“Man is by nature a social animal; an individual who is unsocial naturally and not accidentally is either beneath our notice or more than human…”
― Aristotle, Politics
It is not news that Aristotle saw sociality as a defining point of human nature. Interestingly, his idea is backed by modern evolutionary neuroscientists who claim that exactly complex social behavior is the precursor of the primate brain enlargement process that resulted in the modern human brain. In this blog post, I will try to summarize the milestones of the Social Brain Hypothesis (SBH) and find out whether it really can be counted as the main cause of what we are now.
Social Brain Hypothesis (SBH)
To be honest I was skeptical when first heard about the Social Brain Hypothesis (SBH). Is that possible that our sociality matter that much that it can be counted as the key factor of our brain evolution? Can the number of people with whom we socialize really say anything about our brains?
After taking a look at the current debate on the causes of observed encephalization in primates these questions seem to be valid, so let’s first see what the SBH theoreticians have to say about them.
The social factors of primate brain enlargement first became popular in the 1970s, when primatologists suggested that primate intelligence could be related to the demands of their more complex social life. This idea consequently resulted in the SBH which is based on two basic assumptions: First, cooperative social processes are needed for primates to overcome ecological constraints and second, big brains are required to carry out social processes. Here it is important to note that the starting point of the SBH is not creation of large social groups, but viewing social groups as a possibility to overcome demanding ecological problems. Based on this assumption, individuals living in larger groups have higher fitness. According to neuroimaging studies in humans and macaques, individual differences in social network size appear to be in correlation, particularly with the size of the frontal lobe, the brain region that plays an important role in social interaction. An influential theoretician of the SBH, R. Dunbar, states that causal reasoning, analogical reasoning, comparing alternative outcomes, and the ability to inhibit prepotent responses are the basic functions of all sorts of primate decision making, including social ones. These are the traits characteristic only of simians (anthropoids) and depend on the Broadman area 10 in the frontal lobe present only in anthropoid primates. According to the SBH, exactly the ability to carry out the above-listed executive functions is in correlation with the neocortex volume in primates.
So, what is so special in the social behavior of primates? According to the SBH, the main function of primate social groups is to provide a defense against predators. Hence group members have to coordinate their activities. Sex differences in energy demand, different reproductive abilities, and varying body mass serve as constraints in coordinating group activities. Thus, some individuals have to inhibit their prepotent responses in order to ensure the stability of the group. Another important issue is that living in a social group, besides having its positive sides, might have some flaws as well, e.g. creating competition among a number of conspecifics dwelling on limited space. Those not as physically strong are deprived of the best of food, harassment might cause female infertility, etc. Thus, to deal with these challenges, in anthropoid primates, we have grooming-based coalitions actively protecting their members from harassment when a defender is present in the situation, as well as passively when a harasser avoids attacking individuals with powerful defenders. This last characteristic of a harasser is cognitively quite demanding since it requires having in mind the image of other individuals not present at the moment, foreseeing the consequence of two situations: what would happen in case of executing harassment and what – in case of inhibiting the will, evaluating consequences, making adequate decisions and carrying out inhibitive behavior. Dunbar links this ability of primates with the bases of theory of mind in humans, the ability to mentalize on others’ psychological state. According to him, the mentalizing ability in humans is in correlation with the size of social networks and importantly with the volume of the Prefrontal Cortex (PFC) and temporal lobes. He views mentalizing as the ability to have one’s own perspective on others’ state of mind, like: “I know what I know and at the same time know what A knows”. It is hypothesized that individual differences in mentalizing competences (the number of different mind states one can have in one’s own mind at the same time) in normal human adults correlate with the volume of grey and white matter in the key regions of the theory of mind network, especially in the frontal lobes.
To sum up the SBH, its starting point is that primate social groups are far more complex than those of other animals and dealing with this complexity is the main precursor of observed encephalization. There is a correlation between social group size and neocortex size in primates. The relationship between brain size and group size at least in humans is mediated by mentalizing skills, what is assumed to be the crucial factor of humans’ unusually large PFC and temporal lobes, aka the theory of mind network.
