“When you look at pro-social behaviors,” says David Sloan Wilson, “behaviors that are for the good of the group, they’re not locally advantageous. If you’re an altruist, and you’re doing what’s good for the group, that fundamentally requires time and energy and risk on your part. If your neighbor isn’t doing that, then these behaviors are locally disadvantageous. We have a problem here. How do these traits evolve when they are less fit than the traits that we associate with selfishness and free riding? That’s the dilemma.”

Puzzled?