactions but is not adapted to large-scale complex structures of markets. In my own contributions in the early 1990s, I tried to explain the sunk cost fallacy as a biological adaptation.

However, reflecting on Alós-Ferrer’s paper, this approach would be by far too limited and would commit a serious mistake in treating the functional structure of the brain as fixed, in terms of a legacy of biological evolution. Alós-Ferrer uses the term ‘social brain’ in a much broader sense, that is including the malleability and adaptability of brain structures and functions in the medium and even short term. In other words, the social brain would be conceived as capable to adopt entirely new functions, though partly based on a phylogenetic legacy. In other words, the social brain thesis in the original sense commits the fallacy of genetic determinism in over-estimating the role of phylogeny and underestimating the role of ontogeny and learning. In fact, it would end up with an ‘a-social’ mechanistic explanation in treating the inherited brain functions as a given and as immutable internal causes of behaviour.

But we should not throw out the baby with the bathwater. The last decades of research on general evolutionary theory have shown that evolutionary explanations can hold in different time scales and across different domains of explanation if we generalize over certain basic features of notions such as selection and inheritance (for a classical paper on this, see Hull et al. 2001). Unfortunately, across the various disciplines we observe also widespread resistance against such generalized approaches on evolution, for various reasons, often just because ‘evolution’ seems to smack of biologism, as in the social sciences. I think that a mechanistic and naturalistic approach to brain and behaviour cannot do without a generalized evolutionary theory, and that in this context the sociality of the brain must be recognized in a fundamental sense, fitting to Alós-Ferrer’s approach. This is easy to understand if we consider that all evolutionary approaches are externalist for necessary reasons: If you want to explain a phenomenon, you need to concentrate on external selective determinants of its emergence. In modern evolutionary thinking, this does not mean to downgrade internal factors (such as structural constraints on viable evolutionary trajectories). But certainly, the implication is that any evolutionary explanation must take the interaction between internal and external causal determinants into account when explaining a behavioural phenomenon. Here we are: The social brain is an evolving entity.

Evolutionary explanations introduce a new explanatory category, that is ‘function’. They are not functional explanations in the narrow sense, but if we refer to selection, ate the end of the day we reach explanations that answer to the ‘what for’ question as a specification of the ‘why’ question. I think that this simple step, though philosophically highly contentious (which I feel is making things unnecessarily complex), solves many problems associated with mechanistic explanations in our context. Just to jump to an example: In explaining a behavioural dysfunction such as eating disorders, a mechanistic explanation will certainly strive to identify the neurophysiological generating forces that work within the brain and the body in producing this behaviour. But this explanation will always remain incomplete unless we also explain why this behaviour happens at all. This ‘why’ question needs to include two more specific questions: First, what is the history behind the phenomenon, which means to look at ontogenetical factors, and second, what is the function of the behaviour in the current social context? Eating disorders may have many different functions, and the task of the psychologist and therapist, different from the neuroscientist, is to understand these functions and their genesis in the life-history of a patient. That’s why a narrow internalist mechanistic explanation (such as pharmacological intervention) cannot work in therapy unless supported by interventions that focuses on human purposes and social contexts.

A theory of the social brain is a multi-level evolutionary theory grounded in mechanistic explanations. Mechanisms can be analysed in two dimensions, which directly match with the distinction between functional and evolutionary biology: The first is synchronic mechanistic analysis that studies the causal determinants of a phenomenon and tends to be internalist in the sense that neurophysiological causes are of prime explanatory significance. This needs to be supplemented by functional analysis. But synchronous functional explanations easily fall into the many traps that philosophers have discovered in this approach. Thus, we need the second dimension, diachronic mechanistic explanations. These are mainly reconstructions of selective mechanisms through time, which allow for explaining the emergence of functions in terms of causal histories.

Hull, David L., Langman, Rodney E., Glenn, Sigrid S. (2001): A General Account of Selection: Biology, Immunology and Behavior, Behavioral and Brain Sciences 24(2): 511-573.

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