Normalisation of Deviance (Challenger)

Tom Geraghty · Stories & Cases, Safety & Human Error

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Explores Diane Vaughan's 'normalisation of deviance' (coined in The Challenger Launch Decision): the gradual process by which unacceptable practice becomes accepted as normal, as deviance repeated without catastrophic results becomes the social norm. It isn't about rule-breaking per se, nor intent to cause harm; it's the slow, interpersonally 'acceptable' drift from a safe condition toward greater risk, in a long incubation period before disaster. Every skipped checklist or silenced alarm makes the next one easier because it was fine last time. At NASA, a strong cultural faith in O-ring redundancy turned 'a no-go below 53°F' into 'lower temperatures are in the direction of badness' — and the same pattern recurred with Columbia's foam strikes 22 years later. Vaughan identifies production pressure, scarcity, competition for funds and structural secrecy as drivers, alongside a lack of psychological safety where those who notice the drift don't speak up. The piece extends the concept to the Costa Concordia (the captain's normalised off-route 'salute', promoted by Carnival as marketing) and the Boeing 737 MAX (MCAS issues known since 2016 but not urgent because nothing bad had yet happened). Feynman's image captures it: pulling the trigger in Russian roulette, the gun doesn't go off, so it must be safe to pull again — except we don't even know how many bullets are in the gun. Detecting and correcting the drift takes effort, reflection, and psychological safety; Vaughan notes it's easier to prevent than to correct.

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Read the full article on psychsafety.com →