An oft-repeated bit of wisdom on Wall Street is that markets love to climb a wall of worry. That edifice suddenly looks wobbly as contrarian measures of investor optimism rise.

The weekly Investors Intelligence survey, a poll of stock newsletter writers that has served as one of the best such gauges, exhibits significant complacency. The latest figures show the lowest level of bearishness since the market peak in October 2007 and the highest bullishness since January 2008. Another survey of individual investors' optimism is near its highest level in over a year. Coming just five months after widespread fear of another Great Depression, this seems premature.

Investors are putting their money where their mouths are, too. Inflows into long-term mutual funds were positive for the 20th successive week, while short interest in shares making up the S&P 500 recently fell to the lowest level since February, according to Bloomberg. Another measure, the Chicago Board Options Exchange equity put/call ratio, is near multi-month lows. The ratio's 21-day moving average hit a recent peak near the market's March nadir.