According to Meyer, Evans and Rubinstein, converging evidence suggests that the human "executive control" processes have two distinct, complementary stages. They call one stage "goal shifting" ("I want to do this now instead of that") and the other stage "rule activation" ("I'm turning off the rules for that and turning on the rules for this"). Both of these stages help people to, without awareness, switch between tasks. That's helpful. Problems arise only when switching costs conflict with environmental demands for productivity and safety.Although switch costs may be relatively small, sometimes just a few tenths of a second per switch, they can add up to large amounts when people switch repeatedly back and forth between tasks. Thus, multitasking may seem efficient on the surface but may actually take more time in the end and involve more error. Meyer has said that even brief mental blocks created by shifting between tasks can cost as much as 40 percent of someone's productive time.

The process of switching back immediately to a task you've just performed, as many multitaskers try to do, takes longer than switching after a bit more time has passed, say findings published last fall by researchers from the National Institute of Mental Health. The reason is that the brain has to overcome "inhibitions" it imposed on itself to stop doing the first task in the first place; it takes time, in effect, to take off the brakes. If you wait several seconds longer before switching tasks, the obstacles imposed by that shutting-off process are reduced.Managing two mental tasks at once reduces the brainpower available for either task, according to a study published in the journal NeuroImage. Marcel Just of Carnegie Mellon University asked subjects to listen to sentences while comparing two rotating objects. Even though these activities engage two different parts of the brain, the resources available for processing visual input dropped 29% if the subject was trying to listen at the same time. The brain activation for listening dropped 53% if the person was trying to process visual input at the same time."It doesn't mean you can't do several things at the same time," says Dr. Just, co-director of the university's Center for Cognitive Brain Imaging. "But we're kidding ourselves if we think we can do so without cost."

"The bottom line is that you can't simultaneously be thinking about your tax return and reading an essay, just as you can't talk to yourself about two things at once," he says. "If a teenager is trying to have a conversation on an e-mail chat line while doing algebra, she'll suffer a decrease in efficiency, compared to if she just thought about algebra until she was done. People may think otherwise, but it's a myth.

The mental habit of dividing one's attention into many small slices has significant implications for the way young people learn, reason, socialize, do creative work and understand the world. Although such habits may prepare kids for today's frenzied workplace, many cognitive scientists are positively alarmed by the trend. "Kids that are instant messaging while doing homework, playing games online and watching TV, I predict, aren't going to do well in the long run," says Jordan Grafman, chief of the cognitive neuroscience section at the National Institute of Neurological Disorders and Stroke (NINDS). Decades of research (not to mention common sense) indicate that the quality of one's output and depth of thought deteriorate as one attends to ever more tasks. Some are concerned about the disappearance of mental downtime to relax and reflect. Roberts notes Stanford students "can't go the few minutes between their 10 o'clock and 11 o'clock classes without talking on their cell phones. It seems to me that there's almost a discomfort with not being stimulated--a kind of 'I can't stand the silence.'"

ALTHOUGH MANY ASPECTS OF THE networked life remain scientifically uncharted, there's substantial literature on how the brain handles multitasking. And basically, it doesn't. It may seem that a teenage girl is writing an instant message, burning a CD and telling her mother that she's doing homework--all at the same time--but what's really going on is a rapid toggling among tasks rather than simultaneous processing. "You're doing more than one thing, but you're ordering them and deciding which one to do at any one time," explains neuroscientist Grafman.

The big finding of a 2005 survey of Americans ages 8 to 18 by the Kaiser Family Foundation, co-authored by Roberts, is not that kids were spending a larger chunk of time using electronic media--that was holding steady at 6.5 hours a day (could it possibly get any bigger?)--but that they were packing more media exposure into that time: 8.5 hours' worth, thanks to "media multitasking"--listening to iTunes, watching a DVD and IMing friends all at the same time. Increasingly, the media-hungry members of Generation M, as Kaiser dubbed them, don't just sit down to watch a TV show with their friends or family. From a quarter to a third of them, according to the survey, say they simultaneously absorb some other medium "most of the time" while watching TV, listening to music, using the computer or even while reading.