FOR many it is a reflex as unconscious as breathing. Hit a stumbling-block during an important task (like, say, writing a column)? The hand reaches for the phone and opens the social network of choice. A blur of time passes, and half an hour or more of what ought to have been productive effort is gone. A feeling of regret is quickly displaced by the urge to see what has happened on Twitter in the past 15 seconds. Some time after the deadline, the editor asks when exactly to expect the promised copy. Distraction is a constant these days; supplying it is the business model of some of the world’s most powerful firms. As economists search for explanations for sagging productivity, some are asking whether the inability to focus for longer than a minute is to blame.