Business conditions are so universally dismal that the corporate slogan for most American companies might as well be "We Do More with Less." That places a heavy burden on employees who are often stretched to their limits. Consequently, multitasking—both in the sense of doing more than one task at a time as well as switching among tasks—has taken on an added importance at companies that have experienced either layoffs or hiring freezes or both (usually both). Since these companies are now chronically understaffed, conventional wisdom decrees that those still on the job be as efficient as possible. Hence the need to juggle as many jobs as one can.

But there’s a problem with multitasking. Not only does it take a personal toll on employees, it also doesn’t work.

In a February Wall Street Journal column, writer Sue Shellenbarger cited a growing body of research evidence that indicates multitasking actually erodes, rather than enhances, productivity. As people divide their attention between two even seemingly simple tasks—reading their e-mail, for instance, while talking on the phone—comprehension, concentration and short-term memory suffer. Switching from one job to another doesn’t work any better. Research indicates that that eats up more time than waiting to finish one job before beginning the next—an inefficiency that increases as the tasks at hand become more complicated. Toggling back and forth between a review of the fine print on a vendor’s service-level agreement and a discussion about the amortization of next year’s IT investments with your CFO over the phone? Not a terrific time-saving strategy after all.

Attention Is a Finite Resource

The inutility of multitasking as a productivity tool makes perfect sense when understood in terms of attention and available resources. "Current cognitive models suggest that people have a limited amount of attention available at any moment," says Seth Greenberg, a professor of psychology at Union College. "Attention could be thought of as a fuel that can be dispersed. Thus, tasks can be performed simultaneously with efficiency as long as the required attention for both tasks does not exceed the limit." In other words, a person can multitask effectively as long as any given task doesn’t require too much attention and thereby exhaust his resources.

For example, if one is riding one’s bike through a sylvan glade, one can let one’s mind work on a math problem. The riding can be done on autopilot, and a great deal of attention can be paid to the math. But when conditions change, and piloting the bike demands more attention—say, in rush hour traffic or on uneven ground—the performance of one or both of the tasks breaks down. One can ride safely—or solve the problem—but not both, because the demands upon the finite resource of attention have escalated. And if one insists upon attempting both, the consequence can be a nasty spill.