Betting Your Retirement: Stocks vs. bonds

Q. I'm 35 years old and a diligent saver. I'm torn, however, about whether someone my age should own bonds. If the stock market has never had a negative 10-year span, shouldn't I invest 100% of my savings in stocks and keep it there until I'm within a decade or so retiring? -- Mark N., Austin, Texas

A. With stocks on a roll in recent months, investors who were shunning equities just last year are feeling positively ebullient about them now. But before you plow all your retirement savings into stocks, I have two words of caution for you: Downton Abbey.

What, you may ask, could a soap opera set in England in the early 20th century possibly have to do with your retirement planning? Rather a lot, actually.

As viewers of Downton's third-season premiere will recall, Lord Grantham's lawyer informed him that the Canadian railway into which he had sunk the bulk of his wife Cora's fortune stood on the verge of bankruptcy, jeopardizing the financial health of his estate.

Upon hearing this news, the Earl of Grantham thundered, "Every forecast was certain. Rail shares were bound to make a fortune...It wasn't just me. Everyone said we couldn't lose!"

I'm not saying that putting 100% of your retirement savings in stocks is as rash as Lord Grantham's decision to invest so heavily in just one stock. But tying your retirement prospects to the performance of a single asset class -- and a very volatile one at that -- wouldn't exactly qualify as a prudent move either.

One reason is that while losses in stocks over long stretches are rare, they do happen, despite your supposition to the contrary.

Related: Can retirees beat inflation?

For the 10 years from 1999 through 2008, for example, large-company stocks lost 13% of their value. There have also been a handful of 10-year and even 20-year spans during which stocks had positive returns yet still lagged bonds.

So while I think it's reasonable to expect stocks to outperform bonds over the long term in most instances, there's enough variation so that it pays to hedge. The investing world is too uncertain for all-or-nothing bets.

Besides, you shouldn't be basing your investing strategy solely on expected returns. You've also got to consider your risk tolerance, or how you'll likely react if the market tanks (as it inevitably will many times between now and when you retire).

It's one thing to say you think you should be 100% in stocks because you believe that over long periods equities will rack up the highest gains. It's quite another to stick to that strategy when a plunging stock market zaps the value of your 401(k) by 50% or more, and it's anyone's guess how long it will take for your account to bounce back.

My suggestion: Go to a good online retirement calculator and plug in such information as your age, when you think you might retire, how much you already have tucked away in retirement accounts and how much you plan to save going forward. Then run scenarios with different investing strategies --100% stocks, 90% stock/10% bonds, 80%/20%, etc. -- and see how the probability of achieving a secure retirement changes as you reduce the percentage of your savings you devote to stocks.

Related: 4 ways the market could really surprise you

If you're really the diligent saver you say you are, you may find that you don't have to resort to a high-octane stock mix to have a good shot at a comfortable retirement. You may be able to get by with a more conservative stocks-bonds blend. You might also find that after a certain threshold -- say, 70% or 80% stocks -- adding more equities doesn't improve the odds very much at all.

Of course, it's also true that the more you invest in stocks, the better you'll do if the markets do well. But that upside isn't a given, and even if it materializes it can come with some frightening spills and chills along the way. So it can pay to sacrifice some upside in return for a less jarring ride.

As you near and enter retirement, you'll probably want to gradually scale back the amount you devote to stocks. Research shows that risk tolerance tends to decrease with age. Besides, the consequences of aggressive investing can be more dire later in life. Once you're retired, you no longer have a chance to make up for investment losses by saving more.

That means a big hit to your nest egg could result in you running out of money before you run out of time. For guidance on how you might shift from stocks to bonds as you age, check out this illustration of a target-date retirement fund "glide path."

But whatever you do, don't go all stocks on the mistaken notion that equities are a definite win as long as you remain invested in them at least 10 years. If you do, you may later find, as Lord Grantham did, that no forecast, regardless of how certain it may seem, can ever guarantee that you can't lose.