Why do men talk and women gossip, and which is better for you? Why is monogamy a drain on the brain? And why should you be suspicious of someone who has more than 150 friends on Facebook?

We are the product of our evolutionary history, and this history colors our everyday lives—from why we joke to the depth of our religious beliefs. In How Many Friends Does One Person Need?Robin Dunbar uses groundbreaking experiments that have forever changed the way evolutionary biologists explain how the distant past underpins our current ­behavior.