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Getty ImagesAre you reading this while thumbing through text messages, streaming a TV show online, or scribbling a note to your child's teacher? (Or maybe doing all three?) Don't congratulate yourself.

Even though most people think an amped-up, gadget-dependent lifestyle makes them more nimble, focused, and efficient, that may not be the case. In fact, many researchers believe the human brain can't really perform two or more tasks simultaneously, as the word multitask implies.

Rather, they say, the mind toggles between tasks. And while mindless activities like walking and chewing gum aren't a problem, the brain doesn't fare well when people double up on complex tasks, such as driving and talking on a cell phone.

"Something's got to give," says David E. Meyer, PhD, director of the Brain, Cognition, and Action Laboratory at the University of Michigan, in Ann Arbor. "Either your cell phone conversation will suffer or your driving will suffer."

And it's not just behind the wheel. There's mounting evidence that multitasking can slow you down no matter what you're doing.

In a study published a decade ago, Meyer and his colleagues found that, contrary to popular belief, people are less efficient—not more—when they multitask. That's because it takes more time to complete one of the tasks, especially as they become more complex, versus focusing on a single task.

How multitasking affects memory and attention
Imagine this: You get off the couch to get a snack from the refrigerator, you're interrupted by a phone call, and arrive in the kitchen with no clue why you're there.

That common scenario illustrates how even the simplest forms of multitasking can lead to glitches in the moment-to-moment processing of information known as working memory, says Adam Gazzaley, MD, an associate professor of neurology, physiology, and psychiatry at the University of California, San Francisco.

These types of distractions are becoming more and more a part of daily life, thanks to computers and smartphones, and research suggests they could be taking a toll on our attention.

In a 2009 study, researchers at Stanford University asked students to take an online questionnaire about their media use. From that group, the researchers identified heavy and light "media multitaskers" and compared their performance on three cognitive tests. Their paper found that heavy media multitaskers had more trouble filtering out irrelevant information from their environment. In others words, they were more prone to distraction than their low-use counterparts. The heavy users were also less able to focus and had more difficulty switching tasks.

It's not clear whether multitasking causes these problems, or if preexisting personal differences make certain individuals more prone to distractions posed by the "changing media environment," the authors note.

Next Page: Multitasking may cause more problems for older people [ pagebreak ]Multitasking can be problematic at any age, but it may be more likely to interfere with our working memory as we get older. In a pair of recent studies, Dr. Gazzaley and his colleagues gave a test to groups of young and old adults in which they were shown an image and asked to recall it about 15 seconds later, after being interrupted with another cognitive task.

"Our finding was that older adults are not switching back as easily, and that's why (the interruption) is having a greater impact on their working memory, on holding information in mind for short periods of time," Dr. Gazzaley says.

Are teens better multitaskers?
Of course, kids who grew up instant messaging friends while downloading music and catching the latest viral video on YouTube are more adept at switching tasks than older folks, right? Maybe not.

The jury is still out on whether teens are truly masters of multitasking, says Jay Giedd, MD, chief of the brain imaging unit in the National Institute of Mental Health's Child Psychiatry Branch, in Bethesda, Md. Teens certainly get plenty of practice multitasking, and while they may not pay as high a "tax" for it as adults, they still cannot do multiple tasks at once as well as one at a time, Dr. Giedd says.

Teens and tweens (8- to 12-year-olds), on average, spend more than 7.5 hours a day with various forms of media, but they manage to squeeze 10 hours and 45 minutes of media exposure into that time because they use two or more forms of media concurrently, according to a report by the Henry J. Kaiser Family Foundation.

Dr. Giedd points out that it is especially contentious whether younger teens might be more handy multitaskers than older teens, like the college-age students who had problems switching tasks in the 2009 study of media multitaskers.

Dr. Giedd's research team has performed more than 7,000 MRIs on nearly 3,000 kids over the past two decades to better understand brain development in children and adolescents. So far, so good. "The kids in our study that do a lot of multitasking, they're doing quite well in terms of they're getting good grades, they're creative, they're energetic," he says.

But they may be more stressed. While the link isn't clear, Dr. Giedd notes that kids are sleeping less and that sleep deprivation is a stressor. "It's very tempting to stay up later and to do more stuff," he says, adding that because multitasking is inherently inefficient, kids may have to stay up later just to get all that stuff done.

"By a chain of pretty solid reasoning, one can conclude that multitasking intensely on a regular basis day after day where you really care about your performance and are on the edge of failing because of the challenges to the limits of your information processing will eventually lead to chronic stress," Meyer says.