This article is from the American Pit Bull Terriers Breed FAQ, by Michael Bur with numerous contributions by others.
[Information gleaned from the ADBA phamplet titled "Discover the
American Pit Bull Terrier]
No, they do not have either. 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 comparision
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."
Futhermore, 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."