Of course, investors' circumstances also affect their willingness and ability to take risk. Investors who have accumulated substantial financial assets likely have a greater ability to take risk, but they may wish to preserve their capital and thus be less willing to assume more of it. Investors' ability to take risk is also time-horizon dependent. Recent college graduates contributing to their pension program for the first time have ample ability to take chances, given that they likely have decades to save and invest before they retire. Investors nearing retirement may have relatively less ability to take risk as they transition from accumulating financial assets to relying on them for retirement spending. None of these considerations is captured in standard risk measures.

Investors' experience, be it investing in the market or in their careers, will also influence their relationship with risk. People who have lost vast sums in the market in the past may be uneasy about taking risk in the future. Serial entrepreneurs may be more comfortable with risk than company men.

Attitudes toward risk will shift with the markets and evolve over time. Ask me now, years into a bull market, during a period of relative calm, to rate my willingness to take risk and I'll tell you that I'm happy to pour it on. Ask me nine years and two months ago and I probably would have said, "No thank you." Our attitudes toward risk are always changing. While in March 2009, I'd probably have passed on taking on more risk, the next time the market tanks I'll think back to what a tremendous buying opportunity it was and be more opportunistic (I hope).

How to Manage Risk

There are a number of ways to manage investment risk. I would put them into three buckets: asset allocation, investment selection, and behavior. Ideally, the first two should optimize for the third.

Perhaps the most effective way of managing risk is to select an appropriate mix of assets. Investors with a higher threshold for pain should favor stocks. Those who are more squeamish should lean toward bonds and cash. Striking an appropriate balance is tricky. Just because certain investors don't like the ups and downs of the stock market doesn't mean that they can afford to skip the ride if they want to meet their goals.

At this point in the market cycle it makes sense to revisit your asset allocation. Given their relative performance over recent years, odds are that many investors are heavy on stocks and light on bonds. If this is the case, it may be time to rebalance back to a more appropriate mix. The right combination will ultimately depend on many of the circumstances described above (risk tolerance, ability to take risk, personality, experience, and so on).

Investment selection is another lever investors can pull to ratchet their risk. Large-cap stocks are generally going to be less risky than small caps. U.S. stocks will be less volatile than emerging-markets stocks. Government bonds are safer than corporate credits.

Investment selection extends into strategy selection when it comes to investing in funds. There are a host of exchange-traded funds, such as low-volatility equity strategies, that look to dial risk up or down relative to the market portfolio they draw from. These funds provide the opportunity to maintain market exposure while attempting to reduce their sensitivity to movements in the broader market.

Finally, one of the most important sources of risk is the one we look at in the mirror each day. Poor decisions regarding asset allocation and/or investment selection can easily put us off course. As such, the choices made regarding the first two buckets should optimize for behavior. The best portfolio is the one you're most likely to stick with the next time things get hairy in the markets. Trying to solve for this is the best way to manage the biggest risk of all.