How often have you heard some Wall Street shill talking about all the cash on the sidelines waiting to come back into the market? Or some pundit worrying that too many investors have rushed into stocks, signaling an imminent sell-off?

Well, now, we can safely ignore those claims and others like them. An authoritative new study published recently in the Financial Analysts Journal shows that all investors — individuals and institutions alike — are keeping the lowest percentage of their portfolios in stock in over half a century.

According to three Dutch researchers — Ronald Doeswijk, Trevin Lam and Laurens Swinkels — investors held only 37.7% of the $90.6 trillion in global investable assets in stocks in 2012, the most recent year their data covered.

That and the 37.1% they invested in equities in 2011 were the lowest exposure to equities investors have had since 1959, when records were first kept. It’s considerably below what they held even in the late 1970s, before the Reagan-era bull market began, and in the early 2000s after the dot.com bubble burst.

In fact, there may be cyclical and structural reasons for this shift, according to Lam, senior analyst in quantitative research at Rabobank, based in the Netherlands.

“I do think that the changes in the global multi-asset market portfolio are cyclical,” he told me in an email. “There are periods in which the weight of equities increases at the expense of bonds … [but after the dot.com bust] the weight of bonds rose quickly at the expense of equities.”

Equity ownership peaked at around 64% of the total global market portfolio in 1968 and again in 1999, near the top of two great secular bull markets.

Yet it never exceeded 53% during the mid-2000s cyclical bull market. To me, 2011-2012’s low numbers show that, though the S&P 500 and other indices are hitting all-time highs, investor confidence still hasn’t recovered from the dot.com bubble and the financial crisis.

As I wrote here late last year, surveys showed a steep decline in the percentage of Americans who said they owned stocks or stock mutual funds.

But there are big structural changes as well, Lam told me.

First is the alternative asset revolution, pioneered by David Swensen at Yale University and Jack Meyer at Harvard.

University endowments and other asset managers have drastically reduced their holdings in traditional stocks and bonds, investing instead in such formerly exotic asset classes as private equity, hedge funds and timber land.

According to the study, the market capitalization of assets other than stocks, government bonds and investment-grade corporate credits (for example, real estate, high-yield bonds and inflation-linked securities) rose to 15.6% of total global invested assets in 2012 from 6.2% in 1990.

Hedge funds now manage $2.1 trillion, up eightfold from 1996, and private equity firms have more than $3 trillion in assets under management.

Also, pension funds and individual investors have gone through a massive “de-risking” over the past decade. That reflects not only fear of stocks’ volatility but also big demographic changes.

An aging population worldwide — U.S. baby boomers, of course, but also Europeans, Japanese and even Chinese — is growing more conservative as it approaches retirement.

That’s one reason U.S. individual investors went on a bond-buying spree from 2008 to 2012, pouring $1 trillion into bond funds while yanking $547.9 billion out of U.S. stock mutual funds.

The study pointed out that government and investment-grade bonds comprised 57% of the global portfolio in 2012, a 23.5-percentage-point increase since 1999.

(Lam couldn’t tell me how much the global percentage of stock holdings may have increased in 2013, when the S&P 500 rose 30%-plus, but my own back-of-the-envelope estimate puts the current stock allocation in the low- to mid-40s.)

So what does this all mean?

First, even after five years of a spectacular bull market that has driven the S&P 500 up over 180%, irrational exuberance is nowhere in sight.

Far from inducing “free money” euphoria, the Federal Reserve’s extraordinary bond-buying program (quantitative easing, or QE) has instead provoked deep skepticism from many investors — and not just the doom-and-gloomers I’ve written about over the past couple of weeks.

In fact, that doom-and-gloom thinking itself may be part of the “wall of worry” keeping even more cash out of the market. By that reasoning, stocks could be headed a lot higher before this bull market ends.

But I’m concerned that structural changes — institutions’ push into alternative assets and the aging population’s big shift into bonds — mean that this bull market will end long before world stock holdings go back to their previous peaks at more than 60% of total assets.

Which raises one final question: Why is the financial-services industry still peddling the shopworn advice that individuals should be putting 60% to 70% of their assets into stocks on the verge of retirement, especially when even big institutions have drastically reduced their equity holdings?

Please let me know if you get a good answer, because I haven’t heard one yet.

Howard R. Gold is a MarketWatch columnist and founder and editor of GoldenEgg Investing, which offers simple, low-cost, low-risk retirement investing plans. Follow him on Twitter @howardrgold.

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