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If you ever feel like your brain has low bandwidth – that there are only so many items on your to-do-list you can keep track of at any one time, only so much you can take in before you seem to max out – you would be correct.
There are limits to what our brains can do, and this is never more apparent than when we try to accomplish multiple things at once; when we attempt to multitask.

How well you can actually accomplish two things in tandem will depend on the tasks themselves. Automated tasks, tasks which do not require attentional control or working memory (WM) , are typically good candidates for multitasking. But, when both tasks involve multiple steps or keeping track of new information, performance plummets. If you’ve ever been on the phone with someone while they are typing on a computer, you know what I’m talking about: their response time is slow, they take long pauses, and their IQ appears to have been lowered by 50%. 

The explanation for this difficulty is to be found in the brain. The cognitive functions which allow you to process information and stay on task are your attention and working memory, and in the brain, there appears to be a network of areas which control both of these functions. These areas direct the spotlight of your focus, and can be thought of as a sort of control network, with a limited capacity.

So if the two tasks you are aiming to perform both require activation of this control network, they physically cannot be performed simultaneously without your performance plummeting. There is simply not enough bandwidth in our brains to accomplish two cognitively demanding tasks at the same time. 

This doesn’t mean there aren’t things we can do in tandem–it just limits what they are. It is possible to, for example, listen to music and read at the same time as long as one is done passively. But, if you try to listen for a particular lyric or keep track of a certain progression of notes, your reading comprehension will diminish. 

Some tasks can be automated with practice, removing their reliance on your cognitive control. Take assembling furniture, for example. The first time you build a chair you will need to dedicate neurological activity to holding onto and following the instructions for each step. But say your job is assembling the same type of chair over and over in an assembly line, you will eventually reach a point where you can sustain conversations with coworkers or listen to an audiobook while performing this task.

Some things, like listening to a lecture, cannot be automated and will therefore require your full attention. You can likely doodle and listen to your professor at the same time, but you will find it hard to pay attention if you begin answering your emails. 

In situations where we cannot do two things at once we are forced to switch back and forth between them: serial multitasking. While this almost feels like doing two things at once, the switching requires brain power and is tiring: there is a cost associated with redirection. 

Say you, at work, need to reply to an important email. While you are typing, you need to keep a plan of what you are writing in your mind. This will involve recalling information about the email you have just read, remembering ongoing discussions within the team, as well as your own capacity for problem solving. On the desk in front of you is your mobile phone. It doesn’t buzz or light up, but the simple sight of it triggers a thought process about potential actions: maybe a phone call you need to make, or wanting to check whether your latest instagram post has gotten any comments. Maybe you are able to stave off the impulse of picking up the phone, but the three distracted seconds you spent considering whether or not to do so is enough to disturb your cognition: when you return to the task you were doing, you have to reload all of the information from long-term memory and reestablish your plan in working memory. 

You’ve lost precious mental setup time. 

This sequence of events is repeated over and over. Each time, it’s something minor and probably not cause for concern. But compounded over the course of a full day–or week–they add up. Constant spanners in your mental works. 

It’s easy to think of multitasking as being a desirable skill-set: we all want to be a person spinning twelve plates without dropping a single one. The problem is that our stone-aged brains only have one proverbial hand. So instead of valuing a supposed ability to multitask, we should aim to be one-trick ponies: being really good at accomplishing what we need to do, when it needs doing.