What you call multitasking is really task-switching, said Guy Winch, author of Emotional First Aid: Practical Strategies for Treating Failure, Rejection, Guilt and Other Everyday Psychological Injuries. "When it comes to attention and productivity, our brains have a finite amount," he says.
"It's like a pie chart, and whatever we're working on is going to take up the majority of that pie. There's not a lot left over for other things, with the exception of automatic behaviors like walking or chewing gum." Moving back and forth between several tasks actually wastes productivity, he says, because your attention is expended on the act of switching gears—plus, you never get fully "in the zone" for either activity.
In a 2009 study, Stanford researcher Clifford Nass challenged 262 college students to complete experiments that involved switching among tasks, filtering irrelevant information, and using working memory. Nass and his colleagues expected that frequent multitaskers would outperform nonmultitaskers on at least some of these activities.
They found the opposite: Chronic multitaskers were abysmal at all three tasks. The scariest part: Only one of the experiments actually involved multitasking, signaling to Nass that even when they focus on a single activity, frequent multitaskers use their brains less effectively.