Confidence among affluent investors increased slightly this month, according to a monthly survey.

According to Spectrem Group, a Chicago-research firm, its millionaire investor confidence index rose one point to negative-nine. This represents the index’s sixth-straight neutral reading and follows a six-point decline in December.

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 two points in January to negative-13. This mildly bearish reading follows a five-point decline last month.

"The nation’s wealthiest investors saw their investment confidence improve modestly in January, as millionaires retained their neutral stance and the broader affluent population remained mildly bearish,” said George H. Walper Jr., Spectrem’s president. “With concerns about the economy and stock market still top-of-mind, these investors have begun the New Year with the same cautious mindset they demonstrated throughout 2009."

In response to an open-ended question about the one factor most affecting their investment plans, affluent investors cited: stock market conditions (22%); the economic environment (20%); retirement (8%); household cash flow (7%); the political climate (4%); and household income (4%).

Those choosing stock market conditions fell from 27% in October, the last time this question was asked, while those choosing the economic environment fell from 22%.

Millionaires expressed more concern than the affluent about both stock market conditions (27%) and the economic environment (21%).