Following a difficult August, confidence among millionaire investors saw its largest increase in more than a year.

According to Spectrem Group’s Millionaire Investor Confidence Index, confidence rose 12 points in September to negative-six. This advance, the largest for the millionaire index since May 2009, offsets an 11-point decline in August and returned the index to neutral territory.

Meanwhile, the Spectrem Affluent Investor Confidence Index, which measures the investment confidence and outlook of households with $500,000 or more in investable assets, rose four points to negative-16, a mildly bearish reading.

George H. Walper, Spectrem’s president, said “Nevertheless, despite these September advances the nation's wealthiest investors remain extremely cautious, concerned with both the political climate and the economy.”

In response to an open-ended question about the most serious threat to achieving their household financial goals, affluent investors in September cited: the political climate (27%); the economy (26%); unemployment (7%); market conditions (5%), health-related issues (2%); and inflation (1%). Those citing the political climate rose from 26% in June 2010, the last time this question was asked, and those citing the economy rose from 24%.

Millionaires were more focused on both the economy (31%) and the political climate (28%) than the affluent.

The Spectrem Affluent Investor Confidence Index, which was released Wednesday, is based on 250 monthly interviews with the financial decision-makers in households. The Spectrem Millionaire Investor Confidence Index is based on a subset of overall survey.