Aggression is the most common and most serious behavior problem in dogs. It’s also the number-one reason why pet parents seek professional help from behaviorists, trainers and veterinarians.
What Is Aggression?
The term “aggression” refers to a wide variety of behaviors that occur for a multitude of reasons in various circumstances. Virtually all wild animals are aggressive when guarding their territories, defending their offspring and protecting themselves. Species that live in groups, including people and dogs, also use aggression and the threat of aggression to keep the peace and to negotiate social interactions.
To say that a dog is “aggressive” can mean a whole host of things. Aggression encompasses a range of behaviors that usually begins with warnings and can culminate in an attack. Dogs may abort their efforts at any point during an aggressive encounter. A dog that shows aggression to people usually exhibits some part of the following sequence of increasingly intense behaviors:
- Becoming very still and rigid
- Guttural bark that sounds threatening
- Lunging forward or charging at the person with no contact
- Mouthing, as though to move or control the person, without applying significant pressure
- “Muzzle punch” (the dog literally punches the person with her nose)
- Showing teeth
- Snarl (a combination of growling and showing teeth)
- Quick nip that leaves no mark
- Quick bite that tears the skin
- Bite with enough pressure to cause a bruise
- Bite that causes puncture wounds
- Repeated bites in rapid succession
- Bite and shake
Dogs don’t always follow this sequence, and they often do several of the behaviors above simultaneously. Many times, pet parents don’t recognize the warning signs before a bite, so they perceive their dogs as suddenly flying off the handle. However, that’s rarely the case. It can be just milliseconds between a warning and a bite, but dogs rarely bite without giving some type of warning beforehand.
Classification of Aggressive Behavior
If your dog has been aggressive in the past or you suspect she could become aggressive, take time to evaluate the situations that have upset her. Who bore the brunt of her aggression? When and where did it happen? What else was going on at the time? What had just happened or was about to happen to your dog? What seemed to stop her aggression? Learning the answers to these questions can clarify the circumstances that trigger your dog’s aggressive reaction and provide insight into the reasons for her behavior. You need an accurate diagnosis before you can hope to help your dog.
Aggressive behavior problems in dogs can be classified in different ways. A beneficial scheme for understanding why your dog is aggressive is based on the function or purpose of the aggression. If you think of aggression this way, you can determine what motivates your dog to behave aggressively and identify what she hopes to gain from her behavior.
Dogs’ wild relatives are territorial. They live in certain area, and they defend this area from intruders. Wolves are highly territorial. If a coyote or a wolf who’s not part of a pack invades their territory, the resident wolves will attack and drive off the intruder. Some dogs display the same tendencies. They bark and charge at people or other animals encroaching on their home turf. Dogs are often valued for this level of territorial behavior. However, some dogs will attack and bite an intruder, whether the intruder is friend or foe. Territorial aggression can occur along the boundary regularly patrolled by a dog or at the boundaries of her pet parents’ property. Other dogs show territorial aggression only toward people or other animals coming into the home. Male and female dogs are equally prone to territorial aggression. Puppies are rarely territorial. Territorial behavior usually appears as puppies mature into adolescence or adulthood, at one to three years of age.
Dogs are a social species. If they were left on their own, they would live together in small groups, or packs, of family and friends. If one member of a pack is in danger, the others typically rush in to help defend that individual. This is classified as protective aggression because the dogs are protecting one of their own. Pet dogs may show the same type of aggressive behavior when they think that one of their family members or friends (human or animal) is in peril. Sometimes dogs reserve protective aggression for individuals they consider particularly vulnerable. A dog who has never shown aggression to strangers in the past might start behaving aggressively when she has a litter of puppies. Likewise, a dog might first show protective aggression when her pet parents bring a human child into the family. While this behavior sounds appealing at first glance, problems arise when the protective dog starts to treat everyone outside the family, including friends and relatives, as threats to the baby’s safety. Both male and female dogs are equally prone to protective aggression. Puppies are rarely protective. Like territorial behavior, protective aggression usually appears as puppies mature into an adolescence or adulthood, at one to three years of age.
