How close you consider certain friends to be also affects how often they need to be factored into your routine. Robin Dunbar, a British evolutionary psychologist and professor emeritus at the University of Oxford, claimed that humans can have 150 relationships at a time and that these connections are organized into levels of intimacy: your innermost circle is consists of five best friends, followed by a level of 15 very good friends and a third layer of 50 casual friends. To maintain your relationships with your five closest friends, Dunbar says, you should see them at least once a week. “The next layer, which is your 15 good friend layer,” he adds, “you only see once a month on average, or at least that’s the minimum, to keep them in that circle. The layer beyond, after 50 years that I still consider your backyard barbecue friends … they only need to be seen once every six months.