Its not that you can't focus, it's just that you can't remember what you're doing.
When I began to read this article and do my research for this post, its subject matter could not have been more applicable. I had been struggling through a government paper for hours (an assignment that should have taken me 45 minutes) and it was close to 10:00 on a Sunday night, boy did I know a thing or two about short attention spans.
Well it turns out that our short attention spans might actually result from smaller working memories. Two psychologists, Jennifer McVay and Michael Kane, tested subjects in their study for both working memory capacity and ability to focus on a certain task. They found that working memory capacity was correlated with increased ability to focus.
Thinking unrelated thoughts correlated significantly (r = .368) with poor performance on the task. And unrelated thoughts were negatively correlated with working memory capacity (r = -.217).
The task they used was called a sustained attention to response task or SART. It involves repeating a boring activity for a long period of time and measuring student's performance. Try this SART to see what they're like. You are supposed to tap the desk, or keyboard, or whatever is near you right now, every time the number that pops up is NOT a 4. It's harder than it seems, well if you're distractable like me that is.
Another psychologist named Rita Berto did a study to test the hypothesis that a nicer working environment makes it easier to study. The results of her study are somewhat questionable because of the many variables that might not have been controlled, but it seems that being shown "restorative" photos as opposed to ugly ones makes it easier to focus on the task at hand.