A great advantage of the internet and social media is that they allow us to keep in touch with all our friends, even when they move away. That means our group of friends can carry on expanding indefinitely. Except, says anthropologist Robin Dunbar, it can’t. There’s a limit to the number of friends we can have. It is known as Dunbar’s number. Presented by David Edmonds (Image: Group of friends, Credit: Shutterstock)