The pathological Seattle four-way stop—drivers sitting, gesturing at one another to go ahead rather than taking the right of way—is fundamentally rodentlike. You aren't being polite; you're acting out a deeply programmed instinct to compete for the title of Most Helpful. It's the passive-aggressive peacock show—with brown feathers fluttering in the air. The result: We all sit and wait for someone to be the least nice. Consider the standoff at the four-way stop a gift from our terrified pint-sized furry ancestors. Welcome to the bizarre science behind altruism.

For our little genetic predecessors like the mice, it's always been key to get in a good group. Publicly demonstrating your selflessness—letting everyone else see you'll wait until the end of time for your turn—is a powerful signal that you're ready to cooperate, a way of building a good reputation. Being the most magnanimous mouse buys the goodwill of the group, which comes in handy later. Your actions now determine how you'll be treated in the future. If you aggressively steal someone's turn at the stop, they might give you the finger and tailgate you. If you let someone go first, someone similar might let you merge tomorrow. This sort of direct or indirect reciprocity—a reward or punishment for prior behavior—takes complex thinking. Until some recent experiments, scientists assumed only humans had enough brainpower.

Rodents are essentially teeth and genitals. A mouse can't sit around and think, "Herbert is really great. Next time a hawk comes by, I'll squeak to warn him." Keeping track of many individuals is beyond such a tiny brain. Recognizing the helpfulness of a given group is a much easier task; all it takes is remembering how another last treated you. In a helpful community, we're more likely to contribute. We simply leave when repeatedly treated badly. (Welcome, Southern Californians.) Scientists have found this sort of behavior in animals. A rat helped by another rat is more likely to help a third rat get food—even if all the rats are strangers to one another. Chimpanzees who sense they are in an agreeable group will help another get an out-of-reach object or food—so will an 18-month-old human.

Increasing evidence shows that even simple mammals can feel empathy for the familiar, something also assumed to be limited to adult humans. Mice become agitated if their cagemates are harmed, but stay calm when unknown mice are in pain. (Mice will also eat their cagemates; there are limits.) Empathy for those in our circle seems ground into us. We can only harm another without feeling their pain if we cast them out of our group.

Long attributed to social conditioning, civilization, or religion, our ability to empathize and act altruistically toward others appears in part to be an inherent trait of mammals. Evolution has noted the cost of ruthlessness; cooperation is powerful enough to be written into our genes. Still, when it's your turn at the four-way stop, do the right thing: go.