In 1994 a Purdue University study looked at false accusations in a small town in the mid-west. They counted the number of false accusations. To be considered a false accusation the accuser herself had to admit the claim was false. When possible the researchers verified with the accused that the new version of events now matched the accused's version. 109 cases were examined. Fully 41% of the accusation were found false. The admitted motivations for the false accusations were alibis(50%), revenge(30%) and attention-seeking(20%). The above is a paraphrase of a much longer discussion. Google is your friend.