Professor Robin Dunbar is best known for his work related to how many stable social relationships the human brain can manage. In earlier research, he argued that the optimal number of active relationships is 150 -- now famously known as the "Dunbar Number."
Dunbar is once again delving into the brain's social capacity, but this time he's focused on the size of the orbital prefrontal cortex (aka, the frontal lobe), the part of the brain involved in high-level thinking that sits just above our eyes. Dunbar and collegues have found that the size of this brain area correlates with the number of friendships a person is capable of managing.
The study suggests that we need to employ a set of cognitive skills to maintain a large number of friends, known in psychology circles as "mentalizing" or "mind-reading"-- an ability to understand what another person is thinking, which is crucial to our ability to handle our complex social world, including the ability to hold conversations with one another.
According to Professor Dunbar, as reported by Science Daily,'"Mentalizing" is where one individual is able to follow a natural hierarchy involving other individuals' mind states. For example, in the play 'Othello', Shakespeare manages to keep track of five separate mental states: he intended that his audience believes that Iago wants Othello to suppose that Desdemona loves Cassio. Being able to maintain five separate individuals' mental states is the natural upper limit for most adults."
The researchers took brain scans of 40 volunteers to measure the size of the prefrontal cortex. Participants were then asked to make a list of everyone they had had social (not professional) contact with over the previous seven days. They also took a test to determine their competency in mentalizing.
Dunbar adds, "We found that individuals who had more friends did better on mentalizing tasks and had more neural volume in the orbital frontal cortex. Understanding this link between an individual's brain size and the number of friends they have helps us understand the mechanisms that have led to humans developing bigger brains than other primate species. The frontal lobes of the brain, in particular, have enlarged dramatically in humans over the last half million years."
The study was published in The Proceedings of the Royal Academy of Biological Sciences.
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