Parenting two small children has afforded me a terrific opportunity to watch them and learn about human nature. I have noticed that my kids are happiest in their time together when they play without regard for who gets what, who has to clean up what, etc. When they both enjoy themselves and contribute 100 percent without concern for what the other is or isn’t doing, they tend to be much happier. However, when they begin to bicker over who has to do more or what belongs to who, the result is misery. I think this tendency can be attributed to the human inclination toward selfishness. When my kid...