This is a pretty informative site
American Temperament Test Society, Inc
The American Temperament Test Society, Inc. (ATTS) is a national not-for-profit organization (registered in the state of Missouri) for the promotion of uniform temperament evaluation of purebred and spayed/neutered mixed-breed dogs.
ATTS was established to:
Provide for a uniform national program of temperament testing of purebred and spayed/neutered mixed-breed dogs.
Conduct seminars to disseminate information to dog owners, dog breeders and evaluators (testers) concerning dog psychology, motivation, reaction and other aspects of temperament testing.
Recognize and award certificates to dogs that pass the requirements of the temperament evaluation.
Work for the betterment of all breeds of dogs.
Select, train, prepare and register temperament evaluators.
About Canine Temperament
ATTS Breed Statistics
Pitbulls and staffordshire terriers were among the highest scores of all breeds tested beating out the beagle and bearded collie.
Although the dogs that are being tested though are surely the dogs with responsible owners. You most certainly will not see someone that is breeding the dogs for fighting or that have been mistreating them in some other way bringing them in to be tested.Those people just don't care. I mean if you look at some of the things they do to these poor dogs to train them for fighting would make any breed of dog unstable or vicious.