Asked by: David Pryce Morris
When you’re truly shocked, you don’t cover your mouth; you just scream. Covering your mouth seems to be a secondary reaction, when you’re trying to moderate your initial response. That might be because you have realized there’s no danger or because you don’t want to alarm those around you. It probably evolved to avoid spreading unnecessary panic in groups.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.