Another 1.1 million Americans officially joined the millionaire's club in 2013. That brings the total amount of millionaires in the U.S. to 7.1 million, the most in any developed country. Overall, wealth in the U.S. grew by 16.3% in the last year. And it's picking up steam. Households worth $1 million or more are projected to grow by at least 7.7% a year through 2018. The richest are getting richer, too. Households with assets of $100 million or more held $8.4 trillion in wealth in the U.S. in 2013, which is a gain of nearly 20% over the year before. The primary catalyst for the wealth creation is booming equity markets as well as expanding prosperity in developing economies.