People like to simplify. We like easy-to-follow guidelines and easy-to-implement instructions. That’s why rules of thumb are so popular. The two investing rules of thumb that I see quoted most frequently are to:
- Set your bond allocation (as a percentage of your portfolio) equal to your age, and
- Save and invest 10% of your income.
I really like the first one. I think it’s quite helpful. The second one, however, is a mess.
“Age in Bonds”
Why is the “age in bonds” rule helpful? Because asset allocation comes down to just two variables:
- Your expected holding period, and
- Your tolerance for volatility.
The rule accounts for one of those variables (holding period). So all you need to do is adjust the rule’s prescription based on your volatility tolerance. No problem.
“Invest 10% of Your Income”
In contrast, the question of how much of your income to invest each year involves many more variables than the question of asset allocation. Specifically, it depends upon:
- The age at which you started investing,
- The age at which you plan to retire,
- Whether or not you plan to continue working part-time,
- Whether or not you expect Social Security to pay out at the rate it’s currently promising,
- Whether or not you’ll have any pension income,
- Whether or not you’ll own your home, and
- What you plan to do in retirement. (Do you plan to travel the world? Or mostly just hang out in your hometown with your grandkids?)
The “invest 10% of your income” rule doesn’t account for a single one of those variables! If you ask me, that’s a dangerous oversimplification.
When a Rule of Thumb Doesn’t Cut It
Sometimes there’s really no shortcut. Sometimes you actually have to do the math (or find an advisor or online calculator to do it for you).
That’s the case when determining how much you’ll need to invest each month. That’s the case when determining how much life insurance you need. That’s the case when determining whether you should form an S-corp or C-corp for your business.
You get the idea. When it comes to your finances, rules of thumb can be dangerous. Even if there is a rule that pertains to the question you’re seeking to answer, it’s best to learn the reasoning behind the rule so that you can decide for yourself whether or not you should follow it.