Brian Hare and his wife Vanessa Woods offer a new theory called "self-domestication" which suggests that we have succeeded not because we were the smartest or strongest but because we are the friendliest. Since Charles Darwin wrote about "evolutionary fitness," scientists have confused fitness with strength, tactical brilliance, and aggression. But what helped us innovate where other primates did not is our knack for coordinating with and listening to others. We can find common cause and identity with both neighbors and strangers if we see them as "one of us." This ability makes us geniuses at cooperation and innovation and is responsible for all the glories of culture and technology in human history. But this gift for friendliness comes at cost. If we perceive that someone is not "one of us," we are capable of unplugging them from our mental network. Where there would have been empathy and compassion, there is nothing, making us both the most tolerant and the most merciless species on the planet. To counteract the rise of tribalism in all aspects of modern life, Hare and Woods argue, we need to expand our empathy and friendliness to include people who aren't obviously like ourselves.