Royal families are rarer than they used to be on the global stage. In 1870, monarchs ruled eight of the world's 10 largest economies, and six of them had direct influence over their kingdom's policies. Today, only five of the world's 15 largest economies still have monarchs — Emperor Akihito of Japan, Queen Elizabeth II of the United Kingdom (plus Canada and Australia) and King Felipe VI of Spain — none of whom have any real power. Moreover, each of these three monarchs and the royal families they represent are facing challenges in the coming years that will further test their suitability for the modern world....