It's not the risk of low interest rates or reckless fiscal policy. Nor is it the shaky securitization market or unsustainable GDP growth.

Rather, it's demographics.

Boomers bust the U.S.
People buy stocks to make money. And while investors may love, hate, and brag about their holdings for decades, at some point, it comes down to cashing in. After all, you can't use a share of Berkshire Hathaway to buy a new set of golf clubs.

The problem, however, is when "people" is the Baby Boomers, and "at some point," is, well, now.

The largest generation in U.S. history, the close to 80 million Baby Boomers are currently marching into retirement age. And with more than 50% of the value of U.S. financial market assets in their hands, according to one estimate, the pace at which boomers decide to liquidate retirement accounts could determine the overall direction of U.S. stock prices.

If that sounds dramatic, consider that equity prices, as money manger Ken Fisher has often noted, boil down not to P/E ratios or prevailing interest rates, but to market supply and investor demand.

And so I ask, who's going to step up to put a bid underneath boomers' stock sales?

U.S. birth and workforce participation rates are in decline, which creates a stiff macro headwind. Moreover, the recent financial crisis may have turned off a generation of younger stock investors. Finally, what assets have recently exited money market funds have largely sought the perceived safety of bonds. Twice bitten in the past decade, U.S. investors may remain shy.

Ultimately, while popular domestic names such as eBay (Nasdaq: EBAY), Wal-Mart (NYSE: WMT), and CVS Caremark (NYSE: CVS) may look cheap -- given that all three are trading at a discount to their five-year average price-to-earnings ratios -- years of marketwide selling pressure could see current valuations become 2015's trailing five-year high.

But maybe not
In 2009, the Congressional Budget Office (CBO) produced a research report on this very topic.

Its findings were notable on two accounts. One, financial assets are concentrated among the wealthiest boomers, and historically, the richest Americans rarely spend down financial assets during retirement. Two, the report argues that rising international demand, particularly from Chinese and Indian investors, is likely to prop up U.S. stock and bond prices.

Furthermore, as noted in separate studies, there's a good chance that many boomers will simply end up working longer, thus moderating asset drawdowns.

Nonetheless, I remain cautious. Current and future health-care costs, which aren't captured in previous empirical studies, could force boomers to sell assets more quickly and in larger chunks than expected.

Finally, while net purchases of U.S. equities by foreign investors have generally risen across the past decades, the questionable state of the U.S. economy may soon stamp out that trend.

Accordingly, I recommend that investors not only keep an eye on U.S. equity flows, but also make sure that their portfolios are well-exposed to companies domiciled in nations whose middle class is growing. On that note, CNOOC (NYSE: CEO), MercadoLibre (Nasdaq: MELI), FEMSA (NYSE: FMX), and Tata Motors (NYSE: TTM) all fit the bill.

Berkshire Hathaway and Wal-Mart Stores are Motley Fool Inside Value recommendations. MercadoLibre is a Motley Fool Rule Breakers pick. Berkshire Hathaway and eBay are Motley Fool Stock Advisor recommendations. CNOOC and Fomento Económico Mexicano are Motley Fool Global Gains picks. Motley Fool Options has recommended a bull call spread position on eBay. The Fool owns shares of Berkshire Hathaway. Try any of our Foolish newsletters, free for 30 days.

Fool contributor Mike Pienciak owns shares of CVS Caremark but holds no financial interest in any other company mentioned in this article. The Fool has a disclosure policy.