Your brain size determines how good you're at keeping friends, say researchers.
A new study, led by professor Robin Dunbar of the University of Oxford, has found a link between the number of friends one has and the size of the region of one's brain - the orbital prefrontal cortex - that is found just above the eyes.
This brain region is bigger in people who have a larger number of friendships, the Proceedings of the Royal Society B journal reported.
The study suggests that people need to employ a set of cognitive skills to maintain a number of friends. These skills are described as "mentalising" or "mind-reading" - a capacity to understand what another person is thinking.
The study, for the first time, suggests that a person's competency in these skills is determined by the size of key regions of their brains (in particular, the frontal lobe), say the researchers.
Prof Dunbar said: "Mentalising is where one individual is able to follow a natural hierarchy involving other individuals' mind states. Being able to maintain five separate individuals' mental states is the natural upper limit for most adults."
In their study, the researchers took anatomical MR images of the brains of 40 volunteers at the Magnetic Resonance and Image Analysis
Research Centre at the University of Liverpool to measure the size of the prefrontal cortex, the part of the brain used in high-level thinking.