How long will any client live? Behavioral finance expert Shlomo Benartzi provides an estimate: If 10 high school classmates all make it to age 65 and retire at that time, the first death is likely to occur four years later, while the classmate who buries them all might survive to 99.

If each of them came to you at their 65th birthday and asked for a retirement income calculation, the only option would be to pick a life expectancy - but the true answer may be anywhere from another four to 34 years. As a careful planner, when you pick an age, you might ask, "How certain are you that you won't live longer?" You might then show your client the odds of living to various ages.

The authors of a new study, The Problem With Living Too Long, by researchers at the Institutional Retirement Income Council, an industry research group based in Iselin, N.J., report that half of all 65-year-old females will live to nearly 88.

Leaving aside for a moment all the difficulties of predicting what will happen to one's money and expenses over decades, choosing a life expectancy of 88 gives your 65-year-old female client only a 50/50 chance of not outliving her income. Statistically, a quarter of those women will live another five years, and 10% will endure nearly a decade more. If your client wants 90% certainty she won't live any longer than the age in your calculations, she should pick 98.

However, as the authors note, it's not easy to work for about four decades and accumulate enough money to pay for 30 years or more without earnings. The temptation is to use a younger life expectancy - in effect, planning to die sooner rather than work longer or live on less. This is where the careful planner may feel the need to point out the obvious - that unless your client has special knowledge, she doesn't know when she'll go.

The better choice is for people to work longer, save more, live on less. And as the authors note, we need more options to buy products with our retirement money that "guarantee, or at least virtually assure, that the income will last for a lifetime."

A century ago, the American satirist Ambrose Bierce quipped, in The Devil's Dictionary: "Longevity, n. Uncommon extension of the fear of death." Nowadays, we might define it as: "Uncommon extension of the fear of poverty in old age."