The biological basis of complex human social interaction and communication has been illuminated
through a coming together of various methods and disciplines. Among these are comparative studies
of other species, studies of disorders of social cognition and developmental psychology. The use of neuroimaging
and computational models has given weight to speculations about the evolution of social
behaviour and culture in human societies. We highlight some networks of the social brain relevant to
two-person interactions and consider the social signals between interacting partners that activate
these networks.Wemake a case for distinguishing between signals that automatically trigger interaction
and cooperation and ostensive signals that are used deliberately.We suggest that this ostensive signalling
is needed for ‘closing the loop’ in two-person interactions, where the partners each know that they have
the intention to communicate. The use of deliberate social signals can serve to increase reputation and
trust and facilitates teaching. This is likely to be a critical factor in the steep cultural ascent ofmankind