"Upbringing" and "Training" has nothing to do with Pits and behavioral problems...
I would imagine that would be due to how they were raised, any dog has the capability to be vicious, but for the most part, proper upbringing will do wonders, kind of like people...
Nature or nurture eh? While any animal can be trained, either willingly or unwittingly, to be a nasty specimen it is inarguable that some breeds are more aggressive [also defensive, submissive, and most any other -isive] than others. Having owned Komondorak, Great Pyrenees, Basset Hounds, Laboradors, and others, and having had to deal with an Anatolian Shepard or two, and all manner of herding dogs it is my observation that the passive/aggressive tendencies greatly vary from breed to breed.
The most interesting was the Komondor. This is a a dog that has been breed for well over a thousand years to guard livestock. Not to herd livestock, other dogs do that, just to guard and nothing else. All of the fetching up in the world will not change an iota this breed's instinct to guard it's flock. Which includes the very real physical, perhaps deadly, hazard to anything it and it alone decides is a threat to that flock.
The point being that the Pit Bull, such as is is, likewise is a subspecies that has become popular largely because of it's image and tendencies to be a nasty piece of work. That being the case, the breeding just naturally follows. I should think that anyone owning one of these roughly falls into one of two categories. Those who what to project some sort of 'tough guy' image and those who want to be iconoclasts and demonstrate how popular knowledge is fraudulent.
But the statistics are the statistics and they are difficult to dismiss with self-serving rationalizations. Those statistics clearly indicate that this breed is indeed more likely to take a chunk out of your than any other.
Besides, I find this particular breed to be butt-ugly.