When I was a first-year medical student, my classmates and I used to go down to the hospital cafeteria between lectures to buy snacks. The women from whom we bought them at the check-out counters were all young and sullen, rarely even glancing up at their customers as they rang up purchases. Their customers, in turn, seemed equally uninterested in them. So I decided one day I was going to get them to smile each time they rang up my purchases. To do this, I decided I’d simply start smiling at them myself.

“Hello!” I started saying each time I’d approach. At first, they actually shrank from me physically. But within a few days, they started to smile back at me. And as time passed and I continued to smile and say a bright hello each time I came before them, I noticed something interesting: most of the time they were still wearing the same lifeless expressions. I’d become their “on” switch. Their focus was so concrete that to the person in line right before me they remained their same apparently disinterested, unhappy selves. But as soon as I entered the narrow band of space immediately before them, their eyes would suddenly light up and their mouths would curve into a smile. And I could tell, though we never exchanged any words other than “hello” and they knew absolutely nothing about me, that they were genuinely glad to see me.

Such, I discovered, is the power of a smile, even between strangers. In the intervening years, I’ve found myself wondering why most people don’t smile at people they don’t know. In observing my own reactions, I’ve noticed the following:

1. I’m often lost in my own thoughts, trying to solve a problem, ruminating over one I can’t, planning, or thinking about what I’m about to do. In short, I’m everywhere except where I actually am.
   
2. Even when I think to smile at passing strangers, I can’t always muster up a genuine one. It turns out, according to V.S. Ramachandran in his book A Brief Tour of Human Consciousness, we’re capable of mounting two different kinds of smiles, one genuine and the other forced, which are in fact generated in two separate parts of the brain. They look different, these two smiles, which is why we can always tell one from the other with ease. To produce a genuine smile we must genuinely feel like smiling. To smile at a stranger in a meaningful way, then, requires we muster some kind of real feeling for them—that we care about someone we don’t know, if only in a small way. Thus, for me, smiling at strangers is a small exercise in compassion. The benefit of smiling accrues to me as well as to those at whom I’m smiling, however: studies have also shown that feeling just as often follows expression. That is, when we smile, it actually makes us happier, even, it turns out, if our smile is forced.
   
3. Smiling at strangers might be taken as an invitation I don’t want to offer—for a conversation I don’t want or have time for, or for some kind of entry into my life (however small it may be) that feels invasive. We often guard our privacy intensely and prefer the barriers that exist between strangers to persist, finding ourselves reluctant to break them down even a little bit. But that attitude, I’ve found, often conceals an inability to set appropriate boundaries. If we’re in a hurry, we can simply hurry along. Or excuse ourselves. Or employ any number of socially appropriate reasons to keep a stranger at a social distance we find comfortable.

In the end, of course, I concluded that I really had no good reason not to smile at everyone. Certainly, it takes some amount of attention and energy. But in smiling at strangers, I acknowledge their humanity, and in doing that, in reminding myself of it, I promote peace. How? By bringing joy to others that’s far out of proportion to the investment required—as I learned seven years after I first started my smiling experiment. I’d finished medical school and residency, and had returned to the University of Chicago as an attending physician. One day soon after I’d arrived, I went down to buy lunch in the same cafeteria. And when I approached the check-out line, I found myself greeted by a cashier I didn’t at first even recognize who, wearing a happy, surprised smile, suddenly exclaimed in delight, “Where have you been?”