The Social Brain

Reading faces, understanding other minds, belonging and exclusion: the brain networks for social life, the social brain hypothesis, and why loneliness matters for health.

The social brain hypothesis

Why do primates, and humans especially, have such large brains? The anthropologist Robin Dunbar proposed in the 1990s that brains grew to manage complex social lives: across primate species, the relative size of the neocortex correlates with typical group size. From this he estimated that humans can maintain about 150 meaningful relationships — "Dunbar's number", a rough figure that has been debated.

Reading faces and bodies

Specialised regions respond to faces (the fusiform face area), to bodies and to biological motion (the superior temporal sulcus). Babies prefer face-like patterns within hours of birth. The amygdala helps detect emotional expressions, especially fear, and directs gaze to the eyes.

Understanding other minds

Theory of mind — understanding that others have beliefs, desires and knowledge different from your own — is often tested with the false-belief task: Sally puts a marble in a basket and leaves; Anne moves it to a box; where will Sally look? Most children answer correctly from around age four. Brain imaging consistently implicates a mentalising network including the temporoparietal junction, medial prefrontal cortex and precuneus. Rebecca Saxe and colleagues found the right temporoparietal junction responds selectively when people think about others' thoughts.

Empathy

Observing others in pain activates parts of the anterior insula and anterior cingulate involved in one's own pain — a shared-representation account of empathy (with debates about how literally "shared" it is). Researchers distinguish affective empathy (sharing feelings) from cognitive empathy (understanding them) and from compassion (caring motivation), which training studies suggest may be more sustainable than empathic distress.

Belonging and loneliness

Being excluded hurts. In Naomi Eisenberger's 2003 "Cyberball" study, people excluded from a virtual ball-tossing game showed activity in the anterior cingulate, a region involved in pain distress (later findings have been mixed). Large epidemiological studies associate chronic loneliness and social isolation with higher risks of mortality, cardiovascular disease and dementia; the US Surgeon General declared loneliness a public health epidemic in 2023.