‘150 is “the approximate number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar.” 150 is often referred to as the Dunbar number.

Ten years ago, Robin Dunbar studied the sending of Christmas cards in England. He used the count to measure meaningful social connections. The number sent averaged 153.5, precisely what Dunbar expected. He and other researchers kept finding groupings of 150; self-governing communes, offices of Gore-Tex, etc. Dunbar postulates that this is simply the brain’s limit. Sure, there are outliers, but most people top out at 150 relationships…

“Fundamentally, once you go beyond this number of people you can keep in your head, you begin to filter yourself, you change what you share and how much, you put on your public face.” …

Dunbar plotted the size of the neocortex of each type of primate with the size group that it lived in. The bigger the neocortex, the larger the group. To predict human group size, Dunbar supplanted the ratio of the human neocortex into the group. The result? 147.8, roughly 150…

Dunbar says within the 150, there are other interesting numbers. Three to five are our closest friends. The death of any of our 12-15 closest would devastate us….’ (dirjournal.com).