Heh, I know lots of serious adults who see movies more than once. They're usually fans of a series - Harry Potter any one? - so I think in genera' is not so much a true statement any more.
I don't know any adults who see movies more than once or, rarely, twice. My kids, now older, limit most viewings to three or four times, but used to watch the same movies 15 times. But obviously both your and my experiences are anecdotal, and I don't have any hard data readily available, nor am I willing to scour the web for statistics. So I'll admit my earlier statement was my impression based on observation and things I've read in the past, not on recent extensive research.
And to clarify my earlier statement, the phrase "serious adults" in the sentence "... serious adults rarely see movies more than once or twice ..." stemmed from the fact that we were talking about "serious adult movies" (movies like
Michael Clayton, apparently, as opposed to
Harry Potter). It wasn't a judgment of gravitas, not did it imply anything about the employment history, political involvement, etc., of specific moviegoers.