Hank: Maybe you’re driving home from work and traffic’s a little heavy, so you decide to call your mom. Or maybe you’re watching SciShow while doing some calculus homework. And it seems like both tasks are going well enough, since you might consider yourself good at multitasking. Sound familiar?

Sure, you might think you’re doing well. But as it turns out, you’re just not noticing your mistakes. According to lots of psychology studies – from researchers ranging from the University of Michigan to Stanford – when you think you’re multitasking, you aren’t actually doing two tasks at the same time.

Instead, you’re switching between them quickly so that you don’t even notice. And, in the process of switching, you’re taking longer and making more mistakes on both things than if you were doing one at a time.

So, why can’t we multitask? Your brain just isn’t designed to do two things at once, and that has to do with your brain’s executive control processes.

These processes take place with help from your prefrontal cortex, the brain region you’re using to concentrate right now. And they’re responsible for most of your control and decision-making. And you might be thinking “Hank, how are you so sure I’m concentrating right now?” It’s because you are hearing what I’m saying! And if you are not, you are not concentrating. So I know!

Psychologists think executive control processes have two distinct phases: goal shifting and rule activation. In the goal shifting phase, your brain concentrates on doing Task A instead of Task B. And in the rule activation phase, your brain focuses on turning off the rules it needs for the first task and turning on the rules for the next one.

So, an example: one study asked participants to multitask by solving simple math problems while also classifying different shapes. So first, your brain might work on the math problems instead of sorting shapes. It would focus on the rules for how to do math – like order of operations or how to add numbers – instead of the rules for choosing circles or squares.

And when you switch tasks, first your brain would have to shift goals from math to shapes, then it would have to recall the rules to classify shapes. Going through these stages helps your brain shift between tasks without you realizing it – but it also takes time, even if it’s just a few tenths of a second. Now, that doesn’t seem like very long, but those delays will add up over time.

The same study also showed that multitasking with more complicated or unfamiliar tasks caused the delays to get longer with even more mistakes. Those short delays can have real-world implications – like, a tenth of a second delay while switching between answering your phone and paying attention while driving is about all it takes to cause an accident.

Besides making you less productive, there’s a chance that multitasking might affect, or be affected by, the structure of your brain. One 2014, peer-reviewed study from the Public Library of Science looked at the relationship between how often people multi-tasked and their brain structure.

The participants who multi-tasked the most had less gray matter, the brain tissue that’s mostly neurons, in a region called the anterior cingulate cortex, which is located near the prefrontal cortex and is involved in decision-making and impulse control.

But since it was a correlational study, the researchers were just observing people and weren’t actually manipulating variables. In other words, they couldn’t determine if more multitasking causes the tissue loss or if fewer neurons in that region just happens to be related to the behavior, so more research needs to be done before we can draw any conclusions.

So, the research seems to suggest that we stop multitasking and focus on one thing at a time. But what if it isn’t that easy?

Whenever you complete a small task – like sending out a funny tweet while working on a big project – your brain releases the hormone dopamine, which makes you feel a small sense of reward and accomplishment. Since that’s such a great feeling, it encourages you to keep multitasking.

Even though you might be doing each small task inefficiently or with more errors, your brain keeps releasing dopamine, so the multitasking continues. It’s kind like a vicious cycle.

On top of that, your prefrontal cortex is easily distracted and excited by new things, like kittens and memes, and whole internet's worth of those things, so it makes multitasking even more tempting. But, for the sake of your job or your GPA, maybe finish that project you’re working on, and then then come catch up on SciShow!

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