I am having a hard time digging it up, but I remember reading some reporting on the Netflix Prize that said (before Netflix abandoned the star system) that many users rated things only one-star or five-star.

But, counter to the OP's point, I wouldn't assume this is an attempt to move the average; I would guess this is for a number of reasons, including because it's too much mental energy to decide if a product (film) is worth four or five stars, if you rate something you are often just trying to say "liked it" or "didn't like it."