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For the first time since 1999, all three major U.S. stock indices -- the S&P 500, the Dow Jones Industrial Average, and the Nasdaq 100 -- all closed at record-high levels on the same day, August 11. One component continues to be missing, however, especially for those who remember the late 1990s: the atmosphere of celebration.

But maybe that's not so surprising, given that there are fewer people at the party.

According to a Gallup poll in April, only a little more than half of Americans said they had any money invested in the stock market. The 52% that did fess up to owning stocks matched the lowest level in the 19-year history of Gallup's survey.

Here's a look at the trend:

Percentage of U.S. Adults Invested in the Stock Market

Source: Gallup

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To be sure, many investors and potential and former investors remain spooked by the two vicious bear markets the past two decades, along with the psychological -- and real -- impact of the Great Recession in 2008-2009. Not exactly a confidence booster.

Still, the beat goes on -- many of us who have taken the plunge in recent years have been rewarded. That's perhaps especially true of the buy-and-hold investors among us.

Last year at this time, for instance, almost no one believed that the so-called interest rate-sensitive sectors such as utilities and REITs would outperform the market. The S&P 500 is up 9.6%. And, lo and behold, the Dow Jones Utility Average is up 17.5% year to date (not including dividends), and the Dow Jones Equity REIT Index is up 18.6%.

Investors who bailed out a year ago have little to celebrate. Investors who bailed out seven years ago have even less to celebrate.

That's not to necessarily say the market will continue to march higher over the next year, or month or even week, of course. There will be bumps along the way.

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And investing when the market keeps hitting all-time highs can be especially tricky. But let's leave the market-timing to others. As buy-and-hold investors we know better than to put ourselves in a position where we have to second-guess the market's every next move.

That's why it's important to stay diversified and to rebalance your portfolio periodically.

And for income investors, remember this: right now, there is no better source of investment income than the stock market. Some of us might not like it, as we might feel safer in Treasuries and CDs -- but those instruments, while safe, yield only a pittance.