The SBH may offer an interesting perspective but is still not convincing enough, as if something is missing. So let’s have a look at the issue from a different perspective.
The novel analysis of factors that might have had an effect on observed encephalization in primates claims that not social, but ecological factors are the key aspects in primate brain evolution. Very shortly, those ecological problems that had to be overcome in the process of evolution included: finding food in a seasonally changing environment; storing food and preparing it. From the ecological perspective, exactly these challenges made humans learn how to light a fire, build and use tools, and fight predators.
A current finding in the field openly states that primate brain size is predicted by diet and not by sociality. According to it, frugivores have significantly larger brains than folivores. Interestingly, in their study, Decassien and colleagues used three times more (>140) study sample (non-human primates) than Dunbar and instead of analyzing only the neocortex size, what was mostly done in SBH studies, authors tested the whole brain size, claiming that regions outside the neocortex, namely cerebellum, hippocampus, and striatum also contribute to carrying out complex cognitive functions. According to their results, frugivory is not only beneficial for cognitive functions but also compensates for the energy loss caused by the metabolically expensive brain, since it provides more energy by spending less on digestion.
Another study also rethinks the role of socialization in primate brain evolution. Authors have carried out a re-examination of factors previously assumed to be in relation with primate brain size using two large datasets (the one used by the Decassian et al., 2017 and another published in Stephan et al., 1981 that was used by Dunbar in testing the SBH) and phylogenetic comparative methods. Consequently, they have found a correlation between the brain size and the ecological variables, namely home range size, diet, and activity period, and, in contrast with the SBH, insignificant effect of social group size. Authors assume that this famous correlation that serves as a milestone of SBH might have been caused by the relatively small dataset that Dunbar used to test his hypothesis. This latter statement can be validated by the fact that after matching their dataset to the one used by Dunbar, social group size exhibited significant correlation with the observed encephalization, however when the same analysis was conducted with the larger sample, SBH variable was no more among the best-fitted models. Thus, the fact that the correlation between group size and the brain size is robust only in case of restricting analysis to the sample given in Stephan et al., 1981, suggests that this dataset might be biased to the SBH.
In 2018, there was published one more study conducted by González-Forero and colleagues questioning the role of the SBH. According to its novel computational model, the major role in brain and body growth in primates is that of ecological factors, while social challenges did not show any significant correlation in the observed encephalization pattern. Curiously, the model showed that social cooperation among animals could, on the contrary, lead to a decrease in brain size, as some individuals in cooperative groups could rely upon other group members’ intelligence.
So, what about sociality?
In summing up the results of their computational model, González-Forero and colleagues state that since the model prioritized ecological factor as the driving force of the observed encephalization in primates, whereas social factors more likely played a kind of deterrent role in this process, both factors are important for obtaining the brain size we have today. With other words, according to the model, if there were no social factors, our brains would have been even larger, and here we should keep in mind that big might be good, but oversized can cause some problems.
As for the question why not all species have large brains since all of them have to deal with ecological constraints, the authors’ answer is that ecological problems can lead to human size brain when they are tackled in a more and more developed way. The species that have once started the process of learning never stop it. They accumulate knowledge, create a cultural heritage and transmit it to other conspecifics. So as it seems, challenging ecological environment and cultural heritage of knowledge how to tackle these problems can be assumed as the main producers of the brains we have today.
To recap the current debate in the field, what can be obviously stated is that data quality, the use of statistical methods and employing different variables could be the reason for discrepancies that exist in the studies concerning the main factors of observed encephalization in primates and according to the current findings, the SBH cannot be claimed to be the main correlate of the primate brain evolution.
So, if you enjoy your weekends by being on your own in your room instead of trying to socialize, it is ok, you can relax, since you are not any kind of mutated cyborg, but just another “everyday normal guy” and your not that much social behavior, as well as the sociality of others, is pretty much equally normal human behavior.