In my innovation and growth course at Columbia Business School, I note the productivity-destroying consequences of having people on too many projects at the same time, citing a study reported back in 1992 by Wheelwright and Clark. They found that when software engineers were assigned on to more than two projects at any given time, their productivity fell dramatically. In fact, by the time they were on seven projects or so they were contributing barely 15% of value at work.

My participants were skeptical. That research was done before the Internet, before email, and before the 24/7 nature of working in a global corporation today. Surely we've figured out how to multi-task more effectively in this day and age! So we took up that question in a part of the course I call "nominated topics" which is time dedicated to discussing the topic on which the class decides.

It's Really Task Switching, Not Multi-Tasking.

With a few exceptions (listening to the radio while driving, for instance) what we refer to as "multi-tasking" is actually task switching in rapid succession. That distinction matters. Switching from task to task, even when done quickly, has a draining effect on our ability to actually get the tasks done. Back in 1956, a Harvard Professor named George Miller wrote a famous article called The Magical Number Seven, Plus or Minus One. He found that on average, a normal person is able to keep about 7 elements in short-term memory at any given time. When we jump from task to task, this stresses our ability to keep all the relevant information in one mental place.

Too much Task Switching is the Enemy of Productivity

The results of constant task switching are pretty bad, and no we haven't gotten better at it since 1992. A study sponsored by HP found that IQ scores of knowledge workers distracted by e-mail and phone calls fell from their normal level by an average 10 points: twice the decline recorded for smoking marijuana! Even more seriously, interruptions and task switching when people are trying to work through a process can lead them to forget where they were. The consequences can be dire. Interrupt a nurse mid-way through a process, for example, and the likelihood of it being carried out properly is much diminished. University of Utah psychology professor David L. Strayer has done numerous studies in a variety of contexts and concludes that " takes longer to complete Task A and B concurrently than it would have taken if you had done Task A and then Task B consecutively."

Psychologists also have found that multi-tasking wears us out mentally. A recent piece in Psychology Today notes that "task switching involves several parts of your brain: Brain scans during task switching show activity in four major areas: the pre-frontal cortex is involved in shifting and focusing your attention, and selecting which task to do when. The posterior parietal lobe activates rules for each task you switch to, the anterior cingulate gyrus monitors errors, and the pre-motor cortex is preparing for you to move in some way. The result is that your brain literally gets exhausted from all the switching around, further diminishing effectiveness.

Countering the seductive idea of multitasking

What makes us do so much task-switching in the first place? Email is literally addictive--our brains get a little jolt of short-term pleasure when we receive a message. Informal communications with colleagues can pull us away from what we were trying to focus on. Unexpected demands from others can also throw our days out of whack. What can we do to improve how we cope? Three techniques can help.

First, think of going from continuous processing of ideas and information to "batch" processing. Instead of reading emails willy nilly as they come in, for instance, perhaps you tackle all those on common topic or from the same person at once. If you have a bunch of meetings regarding financials, maybe that's the same day you peruse financially-oriented reports.

Secondly, make it clear to others that taking on a new task inevitably means that something else is not going to get done. The metaphor I often suggest is to think of your day like a fully loaded truck--when you started work in the morning you had a pretty full agenda. When someone else tries to put more items on the truck, it's sometimes wise to ask what will come off.

Thirdly, an old standby, when used diligently, can be enormously helpful. This is to categorize issues by whether they are strategic or not, and whether you are under a deadline or not. Every so often, get that "to do" list categorized along these lines and see where you are really putting your efforts. All too often, the non-strategic but urgent-feeling items take over your agenda.

Wresting control back from the forces that lead to frantic multi-tasking is not easy, but doing so will probably make you happier--and definitely more productive.