I echo this. I read someplace (can't recall now where) that once people see other trash in an area they are more likely to litter there themselves. Few people want to be the first to litter in the park, but if it is full of litter then who cares right?
I think you're remembering the broken windows theory [0], is that right? The a theory that visible signs of crime, anti-social behavior, and civil disorder create an urban environment that encourages further crime and disorder.
