Confidence among high-net-worth investors tumbled in August, according to a monthly survey by Spectrem Group.

The Spectrem Affluent Investor Confidence Index declined four points to a “mildly bearish” negative-20, and the Spectrem Millionaire Investor Confidence Index declined 11 points to negative-18.

This decline in the millionaire index was its largest drop since June 2009, when it fell 18 points to -20. This decline returns the index to “mildly bearish” territory after 12 straight months in neutral, according to the Chicago-based research firm.

The Spectrem affluent index, which measures the investment confidence and outlook of households with $500,000 or more in investable assets, declined for the third consecutive month.

"The millionaires' decline is particularly troubling since it suggests millionaires, typically more sophisticated than the broader affluent population, are reverting to a bearish frame of mind," said George H. Walper, Jr., Spectrem’s president.

In response to an open-ended question about the news story most affecting their economic outlook, affluent investors in August cited: the political environment (18%); unemployment (16%); the economy (11%); stock market conditions (8%); health care reform (5%); the oil spill (2%); and European problems (1%). Those citing the political environment rose from 13% in May 2010, the last time this question was asked, and those citing unemployment rose from 6%.

Millionaires were more focused on the political environment (25%) than the affluent, but less focused on unemployment (13%).

The Spectrem Affluent Investor Confidence Index is based on 250 monthly interviews with households with $500,000 or more in investable assets. The Spectrem Millionaire Investor Confidence Index is based on a subset of the overall group.