From a groundbreaking 2009 report on multi-tasking:

Results showed that heavy media multitaskers are more susceptible to interference from irrelevant environmental stimuli and from irrelevant representations in memory. This led to the surprising result that heavy media multitaskers performed worse on a test of task-switching ability, likely due to reduced ability to filter out interference from the irrelevant task set.  

And now, the author's 2011 ruminations on the same topic:

Historically, when someone tapped on our shoulder, they were necessarily physically next to us. So they knew if we were already holding a conversation with someone else, and could adjust their behavior, or withhold their request. We, in turn, felt compelled to respond to the tap on the shoulder when it came. But now, the incoming chat message, the phone call, and the television announcer, all tap on our shoulder in a sense, trying to get our attention. They are entirely oblivious to each other, and solicit our attention as if they were the only ones. We, on the other hand, feel the same obligation to respond. It may be that our social norms and instincts are not scaling at the rate of communication channels. In this way, media may have brought about a new tragedy of the commons - by aggressively trying to grab our undivided attention, they have threatened the very notion of undivided attention.

I think the moral is that we desperately need to begin exercising more control over our focus and our interactions. Shut off the iPhone. Turn off the twitter feed. Read a book. Have an uninterrupted conversation with the person or people right next to you.

This is not Ludditry: this is retraining ourselves to think with focus in an age dedicated to the disruption of that thought.

(h/t Sullivan)