The Sisterhood of Traveling Pants can only have one more BFF, according to a study by a group of British researchers. They include anthropologist Robin Dunbar, who pinpointed a correlation between a primates' brain sizes and the sizes of their social circles. Humans, for instance, can only form meaningful relationships with up to 150 people. He believes that five of those make up one's closest friends, followed by 10 more in the next layer, 35 in the third and 100 in the outermost one. There are some differences between introverts and extroverts, but they have the same number of what's called the "Dunbar layers."