Should you just buy stocks until you die?

Tina stands for “there is no alternative." Years ago, when interest rates were near zero and investors were acting as if stocks were the only choice, market strategist Jason Trennert of Strategas Research Partners popularized the term “the Tina market."

The latest argument for Tina comes from a newly updated analysis by a team of researchers who find that retirement savers shouldn’t own any bonds at all.

Instead, according to this line of reasoning, investors should keep one-third of their savings in U.S. stocks and two-thirds in international stocks. The researchers advocate that you should hold all stocks, all the time—not only when you’re young and saving for retirement, but even after you retire, for as long as you live.

If only stocks were a sure thing. In financial markets, nothing ever is. ​You can’t just take an analysis of the past, no matter how careful it is, and assume you can extrapolate it into the future.

To reach their conclusion, Scott Cederburg, a finance professor at the University of Arizona, and his colleagues Aizhan Anarkulova at Emory University and Michael O’Doherty of the University of Missouri, analyzed stock and bond returns from 39 countries from 1890 through 2023, taking the longest available period during which each was classified as a developed market. That’s up to 134 years for such markets as the U.S. and the U.K., and as short as four years for Colombia.

Their central finding, across the decades and around the globe: Bonds have historically tended to go up and down in sync with stocks over long periods, making them poor diversifiers—while offering low returns, to boot.

For that full sample, after inflation, Cederburg and his colleagues determined that bonds earned 0.95% annually—far below the 7.74% on U.S. stocks and 7.03% on international stocks.

That means that most people who save for retirement in target-date funds that hold a gradually tapering mix of stocks, bonds and cash could come up short, Cederburg and his co-authors say.

This message comes at a time when investors are already taking Tina to the limit.

At the S&P 500’s record close of 6890.89 on Tuesday, U.S. stocks were valued at 40.5 times their long-term earnings, adjusted for inflation. According to data from Nobel prize-winning economist Robert Shiller of Yale University, that’s the highest ratio in a quarter century—since the bursting of the dot-com bubble in 2000.

“With valuations where they are, this may just be a tougher investment environment for current retirement savers," Cederburg tells me.

“We’re not trying to pretend that putting all your retirement money into stocks isn’t risky," he adds. “It’s an incredibly risky proposition."

In fact, in earlier research, Cederburg and his colleagues looked at more than three dozen global markets back as far as 1841. Over all possible 30-year horizons, investors in those markets who held a domestic all-stock portfolio underperformed inflation 12% of the time.

What about in the U.S.? Edward McQuarrie is an emeritus business professor at Santa Clara University who has studied long-term asset returns back to the 18th century. He found that U.S. stocks outperformed inflation in every 30-year period.

However, they did underperform bonds in 25% of all 30-year stretches, although many of those periods overlapped and most—but not all—were in the 19th century.

All this should point you toward an inescapable but often overlooked fact: Stocks are likely to be the best-performing asset, but even a high likelihood isn’t the same thing as certainty. The odds are in your favor, but you will need luck on your side. We all will.

Consider two hypothetical investors, each with $1 million invested in the S&P 500. They both withdraw 4% a year, in equal monthly installments, for the next 20 years.

One retires on Dec. 31, 1999, right before the bear market of 2000-02; the other on Dec. 31, 2002, shortly after that nightmare ended and a new bull market began.

The first investor would have a little more than $890,000 left after 20 years, calculates financial planner Allan Roth of Wealth Logic in Colorado Springs, Colo.

The second investor would have more than $4 million. (These hypothetical results don’t count fees, taxes and inflation.)

If you happen to retire into a bull market, you could end up with more money every year even after you withdraw to support your spending. If you have the bad luck to retire into a bear market, you could end up running out of money.

That’s why I still own some bonds (in my case, inflation-protected Treasurys), and I think you should, too. The historical odds, and current government policy, are against them. But stocks also are far from a sure thing.

Tina feels friendly right now as stocks flirt with record highs. But over a lifetime, as I’ve written before, investing as if there is no alternative to stocks will demand that you have the patience of a tortoise and the emotions of a stone.

Here’s the ultimate trouble with Tina. Let’s say the odds that stocks will outperform bonds in the future—if, but only if, the future resembles the past—are something like five out of six.

As investing author William Bernstein points out, “That is also how often you win at Russian roulette."

Write to Jason Zweig at intelligentinvestor@wsj.com