The scientific consensus is clear: what most people call multitasking is actually the brain rapidly switching between tasks, not performing them simultaneously. The American Psychological Association explains that each switch imposes a measurable "switching cost" — a delay and increased error rate that compounds over time. Stanford University research found productivity can drop by as much as 40% under multitasking conditions.
Controlled experimental evidence backs this up. A Cambridge Core study in experimental economics found that the productivity effects of multitasking were significantly negative, with a difference-in-differences of −23 points (p=0.04) compared to sequential task completion. The NIH/PMC literature reinforces this, noting that the human brain simply lacks the architecture to process two complex tasks in parallel.
There are minor caveats: roughly 2.5% of people — sometimes called "supertaskers" — appear largely immune to these costs, and very simple or highly compatible tasks (like listening to music while walking) may not suffer the same penalties. But for the kind of cognitive work most people do professionally, the evidence firmly supports the claim that multitasking hurts productivity.