Affluent investors' confidence increased in May for the third consecutive month, according to a survey by Spectrem Group.

The Chicago research company said its Spectrem Affluent Investor Index, which uses interviews with 250 investors to measure the investment outlook of households with $500,000 or more of investable assets, rose 9 points, to a "mildly bearish" reading.

It was the index's highest level since February 2008.

The Spectrem Millionaire Investor Index, gauging a subset of the 250 investors, rose 17 points.

That was the millionaire index's highest level since December 2007.

"Millionaires' investment optimism surged in May, recording the largest monthly increase since we began tracking it in February 2004," George H. Walper Jr., Spectrem's president, said in a press release last week.

In response to an open-ended question about the factor most affecting their investment outlook, affluent investors in May cited: the economy (25%); the political environment (25%); stock market conditions (6%); housing and real estate (3%); and rising oil and gas prices (1%).

Twenty-seven percent of the millionaires Spectrem surveyed said the political environment was their chief concern, and 24% cited the economy.

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