Dogs evolved from wild ancestors who had to compete for food, nesting sites and mates to survive. Even though our pet dogs no longer face such harsh realities, many still show the tendency to guard their possessions from others, whether they need to or not. Some dogs only care about their food. These dogs might react aggressively when a person or another animal comes near their food bowl or approaches them while they’re eating. Other dogs guard their chew bones, their toys or things they’ve stolen. Still others guard their favorite resting spots, their crates or their beds. (Often, these dogs also guard their pet parents’ beds!) Less common are dogs who guard water bowls. Usually a possessive dog is easy to identify because she’s only aggressive when she has something she covets. But some dogs will hide their cherished things around the house and guard them from unsuspecting people or animals who have no idea that they’re anywhere near a valued object. Male and female dogs are equally prone to possessive aggression, and this type of aggression is common in both puppies and adults. For more detailed information about food-related possessive aggression and how to treat it, please see our article, Food Guarding .
When animals and people are afraid of something, they prefer to get away from that thing. This is called the flight response. But if escaping isn’t an option, most animals will switch to a fight response. They try to defend themselves from the scary thing. So a dog can be afraid of a person or another animal but still attack if she thinks this is her only recourse. A fearful dog will normally adopt fearful postures and retreat, but she may become aggressive if cornered or trapped. (Please see our article, Canine Body Language , for more information about what fearful, aggressive dogs look like.) Some dogs will cower at the prospect of physical punishment but attack when a threatening person reaches for them. Fearful dogs sometimes run away from a person or animal who frightens them, but if the person or animal turns to leave, they come up from behind and nip. This is why it’s a good idea to avoid turning your back on a fearful dog. Fear aggression is characterized by rapid nips or bites because a fearful dog is motivated to bite and then run away. Sometimes the aggression doesn’t begin with clear threats. A fearful dog might not show her teeth or growl to warn the victim off. In this kind of situation, the only warning is the dog’s fearful posture and her attempts to retreat. Male and female dogs are equally prone to fear aggression, and this type of aggression is common in both puppies and adults.
Closely related to fear aggression is defensive aggression. The primary difference is the strategy adopted by the dog. Defensively aggressive dogs are still motivated by fear, but instead of trying to retreat, they decide that the best defense is a good offense. Dogs who are defensively aggressive exhibit a mixture of fearful and offensive postures. (Please see our article, Canine Body Language , for more information about what defensive, aggressive dogs look like.) They may initially charge at a person or another dog who frightens them, barking and growling. Regardless of whether the victim freezes or advances, the defensively aggressive dog often delivers the first strike. Only if the victim retreats is the defensively aggressive dog likely to abort an attack. Male and female dogs are equally prone to defensive aggression. It’s slightly more common in adults than in puppies simply because dogs need to have some confidence to use this defensive strategy, and puppies are usually less confident than adults.
Animals who live in social groups, like people and dogs, typically live by certain rules in order to minimize conflict between group members. Canid species, including the dog, adopt a type of hierarchical order that influences which group members get first crack at food, the best resting spots and opportunities to mate. So rather than having to fight for access to valued things each and every time, those lower down on the totem pole know to wait until the higher-ups have had their share before taking their turn. These ordered relationships are frequently reinforced by displays of ritualized aggression. Individuals of high status use aggressive threats to remind the others of their place in the pack. The relationships between people and dogs who live together are certainly more complex than this simplified description, but it’s still important to know that a dog who perceives herself as high in status may show aggression toward family members. (This kind of behavior is sometimes called dominance or status-seeking aggression.) This is why a dog might be perfectly trustworthy with one pet parent but react aggressively toward the other or toward young children in the family. Such dogs are often described as “Jekyll and Hyde” because, most of the time, they’re happy-go-lucky, friendly dogs. But if they feel that someone in the pack has overstepped his or her bounds, these dogs can quickly resort to aggression. An aggressive response is usually provoked by things that a dog perceives as threatening or unpleasant, such as:
- Taking food away
- Taking a chew bone, toy or stolen object away
- Disturbing the dog while she’s sleeping
- Physically moving the dog while she’s resting
- Hugging or kissing the dog
- Bending or reaching over the dog
- Manipulating the dog into a submissive posture (a down or a belly-up position)
- Lifting or trying to pick up the dog
- Holding the dog back from something she wants
- Grooming, bathing, towelling or wiping the dog’s face
- Touching the dog’s ears or feet
- Trimming the dog’s nails
- Jerking or pulling on the dog’s leash, handling her collar or putting on a harness
- Verbally scolding the dog
- Threatening the dog with a pointed finger or rolled-up newspaper
- Hitting or trying to hit the dog
- Going through a door at same time as the dog or bumping into the dog
Social aggression is somewhat more common in males than in females and more common in purebreds than in mixed breeds. Puppies are rarely socially aggressive with people, but they can be with other dogs, particularly littermates. Social aggression usually develops in dogs between one to three years of age.
