With less than a month to go before Iowa's Republican primary caucus, Donald Trump remains atop the presidential candidate polls. His popularity appears unblemished despite brash statements, personal insults thrown at his opponents and rampant speculation over his perceived narcissistic tendencies. Trump surely displays enormous self-regard; in June, for example, he boasted that he'd be "the greatest jobs president that God has ever created." In September, he promised, "We will have so much winning if I get elected that you may get bored with winning." Egoism is a trait Trump seems proud of, in fact. In 2013, he wrote in...