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Who buys stock? This answer may surprise you

Ulrich Baumgarten/Contributor | Ulrich Baumgarten | Getty Images

It's not surprising that the biggest buyers of U.S. stocks are U.S. companies through buybacks, but the sheer size of their purchases may be.

According to Citigroup, companies in the S&P 500, over the past 10 years, bought $4 trillion of their own stock.

GE became the latest Friday when it announced an eye-popping $50 billion buyback and asset sale, goosing its stock. That announcement is on par with the largest ever, Apple's $50 billion buyback announced two years ago.

Read MoreGE unveils massive restructuring, buyback

Compare those trillions to the paltry $100 billion in net inflows into all U.S. equities through ETFs and mutual funds, vehicles used by individual investors. That $100 billion, of course, was affected by outflows, and corporate buybacks are specifically one way.

Citigroup's takeaway: "The lack of US retail investor interest in stocks has been stunning and equity market tops usually consist of overly aggressive individual investor interest in the asset class. In this context, it is challenging to suggest that the S&P 500 is due a major correction barring exogenous shocks."

Meaning, if the class that historically has bought last and panics first isn't a big buyer, will stocks still behave the same way? Their fate, it seems, is now in the hands of CFOs.