When I was growing up, children were commonly taught: "don't talk to strangers." Strangers might be bad, we were told, so it's prudent to steer clear of them.

As it turns out, this is profoundly bad advice. Most people are honest, kind, and generous, especially when someone asks them for help. If a small child is in trouble, the smartest thing he can do is find a nice-looking stranger and talk to him.

The...