In classes at the University of Utah, psychologist David Strayer very often opens with the same question: how many of you consider yourselves good at doing several things at once? Usually about half the room raises a hand. Then he runs them through a test — driving a simulator while solving math problems from memory and recalling words — and the percentage of people who manage to do both without either one suffering is 2.5% [1].
That 2.5% isn’t special, either. Another study from the same university measured the relationship between how well people think they split their attention and how well they actually do, and found exactly the opposite of what you’d expect: the more people multitasked in their daily lives, the worse they performed on objective tests, and yet 70% still rated themselves above average [2]. The ones who scored best on the tests — the top 25% — turned out to be, by a wide margin, the people who switched tasks the least in everyday life and who most preferred doing one thing after another.
Worse still, the more you trust your ability to split your attention, the worse it generally is. People with less capacity to block out distractions tend, for exactly that reason, to jump between tasks more, and they unfortunately carry the false impression that they’re doing it well.
The first thing to clear up is the definition of “mental multitasking” itself. The human brain does not process two complex cognitive tasks in parallel. Ever. What we call multitasking is a trick: the brain jumps from one task to another so fast that the subjective feeling is one of simultaneity (computers do this too), but underneath there’s a queue, a turn, one task waiting while the other holds the stage. That jump has a measurable price in time, in errors, and, as we’ll see, in literal neural activity.
There’s a lab phenomenon with a name that sounds like a bureaucratic form — the psychological refractory period — documented for more than seventy years now, that explains why this is the case at a more basic level. If you ask someone to respond to two stimuli almost simultaneously, the response to the second one always arrives delayed, and that delay doesn’t go away no matter how much they practice. Psychologist Harold Pashler called it the “central bottleneck”: there’s a response-selection stage — deciding what to do with the information you just received — that the brain can only run one at a time [3].
Joshua Rubinstein, David Meyer, and Jeffrey Evans measured that bottleneck under conditions closer to real life: people solving problems while switching between different tasks. They found that each switch involves two separate mental processes — dropping the rules of the previous task and loading the rules of the new one — and that both cost time: fractions of a second per switch, which grow with task complexity and shrink, but never disappear, with practice [4]. And not all switches cost the same: jumping between two similar tasks is cheaper than jumping between tasks that run on different mental rules, like going from mental math to writing something emotionally delicate. Added up over a workday, Meyer went on to estimate that these micro-blocks can eat up as much as 40% of a person’s productive time. Obviously it’s not an exact figure, but it makes the loss of mental efficiency involved pretty clear.
There’s more. When you switch tasks, your brain doesn’t fully move out. Sophie Leroy, a management professor, found that part of your attention stays hooked on the previous task, especially if you left it unfinished [5]. She showed this with a simple experiment: a group worked on word puzzles, and partway through, some were interrupted and moved on to reviewing résumés and making hypothetical hiring decisions, while others were allowed to finish the puzzle before switching. Those who arrived at the second task with the first one half-done performed worse on it — more errors, more slowness — than those who had closed the loop before jumping. Leroy called it attention residue: the brain refuses to close tabs. It keeps running the process in the background, even while you’re already looking at something else.
And this is where Earl Miller comes in, an MIT neuroscientist who has spent nearly three decades implanting electrodes in the prefrontal cortex — the region right behind your forehead, in charge of planning, deciding, and braking impulses — of macaques, to see what actually happens in there when a brain tries to split itself between two tasks. In one of his most-cited experiments, Miller and his team, Jason Cromer and Jefferson Roy, trained a group of monkeys to classify two completely different sets of images: cats versus dogs on one hand, sedans versus sports cars on the other. Each animal was cued at the start of every trial as to which classification was in play at that moment, and had to switch criteria without any steady advance warning — exactly like when you go from answering a text to getting back to a math problem. While the monkeys alternated between the two tasks, the team recorded activity from roughly 500 neurons in the prefrontal cortex [6].
What they found has nuance, and it’s worth telling in full instead of settling for the easy headline. About half of those neurons turned out to be “generalists”: they took part in both the animal task and the car task, which went against the classic idea that each neuron has one fixed, isolated function. You could read this as proof that the brain does know how to multitask at the neural level, and in fact that’s more or less how MIT titled its own press release. But the existence of neurons capable of representing both tasks isn’t the same thing as executing both decisions at once. The flexibility lives in the hardware; the bottleneck still sits in the process of deciding and acting on that information [6]. When two tasks genuinely compete for the same resources, both end up slower and more error-prone. In humans, no electrodes needed — you notice it the moment you have to reread the same paragraph three times because a notification came in and your phone was right there.
Multitasking Experts
In 2009, a Stanford team led by Eyal Ophir, Clifford Nass, and Anthony Wagner compared students who consumed a huge amount of media at once — music, messages, video, social feeds, all simultaneously — with students who rarely mixed formats. The starting hypothesis seemed reasonable: if you’ve spent years training rapid task-switching, you should be better at it. The result was the opposite. “Heavy multitaskers” performed worse at filtering out irrelevant information, worse at managing their working memory, and, most surprisingly of all, worse at the very task of switching focus — the one thing they should have been masters of [8]. Ophir, the lead author, later admitted they looked for what these subjects were better at and found absolutely nothing.
