But Keltner also found the compassionate instinct in the more instinctive and evolutionarily ancient parts of our nervous system: in the mammalian region known as the periaqueductal gray, which is located in the center of the brain, and causes mothers to nurture their young; and in an even older, deeper, and more fundamental part of the nervous system known as the vagus nerve, which connects the brain stem to the neck and torso, and is the largest and one of our most important bundles of nerves. Its long been known that the vagus nerve is connected to digestion, sex, and breathingto the mechanics of being alive. But in several replicated studies, Keltner discovered another of its purposes: When we witness suffering, our vagus nerve makes us care. If you see a photo of a man wincing in pain, or a child weeping for her dying grandmother, your vagus nerve will fire.

Bittersweet by Susan Cain