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In her book “Attention Span,” which came out this year, Gloria Mark, a professor of informatics at the University of California, Irvine, summarizes her research on how we interact with computers. One of the more remarkable conclusions she draws is that our attention spans have been more than cut in half—from 2.5 minutes in 2004 to 75 seconds in 2012, and finally, to 40 seconds in 2017.

So, what is the basis for this claim? Using automated computer tracking software, her team measured how long adults spend on each screen activity before moving on to the next. These measures have shown that, on average, the time spent on each task has more than halved over the last 20 years. Dr. Mark speculates that our computer habits are the culprit and offers some guidance to increase focus: reduce messaging, email, and social media usage. This is sensible advice and echoes what has been advocated by people such as Cal Newport, author of “Deep Work” and the more recent “A World Without Email,” as well as by Tristan Harris, founder of the Center for Humane Technology.

The data is interesting. Switching between tasks has been linked to increased levels of stress. Task-switching also incurs cognitive costs: it takes time to set up and get into a task, loading relevant long-term memories into working memory to think productively and solve problems. One study, cited by Mark, suggests that it takes twice as long to complete two tasks if performed in tandem compared to performing them one at a time.

However, I think there is some confusion here, specifically regarding the equivalence between attention and the time one spends on a task. In my mind, what Mark is measuring as an “attention span” is rather time-on-task and reflects task management. Even if you remove messages, phones, and other distractions from your environment to just look at a single application, this does not necessarily mean you have achieved your optimal focus. What is lacking is a measure of the capacity to control attention.

In cognitive psychology, the capacity to control attention is an individual trait that can be quantified. It varies between people, even in an experimental setting with no external distractions, and each participant spends exactly the same amount of time with the psychological task at hand. Control of attention correlates highly with working memory capacity, and both are predictive of general cognitive ability (IQ) and academic performance.

One way to think about it is that getting rid of distractions and task-switching is necessary but not sufficient; one also needs the capacity to control attention.

There might be interesting interactions between time allocation and control of attention. Cognitive abilities are partly genetic and partly affected by the demands of the environment. We also know that control of attention can be improved by training. Since this ability is affected by the environment, constant task-switching over months or years could affect our capacity to control attention. Conversely, a better capacity to control attention improves our ability to resist distractions and avoid task-switching. Even if digital distractions have negatively impacted our time management, an improved attentional capacity should make us better equipped to handle the demands of 21st-century life.