Good neighborhoods and bad neighborhoods differ in many ways, but the most important may be the trust gap. You can tell a place is nice to live in because children leave their bikes on front lawns, and front doors are often unlocked.

Happy countries also differ from unhappy countries in many ways, but trust might be the most important determinant. Countries such as Finland and Norway, where folks just implicitly trust their neighbors and strangers, are the countries with the happiest people.