It’s important to realize that the complexities involved in social aggression are poorly understood and hotly debated by behavior experts. Some believe that all social aggression is rooted in fear and anxiety, while others believe that it’s motivated by anger and the desire for control. When consulting a professional, make sure you’re comfortable with her treatment recommendations. If the professional’s suggestions consist of techniques for instilling fear and respect in your dog, such as alpha rolls, scruff shakes and hanging, there’s a very good chance that your dog will get worse rather than better—and you might get bitten in the process. Punishment may be appropriate, but only when it’s well planned and limited in application. The judicious use of punishment should always be embedded in a program that’s based on positive reinforcement and trust.
Dogs can be like human children in that when they get frustrated, they sometimes lash out with aggression. A dog who’s excited or aroused by something but is held back from approaching it can become aggressive, particularly toward the person or thing holding her back. For instance, a frustrated dog might turn around and bite at her leash or bite at the hand holding her leash or collar. Over time, the dog can learn to associate restraint with feelings of frustration so that even when there’s nothing to be excited about, she tends to react aggressively when restrained. This explains why some normally friendly dogs become aggressive when put behind a gate, in a cage or crate, in a car, or on a leash. Likewise, a dog who loves people can still show surprising levels of aggression when her pet parent lifts her up so that guests can enter or leave the home. Male and female dogs are equally prone to frustration-elicited aggression, and this type of aggression occurs in both puppies and adults.
Redirected aggression is a lot like frustration-elicited aggression with the exception that the dog need not be frustrated. Redirected aggression occurs when a dog is aroused by or displays aggression toward a person or animal, and someone else interferes. The dog redirects her aggression from the source that triggered it to the person or animal who has interfered. This is why people are often bitten when they try to break up dog fights. When a person grabs or pushes a fighting dog, the dog might suddenly turn and bite. Another example is when two dogs are barking at someone from behind a fence. Sometimes one will turn and attack the other. Male and female dogs are equally prone to redirected aggression, and this type of aggression occurs in both puppies and adults.
An otherwise gentle, friendly dog can behave aggressively when in pain. That’s why it’s so crucial to take precautions when handling an injured dog, even if she’s your own. A dog with a painful orthopedic condition or an infection might bite with little warning, even if the reason you’re touching her is to treat her. The improper use of certain pieces of training equipment, such as the pinch (or prong) collar or the shock collar, can inflict pain on a dog and prompt a pain-elicited bite to her pet parent. Male and female dogs are equally prone to pain-elicited aggression, and this type of aggression can occur in both puppies and adults.
Even though pet dogs rarely have the opportunity to reproduce, intact male dogs will still vie for the attention of females in heat, and females will still compete for access to a male. Intact male dogs sometimes challenge and fight with other male dogs, even when no females are present. Fighting can also erupt between males living together in the same household. In the wild, this is adaptive because the strongest males are more likely to attract females for breeding. Likewise, females living together in the same household might compete to establish which female gets access to a male for breeding. This type of aggression is rare. It’s observed most often in reproductively intact males and less often in intact females. Dogs who were neutered or spayed as adults may still show this type of aggression. If sex-related aggression happens, the dogs involved are usually at least one to three years of age.
