If you list all your friends, and then ask them all how many friends they have, their ­average is very likely to be higher than your friend count.

The reason is bewilderingly ­simple: "You are more likely to be friends with someone who has more friends than with someone who has fewer friends," as the ­psychologist Satoshi Kanazawa puts it. You're more likely to know more popular people, and less likely to know less popular ones. Some people may be completely friendless, but you're not friends with any of them.

Neat.