The neurobiology of human social behaviour: an important but neglected topic.

The past few decades have produced important advances in our understanding of how the brain regulates emotion and cognition. In comparison, research on the neuroscience of human social behaviour is a relatively neglected topic in spite of the importance of social interactions for mental health. In this editorial, I give examples of some of the experimental approaches that have been used to study the neural substrates of human social behaviour in the hope that this will stimulate more researchers to become involved with this fascinating and important topic. Humans are inherently social. We are not special in this way; it is hard to think of any animal for whom the regulation of social behaviour is not important. Something akin to social behaviour may even occur in organisms lacking a nervous system. For example, Science recently published an article titled “Genetic determinants of self identity and social recognition in bacteria.” Different animals, including humans, share many of the same types of social behaviour such as affiliation and aggression, the establishment of hierarchy and territoriality. This can be the case even in species, such as ants, with a primitive brain. Although we may share some of the broader aspects of our social behaviour with more primitive species, human social behaviour is obviously more complex but no less important for our health and survival. Given the importance of social interactions for humans, it is not surprising that most psychiatric disorders involve some disruption of normal social behaviour, and that in several disorders abnormal social functioning is one of the central symptoms. Examples are autism, social anxiety disorder, borderline personality disorder and schizotypal personality disorder. Despite the importance of social interaction, our understanding of the neural factors that control social behaviour is limited. Human social neuroscience is receiving increasing attention, but much of the current work concerns social cognition. For example, studies on the activation of different brain areas in response to faces with different expressions are interesting and important, but they are not central to the regulation of actual social behaviour. If response to faces was an essential determinant of social interaction, then blind people would not be able to form adequate social relationships and the use of text messaging would not be nearly as widespread as it is. The most extensive knowledge on the neurobiology of human social behaviour concerns one particular aspect of social behaviour: aggression. Research on aggression has led to the use of selective serotonin reuptake inhibitors (SSRIs) for the treatment of impulsive aggression, an illustration of how social neuroscience can lead to treatments for disordered social behaviour. However, aggression, although an important societal problem, does not feature prominently in many disorders even though it is required for the diagnosis of intermittent explosive disorder. Furthermore, overt aggression is not a common part of everyday social interactions. Research on the neurobiology of less extreme forms of social behaviour than aggression is limited. Two examples of how research on animals is starting to be applied to human social behaviour follow. In some species of monkeys, serotonin can influence both agonistic-affiliative behaviours and hierarchy. Although low levels of serotonin increase aggressive behaviours, as in humans, increasing serotonin function enhances prosocial behaviours such as grooming other animals. Increasing serotonin function also helps a male to achieve dominant status. Similar results have been reported in a few studies involving humans, carried out both in the laboratory and in everyday life. In the laboratory, healthy participants receiving an SSRI were rated more dominant and more cooperative during a mixed motive game and showed more affiliative behaviours during a dyadic puzzle task requiring cooperation. On the other hand, acute tryptophan depletion to lower serotonin levels caused reductions in the level of cooperation shown by participants when playing the prisoner’s dilemma game. Acute tryptophan depletion also changed behaviour in an ultimatum game in which players had to decide whether to accept or reject fair or unfair monetary offers from another player. Participants with low