Dogs are closely related to wolves and coyotes, both of whom are large predators, and pet dogs still show some classic canine predatory behaviors, including chasing and grabbing fast-moving things. Many dogs love to chase running people, people on bicycles and inline skates, and cars. They might also chase pets, wildlife and livestock. Some dogs bite and even kill if they manage to catch the thing they’re chasing. Predatory aggression is very different from other classifications of aggression because there’s rarely any warning before an attack. A predatory dog doesn’t growl or show her teeth first to warn her victim, so predatory aggression can seem to come out of the blue. Predatory behavior can be especially disturbing if it’s directed toward a human baby. Sometimes the sound of a baby crying or the movement of lifting a baby out of a crib can trigger a lightening-fast reaction from a predatory dog. Fortunately, predatory aggression directed toward people or other dogs is extremely rare in pet dogs. For more information about predatory aggression and how to deal with it, please see our articles, Predatory Behavior in Dogs , Dogs Chasing Bicycles, Skateboards and Other Moving Things , Dogs Chasing Cars , Dogs Chasing Cats , Dogs Chasing Children , Dogs Chasing Runners  and Dogs Chasing Wildlife .
Family Members, Strangers or Other Animals
Determining whom your dog is aggressive toward is essential to understanding her behavior. It’s common for dogs to behave aggressively toward unfamiliar people. Some studies report that as many as 60 to 70% of all pet dogs bark threateningly at strangers and act unfriendly when around them. Aggression toward unfamiliar dogs is also widespread. It’s less common for dogs to direct aggression toward family members or other pets in the home. Most problematic are dogs who are aggressive toward children, especially children in the family. Not only is aggression toward children exceedingly difficult to treat because of safety concerns, the likelihood that a dog with this problem will ever become trustworthy is slim.
Some dogs are aggressive only to a certain category of people. A dog might be aggressive only with the veterinarian or groomer, or with the postal carrier, or with people in wheelchairs or individuals using canes and walkers. In some cases, it’s easy to limit a dog’s access to the people that upset her. For instance, if your short-haired dog dislikes the groomer, you can just groom her yourself at home. But in other cases, the targeted people are impossible to avoid. For example, if you have a dog who dislikes children and you live in a densely populated urban apartment building next to a preschool, it will be difficult to avoid exposing your dog to children.
Aggression toward people, aggression toward dogs and aggression toward other animals are relatively independent patterns of behavior. If your dog is aggressive toward other dogs, for example, that doesn’t mean she’s any more or less likely to be aggressive toward people.
If you’re deciding whether to live with and treat your aggressive dog, there are several factors to consider because you, as the pet parent, are ultimately responsible for your dog’s behavior. These factors involve the level of risk in living with your dog and the likelihood of changing her behavior:
- Size. Regardless of other factors, large dogs are more frightening and can inflict more damage than small dogs.
- Age. Young dogs with an aggression problem are believed to be more malleable and easier to treat than older dogs.
- Bite history. Dogs who have already bitten are a known risk and an insurance liability.
- Severity. Dogs who stop their aggression at showing teeth, growling or snapping are significantly safer to live and work with than dogs who bite. Likewise, dogs who have delivered minor bruises, scratches and small punctures are less risky than dogs who have inflicted serious wounds.
- Predictability. Dogs at the highest risk of being euthanized for aggression are those who give little or no warning before they bite and who are inconsistently, unpredictably aggressive. Dogs who give warning before they bite allow people and other animals time to retreat and avoid getting hurt. As counterintuitive as it might seem, it’s easier to live with a dog who always reacts aggressively when, for instance, every time you push him off the bed than a dog who does so only sporadically.
- Targets. How often your dog is exposed to the targets of her aggression can affect how easy it is to manage and resolve her behavior. A dog who’s aggressive to strangers is relatively easy to control if you live in a rural environment with a securely fenced yard. A dog who’s aggressive to children can be managed if her pet parents are childless and have no friends or relatives with children. A dog who is aggressive to unfamiliar dogs poses little difficulty for pet parents who dislike dog parks and prefer to exercise their dog on isolated hiking trails. In contrast, living with a dog who has recurring ear infections and bites family members when they try to medicate her can be stressful and unpleasant.
- Triggers. Are the circumstances that prompt your dog to behave aggressively easy or impossible to avoid? If your dog only guards her food while she’s eating, the solution is straightforward: Keep away from her while she’s eating. If no one can safely enter the kitchen when your dog’s there because she guards her empty food bowl in the cupboard, that’s another story. If your dog bites any stranger within reach, she’s a lot more dangerous than a dog who bites strangers only if they try to kiss her.
