Robin Marantz Henig ("Driving? Maybe You Shouldn't Be Reading This", in the 13 Jul 2004 New York Times) offers quantitative evidence against the myth that multitasking is efficient:
Last year, psychologists at the University of Michigan reported that when they asked subjects to perform two or more experimental tasks — solving arithmetic problems, say, at the same time they identified a series of shapes — the frontal cortex, the executive function center of the brain, had to switch constantly, toggling back and forth in a stutter that added as much as 50 percent to the time it would have taken to perform the tasks sequentially instead of simultaneously.
In another study, scientists at Carnegie Mellon put subjects in an M.R.I. machine and asked them to listen to complicated sentences at the same time that they mentally rotated geometric shapes. The two tasks activated different parts of the brain, but each region was operating at a suboptimal level. Here, then, was high-tech confirmation of the common-sense wisdom of Publilius Syrus, a Roman philosopher from the first century B.C., who warned, "To do two things at once is to do neither." (Publilius also came up with "Better late than never" and "A rolling stone gathers no moss.")
It's quality that suffers with timesharing — and quality isn't easily measured, especially in thinking and in interpersonal relationships ...
(see also TripleThink (25 Jul 2002), ... )