In the U.S., it might feel like a Golden Age for millionaires, with taxes low and stock prices rising. But in the last year, while the ranks of the super-rich have shrunk around world, the U.S. minted nearly one million new millionaires in the last year. Today, the United States and Japan are home to about 7% of the world's population, but more than 50% of the world's millionaires.