The problem with this research is that the finding hasn’t replicated cleanly. A team led by Meredith Minear repeated a similar design in 2013 with a different sample and did not find the same relationship between heavy media multitasking and worse performance on task-switching tests, although they did find more impulsivity and worse self-control among heavy multitaskers [9]. Other later studies have found mixed results, which may point to a problem in exactly how “media multitasking” is measured and which specific part of task-switching each study is evaluating. Either way, what’s not in dispute, on either side, is that no one processes two complex tasks in parallel efficiently.
Mind the Road
In the supertaskers study, Watson and Strayer confirmed something that research on driving and phones had been pointing to for years: holding a phone conversation delays braking reaction time, worsens object detection, and produces what’s known as inattentional blindness, an effect where the brain, busy processing the conversation, stops converting into usable information up to half of what’s in front of the car, even while the eyes are still looking straight at it [1]. In that same experiment, the average driver took 20% longer to brake and left 30% more following distance while on the phone than while driving without any distraction.
Pay Attention, Students
A team of consumer psychologists showed that the mere physical presence of your own phone on the table — even switched off or silenced, even if you never touch it during the whole session — reduces the cognitive capacity available for the task in front of you, and the effect is larger the more dependent you feel on the device [10]. It doesn’t need to ring, buzz, or light up. It’s enough for your brain to know it’s there, available, for part of your attentional resources to get spent, without you noticing, resisting the urge to look at it. It’s attention residue in its silent version.
You’re probably reading this on a screen right now, with more tabs open than you’d like to admit. Gloria Mark, a researcher at the University of California, Irvine, has been measuring how long people stay on a single screen before switching since 2004. Back then, the average was two and a half minutes. By 2012 it had dropped to 75 seconds. After repeating the study several times between 2014 and 2020, the number settled at 47 seconds [11]. Every one of those screen switches is, technically, a task-switching episode with its own cognitive cost, multiplied dozens of times an hour. And it doesn’t stop there: when something interrupts someone in the middle of a project — a notification, a question, whatever it is — Mark calculates it takes about 23 minutes, on average, to get back to the same level of concentration they had before the interruption [11].
The odds of being one of the people who can actually task-switch, remember, were 2.5% — roughly the same as winning something in a raffle with forty tickets. The rest of us are simply switching tabs very fast and calling it multitasking, phone face-down in one hand — well, next to us — this tab in the other, and the feeling, false but very convincing, that we’ve got it all under control.
References
[1] Watson, J.M. and Strayer, D.L. (2010). “Supertaskers: Profiles in extraordinary multitasking ability.” Psychonomic Bulletin & Review, 17(4), 479-485. With reservations — small sample, no robust replication
[2] Sanbonmatsu, D.M., Strayer, D.L., Medeiros-Ward, N. and Watson, J.M. (2013). “Who multi-tasks and why? Multi-tasking ability, perceived multi-tasking ability, impulsivity, and sensation seeking.” PLOS ONE, 8(1), e54402. Observational
[3] Pashler, H. (1994). “Dual-task interference in simple tasks: Data and theory.” Psychological Bulletin, 116(2), 220-244. Reliable
[4] Rubinstein, J.S., Meyer, D.E. and Evans, J.E. (2001). “Executive control of cognitive processes in task switching.” Journal of Experimental Psychology: Human Perception and Performance, 27(4), 763-797. Reliable
[5] Leroy, S. (2009). “Why is it so hard to do my work? The challenge of attention residue when switching between work tasks.” Organizational Behavior and Human Decision Processes, 109(2), 168-181. With reservations
[6] Cromer, J.A., Roy, J.E. and Miller, E.K. (2010). “Representation of multiple, independent categories in the primate prefrontal cortex.” Neuron, 66(6), 796-807. In primates
[7] Pinotsis, D.A., Fridman, G. and Miller, E.K. (2023). “Cytoelectric coupling: Electric fields sculpt neural activity and ‘tune’ the brain’s infrastructure.” Progress in Neurobiology, 226, 102465. With reservations, hypothesis
[8] Ophir, E., Nass, C. and Wagner, A.D. (2009). “Cognitive control in media multitaskers.” Proceedings of the National Academy of Sciences, 106(37), 15583-15587. Replications with somewhat different results but similar conclusion
[9] Minear, M., Brasher, F., McCurdy, M., Lewis, J. and Younggren, A. (2013). “Working memory, fluid intelligence, and impulsiveness in heavy media multitaskers.” Psychonomic Bulletin & Review, 20(6), 1274-1281. Reliable
[10] Ward, A.F., Duke, K., Gneezy, A. and Bos, M.W. (2017). “Brain drain: The mere presence of one’s own smartphone reduces available cognitive capacity.” Journal of the Association for Consumer Research, 2(2), 140-154. Reliable
[11] Mark, G. (2023). Attention Span: A Groundbreaking Way to Restore Balance, Happiness and Productivity. Hanover Square Press. Book