Affluent investors' confidence increased last month for the second consecutive month, according to Spectrem Group.

The Chicago research company said Wednesday that its Spectrem Affluent Investor Index, which measures the investment outlook of households with $500,000 or more in investable assets, increased 9 points, to negative-21, remaining in mildly bearish territory. It rose 8 points in March. The index is at its highest level since August 2008, when it stood at negative-13.

The Spectrem Millionaire Investor Index also rose for the second time in as many months, climbing 7 points in April, to a mildly bearish reading of negative-19.

Spectrem asked investors in April to pick the one factor most affecting their investments. The most common answers among affluent investors were stock market conditions (30%), the economic environment (18%), retirement (8%), household income (3%) and household cash flow (2%). The most common answers among millionaires were stock market conditions (31%) and the economic environment (18%).