Investing Rules of Thumb: Why They Don't Always Work

Q. I've always heard that 100 minus your age gives you the percentage of your retirement portfolio that you should invest in stocks. Do you recommend that rule of thumb as a good way to invest? -- John Noel

A. There's no question that rules of thumb -- or "heuristics," as behavioral economists call them -- can simplify activities many people find confusing, such as investing. Easy-to-follow rules can even sometimes produce better results than more sophisticated methods.

For example, researchers found that small business owners who were taught basic rule-of-thumb techniques for estimating profits and calculating revenues improved results more than those who were trained in the fundamentals of traditional financial accounting (which may come as no surprise to anyone who's struggled through an accounting course).

Still, I'd be wary about relying exclusively on rules of thumb when it comes to investing, or for that matter, any other aspects of retirement planning, including the 70% rule for estimating how much income you'll need in retirement and the 4% rule for gauging the amount you can safely withdraw from your nest egg after retiring.

One reason you need to be cautious about applying a rule of thumb is that many times there's no real consensus about what the standard is. For example, when I arrived at Money Magazine nearly 30 years ago as a (relatively) fresh-faced writer who still had a full head of hair, 100%-minus-your age was the widely accepted gauge for determining how much stock one should have in a retirement portfolio.

But as investors became more enamored of stocks in the bull market of the late 1980s, you began to see references to a 110-minus-your-age benchmark. And by the time the "Stocks for the Long Run" culture really began to dominate, 120 minus your age was being touted as a more appropriate standard.

Related: Six secrets to a dream retirement

So if you were, say, 30 years old, you could end up with anywhere from 70% of your savings in stocks (100% minus 30) to 90% (120% minus 30), depending on which version of the rule you applied.

But even aside from the question of which benchmark is the accepted one, there's the even more important issue of whether you should be basing your retirement investing strategy on any rule of thumb. After all, by its nature a rule of thumb is a metric that's meant to apply to the average person or typical situation.

You're not an average, though, and your finances may not be typical. You are you, a specific person who has distinct financial needs and preferences.

For example, if you're age 65 and ready to retire, applying the 100-minus-your-age standard would give you a stock stake of 35% of your savings. That may be perfectly fine for many 65-year-olds. But if you have a relatively modest nest egg and you're likely to bail out of stocks if the Dow drops 20%, then investing 35% in equities could be too racy for you.

If, on the other hand, you've got a huge pot of savings that can continue to throw off sufficient income even if the market tanks, or if Social Security and a pension cover enough of your expenses that large fluctuations in your retirement balances don't faze you, then it may make sense for you to boost your stock holdings well beyond 35%.

So what do I propose instead of going with a rule of thumb?

Well, when investing for retirement you want to create a stock-bond allocation that can get you the returns you need without subjecting yourself to a level of risk that you can't handle.

One way to arrive at an acceptable tradeoff between those two aims is to go to a good retirement calculator, plug in your financial information and test run a variety of stock-bond mixes. That will give you an idea of how your chances of achieving a secure retirement may change as you increase or decrease your exposure to stocks.

Related: Social Security's role in your portfolio

Once you've arrived at a blend of stocks and bonds that seems appropriate, you can then go to Morningstar's Asset Allocator tool to see how far that mix might drop over the course of three months if the market tanks. You could also do a quick back-of-the-envelope calculation to determine how large a loss a given blend might have sustained in severe downturns in the past.

From late 2007 to early 2009, for example, stocks lost roughly 50%, while the broad bond market gained about 15%. So an 80-20 mix of stocks and bonds would have sustained a loss of 37% vs. a setback of only 17.5% for a portfolio split equally between stocks and bonds.

Of course, there's no assurance that the future will unfold exactly like the past. It probably won't. But going through the exercises I just described can at least give you a sense of what the possibilities are for different allocations of stocks and bonds -- and offer a much better guide for investing your retirement savings than any rule of thumb.