The only time multitasking does work efficiently, Meyer says, is when multiple simple tasks operate on entirely separate channels—for example, folding laundry (a visual-manual task) while listening to a stock report (a verbal task). But real-world scenarios that fit those specifications are very rare.
This is troubling news, obviously, for a culture of BlackBerrys and news crawls and Firefox tabs—tools that, critics argue, force us all into a kind of elective ADHD. The tech theorist Linda Stone famously coined the phrase “continuous partial attention” to describe our newly frazzled state of mind. American office workers don’t stick with any single task for more than a few minutes at a time; if left uninterrupted, they will most likely interrupt themselves. Since every interruption costs around 25 minutes of productivity, we spend nearly a third of our day recovering from them. We keep an average of eight windows open on our computer screens at one time and skip between them every twenty seconds. When we read online, we hardly even read at all—our eyes run down the page in an F pattern, scanning for keywords. When you add up all the leaks from these constant little switches, soon you’re hemorrhaging a dangerous amount of mental power. People who frequently check their e-mail have tested as less intelligent than people who are actually high on marijuana.
Also covered here and here.