Simply put, you cannot breed aggression into a dog. That goes for aggression toward humans, dogs, and other animals. Aggression is, by definition, a behavior, and behavior is influenced by both genetics and environment. To quote from one of our affiliate’s sites, wherein temperament and behavior is discussed (http://happypitbull.com/basics/nature-vs-nurture), John Paul Scott is a renowned scientific researcher who was interested in the interplay between genetics and behavior. He did experiments with mice to determine whether highly aggressive mice could be created through breeding. “The experiments with mice show us that aggression has to be learned. Defensive fighting can be stimulated by the pain of an attack, but aggression, in the strict sense of an unprovoked attack, can only be produced by training… Heredity can enter into the picture only in such ways as lowering or raising the threshold of stimulation, or modifying the physical equipment for fighting… In considering hereditary effects, we must always remember that the environmental situation is also important…” – John Paul Scott, Aggression.
It’s important to discard the notion that pit bulls today are dog-aggressive as a result of their sordid history. Most of the “pit bulls” out there today are far removed from their history as fighting dogs. It’s like saying Dachshunds are bred to burrow into tunnels after badgers, or Poodles are bred to retrieve game. Most dogs of these breeds don’t do that anymore. They are bred to a physical standard, not for performance. Or they are bred in someone’s backyard as a pet, with minimal, if any, regard for appearance, health, or temperament. While some individual dogs may exhibit a tendency to behave in a way that is stereotypical or traditionally associated with the breed or type, it would be a fallacy to paint the entire breed-type with that brush.
Furthermore, if aggression was something we could breed into a dog, dog fighters would not need to train, condition, and torture their dogs so extensively. Unfortunately, since pit bull owners are continually told that their pit bull is naturally dog-aggressive (by other well-meaning pit bull owners, rescuers, and other misinformed individuals), this can become a self-fulfilling prophecy. Pit bull owners stop socializing their dog or taking it out in public for fear of an incident (a justifiable fear, since the “pit bull” is most likely to be blamed for any incident); they assume the worst about normal dog behaviors like grumbling, snapping, or even rough play between dogs; they fail to act when genuine aggressive behavior does arise, because they believe such behavior is in the dog’s nature and is not changeable; and they are told by trainers that the situation is hopeless. Compare this to an average Lab owner, who is not told from day one that his dog is going to be aggressive. The Lab owner will take his dog everywhere and socialize it, because he believes his dog is going to be a friendly, lovable dog. If and when aggressive behavior occurs, you can bet that owner is going to call up a trainer, because aggressive behavior in a Lab is not considered “normal.”
Respected national trainers like Marjorie Darby, Jean Donaldson, and Janis Bradley stress that dogs become what the owner expects them to become. For instance, despite the fact that “pit bulls” are not herding dogs, Diane Jessup obtained the highest possible herding titles on several of her pit bulls, apparently simply because she thought it would be fun. Yet many people would argue that herding is an “instinct” that is bred into herding breeds!
This is not to say that dogs (all dogs) are only dog-aggressive because their owner screwed up. Again, genetics does play a part; as John Paul Scott says (and Jean Donaldson says something very similar in her book, The Culture Clash), genetics raises and lowers thresholds of stimulation. Some dogs will be quicker to react than others in similar circumstances. It’s up to the owner to learn where those thresholds are, and to work within those thresholds.
However, it’s important to note that we are not capable of controlling those thresholds through breeding except in the very crudest manner—by breeding two dogs with similar temperaments and hoping that the offspring are given that temperament. It rarely works so easily. It is not the same as breeding for coat color (which is controlled by only one or two genes). The resulting puppies may have fairly uniform appearances, but their temperament is theirs and theirs alone, and it’s anyone’s guess how they’ll turn out. Neither dog-aggression nor human-aggression is exclusive to “pit bulls.” It is not a state of being. Any dog can exhibit such behavior if the environment allows for it.