Eyal Ophir, who put out a study on multitasking a couple years back, reflects on what he has learned:
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.
The Dish 