A robust body of peer-reviewed research has examined the effects of multitasking on productivity. The American Psychological Association summarizes findings that switching between tasks, which is what people typically mean by multitasking, consistently leads to more errors and longer completion times compared to focusing on one task at a time.
Stanford University researchers found that individuals who multitask frequently perform worse on tests of attention and memory than those who focus on one task. NIH-backed studies explain this as a result of 'switching costs,' where the brain expends additional effort to move between tasks, directly lowering output and efficiency.
Experimental economics research published in Cambridge Core further confirms that multitasking leads to significantly negative productivity effects, with one study recording a 23-point drop in performance scores. These findings have been replicated across a range of real-world and laboratory settings.