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You may think that as you juggle emails, my-book, twitter, google, work, life, the phone and casual web surfing that you're really doing all of that stuff at once, but what you're really doing is quickly switching constantly between tasks. And switching carries a cognitive cost.

Steven Yantis, a professor of psychological and brain sciences at Johns Hopkins, says:

In addition to the switch cost, each time you switch away from a task and back again, you have to recall where you were in that task, what you were thinking about. If the tasks are complex, you may well forget some aspect of what you were thinking about before you switched away, which may require you to revisit some aspect of the task you had already solved (for example, you may have to re-read the last paragraph you’d been reading). Deep thinking about a complex topic can become nearly impossible.

What if I told you that some people (singletaskers) claim that our most valuable mental habits—things like deep and focused thought—must be learned through concentrated practice.

Nicolas Carr, author of The Shallows: What the Internet Is Doing To Our Brain, comments on his blog:

The fact that people who fiddle with cell phones drive poorly shouldn't make us less concerned about the cognitive effects of media distractions; it should make us more concerned.

And then there's this: “It’s not as if habits of deep reflection, thorough research and rigorous reasoning ever came naturally to people.” Exactly. And that’s another cause for concern. Our most valuable mental habits – the habits of deep and focused thought – must be learned, and the way we learn them is by practicing them, regularly and attentively. And that's what our continuously connected, constantly distracted lives are stealing from us: the encouragement and the opportunity to practice reflection, introspection, and other contemplative modes of thought. Even formal research is increasingly taking the form of “power browsing,” according to a 2008 University College London study, rather than attentive and thorough study. Patricia Greenfield, a professor of developmental psychology at UCLA, warned in a Science article last year that our growing use of screen-based media appears to be weakening our “higher-order cognitive processes,” including “abstract vocabulary, mindfulness, reflection, inductive problem solving, critical thinking, and imagination.”