As The Wall Street Journal reports, investors have poured money into stocks through mutual funds and exchange-traded funds in 2017, with global equity funds posting record net inflows in the week ended March 1 based on data going back to 2000, according to fund tracker EPFR Global. Inflows continued the following week, even as the rally slowed.

"People went toe in the water, knee in the water and now many are probably above the waist for the first time," said JJ Kinahan, chief market strategist at TD Ameritrade.

That brings individual investors increasingly in line with Wall Street professionals. A February survey of fund managers by Bank of America Merrill Lynch found optimism about the global economy improving while investors were holding above-average levels of cash, leaving room for them to drive stocks still higher.

Bullishness among Wall Street newsletter writers reached 63.1% - the highest level since 1987 - a week ago in a survey by Investors Intelligence, before falling to 57.7% this past week.