Doing several things at once makes us worse at all of them: why multitasking doesn't work the way we think it does

Replying to a message while writing an email. Checking a piece of information during a meeting. Studying with several tabs open while the phone lights up every few minutes. At the end of the day, we may feel like we have done many things at once and, therefore, made better use of our time.

The problem is that doing more things simultaneously does not necessarily mean making more progress.

When two activities require attention, memory, or decision-making, what we call multitasking usually consists of something else: temporarily abandoning one task, directing attention to the next, and then returning to the first. That switch may last just a few seconds, but it has a cost.

Cognitive psychology has been studying this for decades. When switching tasks, we usually need time to recall what we were doing, what information was important, and what we had to do next. During that transition, errors increase and response speed temporarily decreases.

A recent study published in Nature Neuroscience adds another piece to the problem: interference begins even before we have switched tasks, but we know we might have to.

Multitasking is not always simultaneous

Not all combinations of activities require the same effort.

We can walk while talking to someone or listen to music while performing a routine task because one of those activities is sufficiently automated. The conflict arises when we try to maintain two tasks at the same time that require similar resources.

Reading a report and maintaining a conversation via messages is a good example. To reply, we have to momentarily leave what we were reading, interpret the message, decide what to answer, write it, and then return to the report.

The document remains open on the same sentence, but our attention does not return exactly to the same point.

The context must be reconstructed: remembering what we were thinking, retrieving the information we had active, and repositioning the goal. This is called the task-switching cost or switching cost, a phenomenon widely described in multitasking studies. Alternating between complex tasks reduces efficiency and increases errors, especially when switches are frequent.

Separately, each interruption seems insignificant. The problem arises when those small restarts accumulate over hours.

Knowing in advance also influences

The new Nature Neuroscience study focused precisely on what happens when we do not know in advance what we will have to do.

The researchers used visual tasks in which participants had to focus on different properties of the same image. In some trials, they knew from the start what information they would need to respond; in others, they did not find out until later which they would have to focus on.

When that uncertainty existed, both the human participants and the studied macaques made less precise decisions.