Multitasking
Why the human brain is poorly designed for doing several things at once — and what the evidence says about the consequences
A. Multitasking — the simultaneous performance of two or more tasks — is widely regarded in contemporary working culture as a desirable and even admirable skill. The ability to manage multiple streams of work simultaneously is treated as evidence of professional competence, and the technologies of the digital age — smartphones, instant messaging, email, and social media — have created an environment in which attention is constantly divided between competing demands. The scientific evidence on whether human beings can genuinely multitask — and at what cost — is considerably less flattering to this cultural ideal than the ideal itself would suggest.
B. The brain does not truly perform two cognitive tasks simultaneously. What is experienced as multitasking is more accurately described as rapid task switching — the serial execution of tasks in rapid alternation, with the focus of attention shifting between them rather than being divided between them. Each switch between tasks involves a cognitive cost: a period during which performance on the new task is reduced as mental resources are reallocated and the mental context of the previous task is suppressed. This 'switch cost' is small for simple, well-practised tasks but substantial for complex tasks requiring sustained concentration, where it can amount to the equivalent of several seconds of full processing time.
C. Research consistently finds that attempting to perform two cognitively demanding tasks simultaneously reduces performance on both compared to performing each task sequentially. Studies of drivers who use mobile phones — even hands-free devices — consistently find impaired reaction times, reduced hazard detection, and poorer lane- keeping compared to undistracted drivers. The impairment is not caused by the physical act of holding a phone but by the diversion of cognitive resources to the conversation: a passenger in the car does not produce the same impairment because the passenger, unlike the remote caller, naturally adjusts their conversational demands when the driving situation requires it.
D. Individual differences in multitasking ability have been extensively studied, with the consistent finding that those who consider themselves to be the best multitaskers — and who multitask most frequently — tend to perform most poorly on laboratory measures of multitasking ability. Self-assessed heavy multitaskers show poorer performance on tasks requiring the filtering of irrelevant information, the updating of working memory, and the management of task-switching compared to light multitaskers. This counterintuitive finding has been interpreted as suggesting that heavy multitaskers have become accustomed to dividing their attention to the extent that their ability to sustain focused attention has been diminished.
E. Not all multitasking is equally demanding. Combining a cognitively demanding task with a routine physical one — listening to a podcast while doing household chores, or holding a simple conversation while walking — involves little or no performance cost because the tasks recruit different cognitive and motor systems that operate largely independently. Problems arise when both tasks compete for the same limited cognitive resource — particularly sustained attention, working memory, and the executive control functions of the prefrontal cortex. In these cases, the tasks interfere with each other at the neural level and performance costs are unavoidable.
F. The implications for workplace productivity and education are significant. Research on the impact of task interruptions — often used as a proxy for multitasking in naturalistic settings — finds that after a single interruption, the average knowledge worker takes more than twenty minutes to return to the task at the same level of deep engagement. Accumulating interruptions across a working day produce a cumulative loss of productive time that substantially exceeds the duration of the interruptions themselves. In educational settings, the presence of smartphones and the temptation to engage with social media has been found in multiple studies to reduce not only the performance of multitasking students but also the performance of nearby students who are distracted by observing others using devices.