It is among the most comforting commonplaces of personal finance, to be found illustrated in mutual fund brochures, on wall posters, even on coffee mugs: The longer you plan to hold your investments, the better off you are with common stocks. A common rule of thumb has it that if you are 30 years old, you should put 70 percent of your nest egg in stocks and 30 percent in bonds; by the time you turn 70, you should have reversed the allocation.

The ultimate dependability of this view long has been doubted by the best economists, Paul Samuelson and Robert Merton in particular. Their arguments-that short-term riskiness will not necessarily always be washed out by a long-term horizon-have been highly technical, based on the theory of expected utility maximization. They are to be found in technical papers such as "Fallacy of the Log-Normal Approximation to Portfolio Decision-making over Many Periods." But they have persuaded relatively few professional pension managers.

Now a Boston University finance professor, Zvi Bodie, has come up with an argument that may prove to be more compelling to money managers. If the conventional wisdom is true, says Bodie, if stocks become less risky the longer you plan to hold them, then it ought to be relatively easy to buy insurance against the possibility that they might earn less than if you had parked your investment in long-term government bonds. Guess what? It isn't. In fact, such insurance costs more, not less, the farther out in time you go.