According to British anthropologist Robin Dunbar, the “magic number” is 150. Dunbar became convinced that there was a ratio between brain sizes and group sizes through his studies of non-human primates. This ratio was mapped out using neuroimaging and observation of time spent on grooming, an important social behaviour of primates. Dunbar concluded that the size, relative to the body, of the neocortex – the part of the brain associated with cognition and language – is linked to the size of a cohesive social group. This ratio limits how much complexity a social system can handle.

Dunbar and his colleagues applied this basic principle to humans, examining historical, anthropological and contemporary psychological data about group sizes, including how big groups get before they split off or collapse. They found remarkable consistency around the number 150.

According to Dunbar and many researchers he influenced, this rule of 150 remains true for early hunter-gatherer societies as well as a surprising array of modern groupings: offices, communes, factories, residential campsites, military organisations, 11th Century English villages, even Christmas card lists. Exceed 150, and a network is unlikely to last long or cohere well. (One implication for the era of urbanisation may be that, to avoid alienation or tensions, city residents should find quasi-villages within their cities.)