Optimism about U.S. stocks increased by the most since July as economic reports emboldened investors following three straight weekly losses in the Standard & Poor's 500 Index, according to a survey from the American Association of Individual Investors.

The proportion of investors who anticipate a gain in the next six months jumped to 30.8 percent in the week ended Sept. 1, the biggest rise since the period that ended July 15.

That compares with the 17-month low of 20.7 percent the previous week and the historical average of 39 percent, according to the Chicago-based company, which has tracked individual investors' projections since 1987.

Bearish sentiment, or expectations that stock prices will fall over the next six months, fell 7.3 percentage points to 42.2 percent, according to AAII.

Bullishness on U.S. stocks last week fell to the lowest level since March 2009, when the S&P 500 began an 80 percent rally. Investors had lost faith in equities as the U.S. government said the economy grew less than previously forecast, home sales plunged by the most in a decade and jobless claims rose to the highest since November.

"Sentiment was so bad," said Richard Campagna, chairman of 300 North Capital LLC, which manages $600 million in Pasadena, Calif.

"The level of bearishness and fear in the market a week ago was so high that anything that makes it appear that the economy's not getting worse is a wild positive."