They like to kill dogs... AND people.  Right?

Wrong.  Yes, the American Pit Bull was trained and bred to fight numerous things. Bears, Bulls, vermin, hogs, and of course, other dogs.  Today, they are known for the high occurence of dog aggression within the breed (making them unsuitable for dog parks), and often the presence of a high prey drive.  They often do best in single pet homes, and would do even better when owned by an person either experienced with dogs, or willing to educate themselves on the breed.

Never, until very recently, was any human aggression bred into the breed.  There are some very sick sites out there advertising the Pit Bull as personal protection dogs.  They choose some of the most poorly-bred Pit Bulls and Bandogs, and breed them to bring out the traits of human aggression.  The problem?  These dogs are finding their way more and more into the muddled gene pool of the American Pit Bull.  Is it the dog's fault?  No, but it puts them in the spotlight.... resulting in more and more cities banning them from their streets.
 
Generally speaking, all dogs are, by definition, territorial predators. All dogs are “naturally aggressive” to some extent, as it is through “aggressive” behaviors like fighting, defending, hunting, guarding, barking, biting, and posturing that dogs’ predecessors survived. These behaviors are also the reason why humans domesticated dogs in the first place. Dogs have long been bred and used for hunting, protection, fighting, and guarding. Thus, aggression is a natural component of all dogs’ behavior.

However, it is important to stress that aggression is not a state of being or a temperament (which would be solely affected by genetics). Aggression is a behavior. Behavior is the product of both genetics and environment (the way a dog is or is not raised and trained, and the circumstances a dog is put in by its owner). Therefore, a dog’s breed alone does not and can not shape behavior.  Socialization, training, management, and proper care–or lack thereof–can have a strong influence on a dog’s behavior. Because a dog’s owner determines the situations a dog faces, the dog’s owner has total control over whether a dog has the opportunity or the inclination to exhibit aggression.

It is also important to differentiate between different types of aggression. Aggression is a complex behavior that has many different causes and manifests in a variety of ways. Aggression does not necessarily equal danger to the public. Dogs are predators, and many will kill “prey” such as birds, cats, and rodents. This type of aggression does not automatically translate to aggression toward humans (including babies and children), though the media is filled with quotes from fearful citizens saying “It was a cat this time, but it could have been my grandbaby!” Many dogs are protective of their food or toys; this does not mean they are also prey-aggressive or human-aggressive.
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A LINK TO AN ARTICLE ADRESSING THE MYTHS SURROUNDING THE BREED

WHY DOGS BITE