When other people whisper, we tend to lower our own voices. When we’re around an older person, we’re prone to walking more slowly. If we’re seated on an airplane next to someone with a pronounced accent, many of us unconsciously begin to imitate it. I can remember visiting in Moscow back in the cold war days, and being struck that there were no colors anywhere in the city. The sky was gray, the houses were gray, the cars were gray, and the faces of the people I passed on the streets were unrelentingly pale. But what really stood out for me the most was that virtually no one was smiling. As I walked along, I’d give the other pedestrians in Mos cow a quick smile of acknowledgment, and time and again, I’d get back nothing in return. At first, this was amusing (because it was so strange), but after about an hour, I started to realize the effect it was having on me. My mood changed. I wasn’t feeling my usual lighthearted self. I’d quit smiling. I felt borderline grim. I felt gray. Physically and psychologically, without even realizing it, I’d been mirroring everyone else around me.