Lack of evidence = lack of evidence.
Though there is plenty of evidence that John R. Lott, Jr. the man behind this 'research' has a history of inventing 'data' to prove all sorts of things. Along with inventing a certain "Mary Rosh" sockpuppet to tell the world what a marvellous person John Lott was, and how his research was impeccable.
http://reason.com/archives/2003/05/0...of-mary-rosh/1