Plenty of people believe that doing several things at once makes them more productive. In fact, the opposite is true.

By Anne Fisher, contributor

FORTUNE -- Dear Annie: Since my department was shrunk by layoffs a couple of years ago, I have gotten an in-house reputation as a champion multitasker, able to handle half a dozen projects at one time (and leap tall buildings in a single bound). Working this way is not only exhausting, but I think it makes me less competent. Everything takes much longer than it would if I could just focus on one task at a time.

So I made a New Year's resolution to cut it out, or at least do less multitasking. The problem is my boss, who is constantly piling more stuff on me to do "with my left hand," as he puts it, when I'm already busier than a woodpecker in a lumberyard. Can you or your readers suggest any way to convince him that we'd all be better off tackling one thing at a time? — Frazzled

Dear Frazzled: A heap of academic research has demonstrated conclusively that the human brain needs time to refocus every time we turn our attention in a different direction. A famous experiment at Stanford in 2009, for instance, found that multitaskers are "more susceptible to interference from irrelevant environmental stimuli" -- that is, more easily distracted and less efficient at what they're trying to do.

So your impression that doing several things at once slows you down and trips up your productivity is correct. But if your boss isn't impressed with academic research, Sanjeev Gupta, CEO of consulting firm Realization, suggests a quick game to prove the point.

Here's how it works: Get a stopwatch, or any watch with a second hand, and time how long it takes you (or your boss) to write "multitask 123456789." Next, time how many seconds it takes to write the same thing, but this time with the numbers interspersed between the letters: "m1u2l3t4i5t6a7s8k9."

Simple, right? When I tried this, it took me 9.5 seconds to write "multitask 123456789" -- and 24 seconds, or more than twice as long, to write the version that requires switching back and forth from letters to numbers.

Okay, so maybe I'm just lousy at multitasking but, says Gupta, even master multitaskers (like you, perhaps) will see a dramatic difference. "It seems counter-intuitive, but if you give two employees two identical tasks, the one who focuses on one task at a time will always finish faster and with fewer errors than the one who multitasks," Gupta says.

"Many people boast about their multitasking prowess on their resumes and in job interviews," he adds. "But in reality, this is like saying, 'I don't get things done as quickly, or as well, as others do.'" More