- Ease of motivating your dog. The final consideration is how easy it is to motivate your dog during retraining. The safest and most effective way to treat an aggression problem is to implement behavior modification under the guidance of a qualified professional. Modifying a dog’s behavior involves rewarding her for good behavior—so you’ll likely be more successful if your dog enjoys praise, treats and toys. Dogs who aren’t particularly motivated by the usual rewards can be especially challenging to work with, and the likelihood of such a dog getting better is small.
Always Work with Your Veterinarian
Some aggressive dogs behave the way they do because of a medical condition or complication. In addition to acute painful conditions, dogs with orthopedic problems, thyroid abnormality, adrenal dysfunction, cognitive dysfunction, seizure disorders and sensory deficits can exhibit changes in irritability and aggression. Geriatric dogs can suffer confusion and insecurity, which may prompt aggressive behavior. Certain medications can alter mood and affect your dog’s susceptibility to aggression. Even diet has been implicated as a potential contributing factor. If your dog has an aggression problem, it’s crucial to take her to a veterinarian, before you do anything else, to rule out medical issues that could cause or worsen her behavior. If the veterinarian discovers a medical problem, you’ll need to work closely with her to give your dog the best chance at improving.
Always Work with a Professional Behavior Expert
Aggression can be a dangerous behavior problem. It’s complex to diagnose and can be tricky to treat. Many behavior modification techniques have detrimental effects if misapplied. Even highly experienced professionals get bitten from time to time, so living with and treating an aggressive dog is inherently risky. A qualified professional can develop a treatment plan customized to your dog’s temperament and your family’s unique situation, and she can coach you through its implementation. She can monitor your dog’s progress and make modifications to the plan as required. If appropriate, she can also help you decide when your dog’s quality of life is too poor or the risks of living with your dog are too high and euthanasia is warranted. Please see our article, Finding Professional Help , to learn how to find a Certified Applied Animal Behaviorist (CAAB or ACAAB), a veterinary behaviorist (Dip ACVB) or a Certified Professional Dog Trainer (CPDT) in your area. If you choose to employ a CPDT, be sure that the trainer is qualified to help you. Determine whether she has education and experience in treating canine aggression, as this expertise isn’t required for CPDT certification.
Can Aggression Be Cured?
Pet parents of aggressive dogs often ask whether they can ever be sure that their dog is “cured.” Taking into account the behavior modification techniques that affect aggression, our current understanding is that the incidence and frequency of some types of aggression can be reduced and sometimes eliminated. However, there’s no guarantee that an aggressive dog can be completely cured. In many cases, the only solution is to manage the problem by limiting a dog’s exposure to the situations, people or things that trigger her aggression. There’s always risk when dealing with an aggressive dog. Pet parents are responsible for their dogs’ behavior and must take precautions to ensure that no one’s harmed. Even if a dog has been well behaved for years, it’s not possible to predict when all the necessary circumstances might come together to create “the perfect storm” that triggers her aggression. Dogs who have a history of resorting to aggression as a way of dealing with stressful situations can fall back on that strategy. Pet parents of aggressive dogs should be prudent and always assume that their dog is NOT cured so that they never let down their guard.
Are Some Breeds More Aggressive Than Others?
It’s true that some breeds might be more likely to bite if we look at statistics gathered on biting and aggression. There are many reasons for this. One likely reason is that most dog breeds once served specific functions for humans. Some were highly prized for their guarding and protective tendencies, others for their hunting prowess, others for their fighting skills, and others for their “gameness” and tenacity. Even though pet dogs of these breeds rarely fulfill their original purposes these days, individuals still carry their ancestors’ DNA in their genes, which means that members of a particular breed might be predisposed to certain types of aggression. Despite this, it’s neither accurate nor wise to judge a dog by her breed. Far better predictors of aggressive behavior problems are a dog’s individual temperament and her history of interacting with people and other animals. You should always research breeds to be sure that the breed or breed mix you’re interested in is a good fit for you and your lifestyle. However, the best insurance policies against aggression problems are to select the best individual dog for you (please see our article, Choosing a Puppy from a Litter , for more information) and to provide her with appropriate socialization as a youngster (please see our article, Socializing Your Puppy ).