If you’re an avid (read: addicted) multitasker and flipping between tasks even as you read this, you might want to put down the Twitter feed for a sec. Because while you may think you’re really good at it, there’s scant proof of that. Just ask science. Because there’s mounting evidence that multitasking does just the opposite—not only makes you less effective, but could quite possibly make you more depressed.

I know this. I do. And yet I feel pulled, as you do, in a thousand directions at once, every day, all the time. No matter what I read or watch I feel I should be doing something else. It’s the blight of the modern professional. We have more competing for our attention than ever, and the seduction of “do everything at once” will cost us.

To come to terms with our multitasking addiction, we have to face up to some of the lies we tell ourselves to defend it. Here are some of the biggest lies you and I tell ourselves about multitasking, exposed to the cold light of scientific research.

LIE: I’m really good at it.
TRUTH: No one is.

Multitasking isn’t like tennis or learning French; you don’t get better the more you do it. Your brain isn’t designed to focus on more than one thing at a time, and an attempt to do so results in splintered focus, and, according to this interesting research out of Stanford University, cognitive damage. (Travis Bradberry did a nice distillation of the study on Forbes.)

In it, researchers looked at how heavy multitaskers (people who regularly do a lot of media multitasking) fared against light multitaskers (those who don’t do it regularly). And the heavy multitaskers came up short every time. When test subjects were shown letter and numbers and told what to focus on (say, a number and whether it was even or odd, or a letter and whether it was a vowel or consonant), the heavy multitaskers underperformed. One of the researchers explains that it’s because they couldn’t ignore what they were told to ignore, and paid attention to everything at once.

LIE: It makes me more productive!
TRUTH: It makes you less effective.

When you flip from window to window and task to task during the day, you may feel you’re working faster, but what you’re doing is switching a lot, but that doesn’t make you effective. Chances are that what makes you good at your job is your ability to make good decisions. And while this particular study didn’t measure decision-making, it did consider the participants’ ability to prioritize. One of the key findings of the study is that heavy multitaskers showed a marked inability to focus on one thing or, more interestingly, prioritize. Communication Professor Clifford Nass, one of the researchers whose findings are published in the Aug. 24 edition of the Proceedings of the National Academy of Sciences, says that heavy multitaskers are “suckers for irrelevancy” because “everything distracts them.”

An inability to filter information means an inability to effectively prioritize. And if you can’t prioritize well, you can’t make good decisions. I’ll add this: When the researchers tested a theory that perhaps heavy multitaskers had better memory function which allowed them to organize information? That theory did not pan out. In fact, multitaskers fared worse in the memory department.

If you can’t remember what you were doing a minute ago, let alone what you have to do next or later, and a compromised ability to know what’s worth paying attention to, you’re going to have some real problems getting things done, let alone getting ahead.

LIE: It’s fun!
TRUTH: It’s the destroyer of fun.

There are some kinds of multitasking that are utterly benign: folding laundry while watching Law & Order: SVU. Doing some light administrative work (or toilet scrubbing) while listening to Serial. One of the reasons why we think we enjoy multitasking may be at least partially explained by this study published in Computers In Human Behavior, in which researchers found that it gives the sense that time is passing more quickly (which feeds the sensation that you’re “getting things done”). (Read more on why busywork makes you happy.)

But I’m far more disturbed by this study, published in Cyberpsychology, Behavior, and Social Networking, in which researchers found an association between media multitasking and psychosocial dysfunction, which suggests that “the growing trend of multitasking with media may represent a unique risk factor for mental health problems related to mood and anxiety.” Yikes.

I discovered this myself when recently I decided that one of my favorite shows wasn’t good anymore. It felt tedious, plodding, and not compelling. But then I looked at what I was doing differently: I had started to catch up on admin stuff while watching it. It wasn’t the show’s fault at all—my splintered attention had sucked all the fun out of that one hour of TV I was saving for myself that night.

Multitasking doesn’t give you or me a leg up on anything. But only when we can shift our cultural perception of productivity to focused over fast, we won’t be able to do or think or even fully live the way we most wish we could.

Read more about how being busy can be a problem.