Multiple scientific studies have measured the impact of multitasking on productivity. According to research cited by the American Psychological Association and Stanford University, switching between tasks can reduce productive output by as much as 40%. Most people experience increased errors and slower work compared to focusing on tasks one at a time.
These findings have been confirmed in controlled experiments, such as those published in Cambridge Core's Experimental Economics (difference-in-differences of −23 points) and NIH/PMC reviews. The consensus is that the brain is not wired for parallel processing of complex tasks and incurs measurable 'switching costs.'
While a very small percentage of 'supertaskers' may be less affected, for the vast majority, multitasking leads to substantial productivity losses. This effect is most pronounced for complex or attention-demanding activities, and the statistics are robust across multiple studies and reputable sources.