U.S. millionaires population expanded by 8% in 2010

``What recession? The millionaire population jumped in the U.S. by 8% last year, fueled by the stock market recovery, according to an industry report on Wednesday. The number of U.S. households worth at least $1 million rose to 8.4 million in 2010, compared to 7.8 million the prior year, according to a report by Spectrem Group... But [they] added that many millionaires are still operating under a cloud of caution... "Our ongoing polling and research indicates that investors remain unconvinced that we are back on solid ground."''