The behavior: Physical attacks (including biting) on a person or another animal, or threatening behavior, including growling, snarling, snapping, loud rapid barking, and lunging.

Why dogs do it: Usually aggressive behavior arises from fear. The dog feels threatened by the presence or approach of another animal or a person and lashes out to defend himself. A dog may also display aggression to protect his territory (guarding the owner's house or car), to warn others away from his food or toys, or to assert dominance.

Aggressive behavior may be influenced by many factors, including:

• Genetic predisposition. Some breeds have been selectively bred for protective behavior.

• Insufficient social experience. A dog who had few opportunities to interact with other dogs and people as a puppy may overreact to strangers as an adult.

• Maturity. Some dogs are quite friendly as puppies but begin displaying guarding, threatening, and other aggressive behaviors on reaching adulthood (between 2 and 3 years of age).

• Unwitting encouragement from owners. Many people don't realize that if they try to soothe and reassure their dog when he gets belligerent, they're reinforcing the behavior. Other owners back off at any sign of aggression toward themselves, teaching the dog that he is in charge.

• Territoriality. A dog may guard his bed, toys, food, house, yard, family members, car, and the street he regularly walks on. In a new environment he may react aggressively to anything approaching him and even to being looked at by another dog or a person.

• Dominance. Aggressive behavior may arise during encounters with other dogs in an effort either to assert dominance or to resist being dominated. Similar status conflicts may also crop up between a dog and his human family members.

• Roughhousing. Play fighting between puppies is normal and includes playful growling, chasing, nipping, poking, biting, and wrestling. All this fun helps puppies figure out their place in the pack. Older dogs enjoy play fighting, too; it's a way to bond and get exercise. But if a roughhousing person gets into the mix, aggression can escalate, particularly if the dog wins the match. Innocent rough play between person and puppy can develop into serious aggression as the dog matures.

• Inability to escape. A dog who's cornered or tied up will react much more strongly to a perceived threat (such as a stranger approaching) than he would if he were free to move away.

• Pain. If a dog is sore or has an injury, he may react aggressively if someone touches that part of his body. This is a reflexive response caused by pain.

• Senility. Confusion or diminished senses (such as smell and sight) may interfere with an older dog's ability to recognize people, to anticipate being approached or handled, and to judge whether he's safe in a particular situation.

• Predatory instincts. Dogs are hunting animals. Most have an innate urge to chase anything moving away from them, whether it's a tennis ball or a delivery truck. Behaviorists don't classify hunting and chasing as aggressive behaviors unless they are directed toward people.

Training Tips

Seek professional help. Aggressive behavior is a serious problem and requires a thorough yet swift response from owners. See your veterinarian first: A thorough medical evaluation (including a thyroid test) will show whether any physical conditions might be lowering your dog's aggression threshold. Next, consult a professional animal behaviorist, who will pinpoint circumstances that trigger belligerence and customize a treatment plan for your dog.

Keep a journal. Record all the occasions in which aggression occurred. Note exactly what happened right before, during, and after the flare-up, including who was present, how they responded, and the time of day. If you have a video camera, keep it handy for documenting incidents. Involve your entire household in the detective work. Show the list to a behaviorist.

Avoid provocation. Once you have your inventory of situations that set your dog off, share it with your household. You should all make every effort to avoid these circumstances. Don't let your dog practice his aggression skills! For example, if he's threatening when people come to the door, don't let him greet visitors; put him in a safe place, such as his crate (he should be properly crate-trained), and let him join in only after he quiets down.

Never punish for aggressive behavior. Punishment will only intensify your dog's fear, exacerbating the aggression. In tense situations, remind him you're the leader, command him to sit, and praise him for doing so.

Go through basic training. An obedience course will give your dog discipline as well as something constructive to do with the energy he's using to be aggressive. Once he responds consistently to a few commands – for instance, he will sit to greet somebody – you can put him through his paces whenever he might be getting snappish. This kind of work is also great for your relationship, reinforcing your position as leader of his “pack.”

Teach your dog respect. Your dog needs to know that you're the leader of his “pack.” If you play with him, give him toys and treats, and pet him no matter how he behaves – just because he is fluffy and adorable – why would he ever clean up his act? Here are some tips on how to show him you're top dog:

• Make a list of your dog's favorite things: walks, meals, toys, treats, chasing balls, getting brushed, and so on. Every time you want to do one of these nice things for him, command him to “sit” or lie “down” first. It's as if you're having him say please.
• Remember to praise him every time he obeys you. Your feedback lets him know he's on the right track.
• Be consistent about showing him that you – and other human members of the household – control all the valuables in his life. To get what he wants, he must first do something you request. This call for a firm change in your behavior – not just for a few days but for the rest of his life.
• Don't allow him to rest on your bed, the couch, or any elevated surface (the position suggests higher status). If you can't bear not to let him up, make him sit for you first.

Spell out the rules of the game. You – not your dog – should always direct your play sessions. If you're going to play fetch, for example, begin by telling him to sit. When he does, throw the ball. Teach him to bring it back by offering to swap it for a treat. If he tries to play “keep away,” turn your back and ignore him until he drops the ball. Always be the one to end any game: Put the ball or other toy away, and tell him the game is over. Stop playing while he still wants more, before he gets tired and quits on his own. Remember: You are in charge.

Tools You Can Use

Gentle Leader. This head halter gives you better control of your dog than a standard collar does, and it even applies pressure in spots on the neck and muzzle that can trigger a more passive response. If your dog pulls or lunges while wearing the Gentle Leader, the leash will close his mouth and turn his head away from his target. You can prevent potential brawls by steering him away from dogs, kids, or other triggers before anything starts. Stop and change directions frequently, so your dog has to follow your lead.

Basket muzzle. If you must expose your dog to a volatile situation before he's trained to handle it, put a basket muzzle on him. He'll be able to pant and even bark, but he won't be able to bite. Use this device only in high-risk situations or while you're working on a treatment plan.

Balls and Frisbees. Aggression is often related to stress, and exercise is the universal stress reliever. Interactive play with a ball or Frisbee is great exercise and can strengthen your bond with your dog. If he's socialized, agility classes and other dog sports can also provide a fun workout for both of you.