The way some people talk, you'd think that a flat tax system -- in which everyone pays at the same rate regardless of income -- would make citizens feel better than more progressive taxation, where wealthier people are taxed at higher rates. Indeed, the U.S. has been diminishing progressivity of its tax structure for decades. But a new study comparing 54 nations found that flattening the tax risks flattening social wellbeing as well. "The more progressive the tax policy is, the happier the citizens are," says University of Virginia psychologist Shigehiro Oishi, summarizing the findings, which will be published in...