None. There is an urban legend that 'Pit Bull Type Dogs' cannot release their jaws once they bite down. This is absolutely not true. 'Pit Bull Type' dogs wildly include any dog that makes the local media. Especially, American Pit Bull Terriers and Staffordshire Bull Terriers have been selectively bred to fight in dogfights, bait bulls and do any number of obscenities, unfortunately, and the tendency to go for the neck and hold has been highly valued by the scum that breed animals for fighting as opposed to the usual predatory response of grabbing and shaking. This is probably the original beginning of the urban legend. "The infamous locking jaw is a myth. The American Pit Bull Terrier and related breeds are physiologically no different from any other breed of dog. All dogs are from the same species and none have locking jaws. Dr. I Lehr Brisbin of the University of Georgia states, "To the best of our knowledge, there are no published scientific studies that would allow any meaningful comparison to be made of the biting power of various breeds of dogs. There are, moreover, compelling technical reasons why such data describing biting power in terms of 'pounds per square inch' can never be collected in a meaningful way. All figures describing biting power in such terms can be traced to either unfounded rumor or, in some cases, to newspaper articles with no foundation in factual data." Furthermore, Dr. Brisbin states, "The few studies which have been conducted of the structure of the skulls, mandibles and teeth of pit bulls show that, in proportion to their size, their jaw structure and thus its inferred functional morphology, is no different than that of any breed of dog. There is absolutely no evidence for the existence of any kind of 'locking mechanism' unique to the structure of the jaw and/or teeth of the American Pit Bull Terrier." (http://www.austinlostpets.com/kidskorner/2October/pitbull.htm) - You'll find a lot of impartial information on this site: http://www.pbrc.net/home.html And here: http://www.austinlostpets.com/kidskorner/2October/pitbull.htm I have a pitbull. It is much better natured than your average Golden Retriever or Cocker-Spaniel, and has never exhibited any aggressive traits whatsoever. It's all in the training. In fact, Pit Bull Type Dogs consistently rate as good or better in temperament compared to most common purebred home companion dogs. http://www.atts.org/stats1.html