William Bernstein, investment adviser and author, says the worst mistake is not knowing when to take money off the table.

NEW YORK (Money Magazine) -- William Bernstein has a gift not only for grasping the complex but for helping the rest of us get it too.

He spent the first chunk of his career as a neurologist practicing on the coast of Oregon but cut back on his work hours in 1990. A few years later he focused on a new fascination: investing. He launched an online journal (a sort of proto-blog) called efficientfrontier.com and wrote "The Intelligent Asset Allocator," the first of several books. (He has also written for MONEY.)

Now he's an investment adviser for a handful of high-net-worth clients. Bernstein's writing often explores academic financial theory, but he manages to turn it into practical, plain-English advice.

His latest obsession, resulting in the short e-book "The Ages of the Investor," is what economists call the life-cycle theory, which dictates that your asset allocation should be tied to your earnings power throughout your career.

Bernstein, 64, spoke with senior editor George Mannes; their conversation was edited.

There's a debate going on now among economists about how much exposure people should have to stocks. What made you weigh in?

It's almost like a political issue. There's a "right wing" of very smart, authoritative people who think that savers and retirees should be investing conservatively because stocks are so risky. And then there's a "left wing" of equally smart and authoritative people who believe the opposite.

I was trying to reconcile the two views. Plus, I wanted to deal with what happened in the 2008 financial crisis, which changed how people, myself included, think about risk.

How so?

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