In the hobbies of dog fancy and dog breeding, the term dog aggression describes canine-to-canine antipathy.
Aggression itself is usually defined by canine behaviorists as "the intent to do harm". Many dogs will show "displays of aggression" such as barking, growling, or snapping in the air without actually having any aggressive intent.
Dog aggression is a common dog behavior, and can be seen in all breeds of dogs, although some dog breeds have a predisposition to display such aggression. The breed standard usually spells out whether dog aggression is common in the breed and to what degree it is allowed. Most of the terrier breeds and the bull breeds have a higher likelihood of developing dog aggression. Individual dogs may or may not display the level of aggression that their breed standard suggests.
Factors contributing to the likelihood of the development of dog aggression include:
Dog aggression does not usually become a serious behavior in most dogs until they reach the age of 1.5 years. Prior to that age, most dogs show only the seeds of developing aggression, such as fear and/or nervousness around other dogs, displays of aggression only under certain circumstances (while on leash, in the presence of food, in the presence of the owner, etc.), or most commonly, over-the-top play behavior. Play behavior such as tackling, chasing, mouthing, nipping, pawing, and wrestling are all normal canine behaviors that serve the evolutionary function of preparing the young dog for later combat and hunting. Young dogs that engage in excessive amounts of these behaviors are much more likely to develop dog aggression as they age.
Dog aggression should not be confused with human aggression, which is allowed in some breed standards as well, as long as the aggression springs from a desire to defend the dog's owner or territory.
The United States has the highest reported incidence of dog aggression problems of any country in the world. One of the major contributing factors to the development of dog aggression is living as part of a multidog (more than one) household. More than a third of dogs in the United States—a higher percentage than any other country in the world—live as part of multidog households.
In order to really enjoy a dog, one doesn't merely try to train him to be semihuman. The point of it is to open oneself to the possibility of becoming partly a dog.
Paper and canvas prints of "Growing Up Chinese Shar-Pei" by Barbara Keith are available online.