Deep Work: Rules for Focused Success in a Distracted World · chapter 5 · id deep-work-c5-01-people-who-multitask-all-the-t

“People who multitask all the time cannot filter out irrelevancy, cannot manage working memory, and are chronically distracted, based on Clifford Nass's Stanford research.”

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Receipts

Ophir, Nass & Wagner (2009). Cognitive control in media multitaskers. PNAS, 106(37), 15583-15587.paywalled — manual check queued

Heavy media multitaskers performed worse on tests of task-switching ability, filtering irrelevant information, and working memory compared to light multitaskers.

Stanford Report: A decade of data reveals heavy multitaskers have reduced memory (2018)source alive

In about half of subsequent studies, heavy media multitaskers significantly underperformed on tasks of working memory and sustained attention; the other half showed null results.

This claim is a stable, citable object. If you can falsify a verdict, tell us — corrections are loud here.