Despite volatile financial markets and slow economic growth, the U.S. added 300,000 new millionaires in 2015, bringing the total to a record 10.4 million, according to a new report.

The number of American households with assets of $1 million or more, not including their primary residence, increased 3 percent last year, from 10.1 million, according to Spectrem Group, a market research and consulting firm.