The U.S. now has a record number of millionaires after 300,000 more people joined the club in 2015 to bring the total up to 10.4 million, reports Robert Frank of CNBC.

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

"Despite ongoing concerns about market volatility and the direction of the U.S. economy, the number of affluent American households expanded in 2015," said George Walper Jr., president of Spectrem Group.

While the millionaire population has been growing throughout the recovery, thanks to a flood of liquidity from central banks and rising markets, the strongest growth has been at the very top of the wealth ladder.

The number of American households worth $25 million or more has grown 73% since 2008, compared with growth of 54% for millionaire households.

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