The 7 new rules of financial security

Old thinking: The longer your time horizon, the more stocks you should own.
New rule: Time isn't everything. You must also consider your earnings potential.

It's one of the basic rules of thumb: The more years you have to recoup losses, the more aggressive you can be. Unfortunately, the math isn't so clear-cut.

Here's a better way to think about how aggressive your portfolio should be: Imagine that it includes not only stocks and bonds but also your human capital, meaning your ability to earn income by working. The safer it is, the more chances you can afford to take with your other assets - that is, your portfolio.

This doesn't mean that time no longer matters. As you age, the value of your human capital declines, and you'll need to secure more of your savings. So the conventional advice to hold a lot in stocks when you are young and gradually trim back can still make sense.

But not for everyone. The nature of your career may make your human capital more bond-like or more stock-like, says finance professor Moshe Milevsky of York University in Toronto. Tenured professors like Milevsky have human capital that resembles a triple-A-rated bond, especially when they have a solid pension plan. Those lucky souls can dive aggressively into stocks and even stay there as they approach retirement, he says. The human capital of a commission-based mortgage broker, on the other hand, is pretty clearly a stock - and it's not a blue chip. That person should own a fair amount of bonds, even when young.

What to do: Assess your human capital. A typical worker's income is about 70% like a bond and 30% like a stock, says Thomas Idzorek, chief investment officer for Ibbotson Associates. Use that as your baseline and then think about how long you'll be working, the stability of your current job, and your ability to change careers if you have to. You've probably realized in the past few months that your human capital is not as secure as you once thought. If you've been an aggressive investor, that alone may be a reason to shift more of your assets to safer ground.

NEXT: Rule No. 4: Borrowing