One study calls conservatives indecisive and inhibited. Another says liberals see more crises in the world and are pessimistic. The sweet spot might be in the middle. In our bitterly polarized society, liberals and conservatives not only argue over the right approaches for public policy but also do battle over which ideology more reliably produces private happiness. The left recently embraced an academic study that says right-wingers were whiny kids who grew into insecure adults, while conservatives trumpeted surveys showing that it's self-identified liberals who count as self-pitying and pessimistic. The latest salvo in the "Happiness Wars" came from the...