When your dog demonstrates aggressive behavior, you need to take immediate action to mitigate the behavior before it continues or escalates. Two training methodologies are recommended eliminate aggression: operant conditioning and classical conditioning. In both cases, the focus is not on punishing aggressive behavior. Instead, the focus is on replacing undesirable behavior with a more desirable behavior.
Dogs display aggressive behavior for a range of reasons. Provocation or antagonization can make a dog become aggressive. Likewise, some dogs who are afraid of certain stimuli can become aggressive. The first step in dealing with aggression is identifying two things: What triggers your dog to become aggressive, and what his aggression signals are. Examples are baring teeth, growling, body stiffening, and other behaviors.
Operant conditioning hinges on the idea that animals will learn to behave in a manner that earns them a reward and helps them avoid punishment. The dog forms an association between a behavior and a consequence. When a dog faces a stimulus that causes aggression and he does not react, you give him a reward -- usually praise and a dog treat. For this to work, the dog must be presented with the stimulus from a distance at first. The distance depends on the stimulus and the dog's usual reaction, but the goal is to start with the dog and the stimulus far enough apart that the dog does not react. Reward the dog for not reacting. Then, gradually move the stimulus closer. Each time the dog doesn't react, he earns a reward. If he does react, move away from the stimulus and try again.
Classical conditioning creates an automatic response to a stimulus. The best example of this is the well-known experiment with Pavlov's dogs. His dogs learned that a ringing bell brought dinner, so they would salivate when they heard the bell. With aggressive dogs, the goal is to create an automatic response when the dog is faced with the stimulus that causes aggression. For example, a dog that becomes aggressive in the presence of other dogs should be taught to sit anytime he sees another dog. When he sits -- instead of reacts to the stimulus -- he earns a reward. Over time, with repetition, eventually the dog learns that sitting when he sees another dog earns him a reward, while acting aggressive does not.
Handling an aggressive dog takes skill and experience. If your dog displays signs of aggression, you can change that behavior. However, it is imperative that you work with a qualified, professional dog trainer. Seek out a trainer who uses positive reinforcement. Avoid trainers who use physical corrections. In a University of Pennsylvania School of Veterinary Medicine study published in early 2009, researchers found that type of training can actually exacerbate aggression. If not addressed immediately, aggression can escalate. Work with a trainer at the first sign of aggression.