"Right this way, folks!" shouts the carnival barker. "REDUCE YOUR BRAIN POWER WHILE INCREASING YOUR STRESS LEVELS!" Are you tempted to follow him? Of course not. And yet, if you spend a lot of time multitasking, you're achieving the same result.
Many people think they can multitask. It's true that you can walk and chew gum at the same time, but the reason is that these two tasks don't require your attention. Tasks that involve language processing or decision making need your attentional focus, and when you try to do two such tasks at the same time, you end up switching your attention back and forth.
One reason multitasking (or task-switching) is so hard is that it calls upon working memory—a brain resource that's extremely limited. Every time you switch to the other task, it's hard to hold that first task in memory so it's there when you come back. If it's not there, you lose your train of thought. Constantly answering the question, "now where was I?" is a big waste of time and energy.
If you want to feel a hint of the stress and energy expended in task-switching, try the Stroop Test. In this test, pictured at left, you need to ignore what the letters say, and instead quickly call out the color of the letters each word is written in. For each word, you have to switch from word-reading mode to color-identifying mode, and this is both inefficient and taxing.
A study out of UCLA illustrates some of what's lost when you multitask. Participants engaged in some learning trials while single-tasking and other trials while dual-tasking. When they had multitasked, the participants could perform the learned behavior, but they were much less able to identify the rules underlying what they were doing. Importantly, brain imaging revealed that different areas of the brain were active under single-tasked as opposed to multitasked learning. Learning while multitasking involved implicit processes similar to forming a habit without consciousness of what is being learned. Learning while single-tasking involved utilizing working memory, and what was learned was more flexible and involved more abstract, generalizable knowledge.
To apply these results to another common situation, reading something doesn't involve just consuming words; to make what you've read useful, you must relate the new information to what you already know. Apparently, multitasking interferes with this process.
So if you swear by multitasking and think you can do it as well or better than single-tasking, the research has bad news for you: Performing two tasks at once, instead of sequentially, multiplies trouble. Multitasking hurts in terms of speed, accuracy, quality of output, and energy consumption. In essence, when you're multitasking, you're dimming your bulb, de-powering your brain. You're so much better off single-tasking. Unless, of course, low-power processing is what you're after.
So try single-tasking one thing that you usually multitask. You'll be amazed at how much more easily and quickly you get it done, and with much better quality than you expected. You may even be tempted to try it again.
My next post: FIVE REASONS WE MULTITASK ANYWAY
 Medina, J. (2008). Brain rules: 12 Principles for surviving and thriving at work, home, and school. Pear Press.
 Klingberg, T. (2009). The overflowing brain: Information overload and the limits of working memory. (Translated by Neil Betteridge). London: Oxford University Press.
 Foerde, K., Knowlton, B., & Poldrack, R. A. (2006). Modulation of competing memory systems by distraction. Proceedings of the National Academy of Sciences. www.pnas.org/cgi/doi/10.1073/pnas.0602659103
 Cantor, J. (2009). Conquer CyberOverload: Get More Done, Boost Your Creativity, and Reduce Stress. Madison, WI: CyberOutlook Press.