have you ever noticed...

that when watching anime comedy in a group of people, when a funny line comes onscreen, they won't laugh as soon as they read the subtitle? They'll wait until they hear the voice actor reach the part of the line with the proper inflection to laugh, even if they don't understand what he's saying.

I wonder why that is? Do people feel it inappropriate to laugh at something they've read, preferring to wait for the audio cue? Or maybe they just all finish reading the line at different times, and don't want to laugh alone, so they wait for a common starting point before laughing.