Fiction.  Aggression in dogs is caused by many factors, including, but not limited to, inbreeding, lack of socialization, poor health, poor diet and nutrition, and abusive handling. A recent study of fatal dog attacks on humans found that 90% of the dogs that attacked were intact males, 36% of attacks occurred near a litter of puppies, and 19% of dogs that attacked were tethered or chained.

The American Temperament Test Society, as of February 2012 found American Pit Bull Terriers had a passing score of 86.8%.  This is a VERY challenging test, and 839 pit bulls were tested.  By comparison, they scored higher than Collies (80.1%), Golden Retrievers (85.2%), Malteses (81.3%), and Schnauzers (78.9%).  They scored exactly the same as Standard Poodles.

When Michael Vick's 51 pit bulls were seized from his illegal dogfighting operation, only one dog was euthanized after being deemed to dangerous to be re-introduced into society.  A cream colored pit bull named Hector currently works as a service dog. These dogs were all subjected to unthinkable cruelty, in addition to the factors mentioned above.