Sociologist Robert Wuthnow of Princeton University has explored how it is that people make everyday ethical decisions. Many people, he found, perform deeds of compassion, service, and mercy because at some point in their past someone acted with compassion toward them. He wrote, "The caring we receive may touch us so deeply that we feel especially gratified when we are able to pass it on to someone else."

He tells the story of Jack Casey, who was employed as an emergency worker on an ambulance rescue squad. When Jack was a child, he had oral surgery. Five teeth were to be pulled under general anesthetic, and Jack was fearful. What he remembers most, though,…