Robin Dunbar first noticed it in the grooming patterns of primates.
In the 1990s, Dunbar, a British anthropologist, was trying to understand primate grooming behavior — why did these animals spend so much of their time on the practice, and how did they choose whom to groom? He theorized that a primate’s number of grooming partners was connected to the size of its brain. Basically, bigger-brained primates could have larger social groups. Dunbar eventually realized that his findings could also apply to the biggest-brained primate: you.