Dunbar’s Number and Group Dynamics

Military units have typical sizes that promote strong and effective working relationships and make group management easier to accomplish. For instance, a platoon might consist of 12 to 15 individuals and have one or two officers in charge. In the early 1990s, an English anthropologist named Robin Dunbar theorized that the maximum size for any stable group of primates was determined by brain size—specifically, the size of the neocortex. The neocortex is involved in sensory perceptions, spatial reasoning, conscious thought and language. The larger the neocortex, he reasoned, the more individuals with whom they could maintain personal relationships. Dunbar compared primate brains to human brains and used the differential to predict the ideal size for a group of humans. The number he came up with was 147.8 people. Rounded up to 150, it became known as Dunbar’s number. That number, 150, has also been called a friendship circle, a limit of the number of (functionally active) friends and acquaintances you can have in your life at any time. Funeral directors know this and typically will print 200 programs for a memorial service. A U.S. Army company has about 150 individuals, give or take a few.   

A survey of ethnographic data found that hunter-gatherers around the world lived in shifting communities that ranged from 90 to 221 people, with an average of 148. Neolithic villages in Mesopotamia were thought to have around 150 people. The Roman army of the classical period used a formation of 130 men—called a maniple or double century—in combat. Hutterite communities in South Dakota split after reaching 150 people because, in their opinion, anything larger cannot be controlled by peer pressure alone. Hutterites, by the way, are one of three religious groups that trace their roots to the Radical Reformation in Europe in the 16th century. The other two groups are the Amish and the Mennonites.   

from Threes, Chapter 5, “Threes in Psychology and Sociology”