Evolution of the 'Social Brain' in Humans: What Are the Benefits and Costs of Belonging to a Social Species?

Because the human brain has become so large and sophisticated in terms of the social computations it supports, it takes a very long time for it to develop fully.
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Humans are highly social beings. We like to be surrounded by friends and share our personal experiences with others. The recent appearance of various social networking tools, and their adoption at a virtually explosive rate, nicely illustrate the strong and fundamental human desire for social belonging and interpersonal exchange.

Not surprisingly, there is emerging evidence that evolutionary processes have favored the development of complex social behaviors in humans, along with the brain architecture that supports them. The human brain, and particularly the neocortex (which constitutes its outmost layer), is much larger in humans as compared to other primates and mammals of similar size. This is particularly interesting because the neocortex comprises many of the brain areas involved in higher social cognition, such as conscious thought, language, behavioral and emotion regulation, as well as empathy and theory of mind -- the ability to understand the feelings and intentions of others. We are, so to speak, biologically hard-wired for interacting with others, and are thus said to be endowed with a "social brain."

The reasons for the evolution of the human social brain are not yet completely understood. There is, however, growing consensus that two processes likely played key roles in triggering the observed dramatic increase in brain, and particularly neocortex, size. These were the development of (i) socially monogamous pair bonds, and (ii) paternal care / the involvement of the father in rising children (see here for additional information). Both of these processes offered additional defense mechanisms against infanticide and predation on offspring. In a nutshell: if the father stuck around long enough with his partner, and vice versa, the common children had a higher chance of survival, which in turn increased both the female's and the male's reproductive success. And that is what ultimately counts in evolution: promotion of survival of the fittest.

In terms of brain development, the emergence of socially monogamous pair bonds and bi-parental care imposed unprecedented needs for extensive social coordination and synchronization between the two partners and parents. For example, who is responsible for which aspects of childcare? Or how can both partners optimize the time used for searching food versus providing shelter to their offspring? These newly appearing requirements are thought to have laid the foundations for human social evolution as reflected in the development of advanced social cognition and skills.

It is believed that eventually, the cognitive mechanisms necessary for pair bonding and coordinated bi-parental care generalized to non-reproductive individuals. This generalization likely helped the formation of larger social groups, such as extended families, same-sex alliances, large coalitions, etc., again promoting higher rates of survival and reproduction. The increasing size of such expanded social groups in turn entailed further social brain development, ultimately giving rise to the present form of the human social brain.

The obvious benefits of social brain evolution are that humans are, today, equipped with a highly sophisticated social processing machine that enables us to engage in complex social interactions, and to maintain relationships to a great number of different individuals as well as groups. Our brains are further wired in a way that we experience reward during mutual social interactions, and feel sensations similar to physical pain when we are socially rejected or disapproved. It therefore looks like evolution has provided us with the perfect hardware for living in a world that is becoming ever more crowded.

Social evolution, however, does not only come with benefits. It also has its costs. Because the human brain has become so large and sophisticated in terms of the social computations it supports, it takes a very long time for it to develop fully. Compared to other mammals, human children have a very long developmental period and are highly dependent on care by adults. Human parents not only have to nurture their children until their brains are fully operational biologically, but they also have to provide an extended and stable context within which their children can safely acquire all the skills necessary for understanding their social surroundings. And this process continues far beyond childhood. For example, some social skills can only be learned by means of peer activities during adolescence, and throughout this period parents still have important protective and sheltering roles.

Recent research on attachment style in humans is starting to reveal some of the underlying behavioral and neural correlates of such guided social learning, and particularly the consequences if social learning does not occur in a sheltered environment. An individual's attachment style is a measure for the quality of his/her social bonds with others. It is crucially shaped through interactions with caregivers in early life, such as a child's parents. If others close to a child are responsive and caring, the child develops a secure attachment style. If they are unresponsive or inconsistent in their behavior towards the child, however, the child develops an insecure, either avoidant or anxious attachment style. Once acquired, the attachment style of a person is believed to remain rather stable throughout the lifespan, and to even be transmitted from one generation to the next. It is therefore likely to circularly influence many of the steps involved in social brain development and skill acquisition during childhood, adolescence, and even adulthood. We now have preliminary evidence that an insecure attachment style prompts people to respond either too strongly or too weakly to social information. This is especially striking in the case of attachment avoidance, which appears to considerably decrease both behavioral and brain responses within positive social contexts normally experienced as strongly rewarding. In other words, we know that a person's behavioral and brain responses to social information can be importantly altered if social brain development and skill acquisition do not occur in a protected setting.

Becoming social has made us who we are today. Evolution has provided us with the best tools possible for successfully engaging in social interactions. We should, however, not forget our responsibilities to ensure that future generations can learn in a protected and sheltered environment how to properly use these tools for their own lives.

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