"I'm good at it"

Perhaps all that applies to other people. You've always been someone who can keep many plates spinning. You've been told so in appraisals. You may even feel you work best that way.

There's a study aimed squarely at this belief. David Sanbonmatsu, David Strayer and colleagues at the University of Utah measured three things in the same people. They measured how much the people multitasked in daily life, including using a phone while driving. They asked how good the people thought they were at it. And they tested how good they actually were, using a demanding task that requires holding information in mind while doing something else.

The results were uncomfortable. The people who multitasked most were not the most capable. Actual ability and amount of multitasking were negatively related. The heaviest multitaskers tended to be the worst at it. Most people's rating of their own ability was significantly inflated. And multitasking went with impulsiveness and with a weaker ability to block out distractions. The authors' suggestion is that many people juggle because they find it hard to stay on one thing, and they aren't doing it out of any skill.

I'd add a reason of my own why self-assessment goes wrong here. The costs are hidden from the person paying them. You don't experience the restart as a cost. It just feels like writing the report. You don't see the errors until later, if at all. What you do feel is the juggling itself, which is lively and feels competent. So everyone has reason to believe they're good at it, and the evidence that would correct them is hard to see.

A small number of people may be real exceptions. Jason Watson and Strayer tested two hundred people driving a simulator while doing a hard memory task that they listened to. Nearly everyone got worse at both. Five people, or 2.5 per cent, showed no decline at all. The researchers called them supertaskers. It's possible you're one, but the odds are about forty to one against. The Utah study also suggests that the people who feel surest they're exceptions are, if anything, less likely to be.

You may have read that heavy multitaskers damage their ability to concentrate. That claim comes from a well-known 2009 study by Eyal Ophir, Clifford Nass and Anthony Wagner, which found that heavy media multitaskers were more easily distracted on laboratory tests. I'd treat it with caution. Wisnu Wiradhany and Mark Nieuwenstein repeated the work and pooled every published study they could find. Most of their own tests didn't reproduce the effect, and across the pooled studies the link was weak and disappeared after a standard statistical correction. So I won't tell you that juggling has harmed your brain, because the evidence doesn't support it. The costs of switching are well supported, and you pay them every day you work this way.