Aggression is a serious problem in dogs and one that is becoming increasingly common. The consequences from aggression are quite severe; for the victim and their family, the dog’s owners and the dog itself. 
Aggression!The potential for serious emotional and financial cost is high. The victim may experience severe lacerations, disability or loss of life. The dog’s owners may have to pay several thousands of dollars in hospital bills, lawsuits and fines. The dog may be impounded or euthanized.
As with many things in life; an ounce of prevention is worth a pound of cure. 
About 50% of dog bites involve children. Children, especially young children, tend to be low to the ground, make quick movements (which may frighten a timid dog), run (which will set off a dog’s chase instinct), squeal, scream or otherwise make loud noises (which may set off a dog’s prey drive). Children who are used to dogs tend to run towards dogs (which a dog may perceive as charging), make eye-contact with them (which a dog may see as a threat) and smile (which to a dog, might look as if they are bearing their teeth).   
Unsupervised toddlers may wander towards a dangerous dog or behave inappropriately to an unsocialized dog. NEVER LEAVE A YOUNG CHILD UNATTENDED WITH A STRANGE DOG. 88% of fatal dog attacks among toddlers occurred when there was no adult supervision.(1)Teach your child how to properly greet a dog and keep a watchful eye that everyone is playing safe. 
“Aggression” can encompass a wide variety of behaviours that can arise for any reason, under any circumstance at anytime. Always remember that all dogs bite. Saying a dog will never bite is like saying a human being will never hit. They will in the right circumstances and it is up to us to minimize the chance of it happening. Dogs (and all animals) can only respond to danger in one of two ways: either to flee or to fight. A leashed or tethered dog can not flee, he will have little choice but to bite. This is often mistaken as aggression, but it’s more ‘fear-aggression’ A fearful dog can be far more dangerous than a confidant, dominant dog because he perceives danger but cannot always run. 
Typically, a dog will give warning before aggressing. 
+Stiff body posture, especially if he leans towards or away from you.
+Deep chest growl
+Lunging or charging
+Baring teeth.
Dogs almost always warn before biting (even if it is only a matter of seconds between the warning and the bite) the exception is a dog who has been punished in the past for growling. Never strike a dog who growls or barks, never use choke chains, prong collars or shock collarsto correct a dog who displays these signs as these will create a dangerous dog. Many times people don’t recognize warning signs so they believe that the dog bites ‘out of nowhere.’ Always make sure that you understand how to read a dog’s warning signs. 
Aggression!Determining whether or not a dog is aggressive is not a precise science since there are eight functionally classified types of aggressive behaviour: fear, dominance,* possessiveness, protectiveness, predation, punishment, pain and intraspecific. (2) It is interesting and should be of note that while mammals all possess the same core emotions  many of these categories show different neurobiological mechanisms in dogs or other species. These mechanisms affect the probability of a bite when a dog is restrained, in competition (over resources) or encounters fearful stimuli. One can not refer to a dog as “aggressive” as this is grossly inaccurate. One can only say whether a dog displays aggressive behaviours and ideally, what caused the bite should be classified among the category that best describes what sparked the event. (Fear response, possessiveness, etc.) 
*In the years prior to Moyer’s research it has come to light that so-called dominance aggression is indeed a misclassification as truly dominant dogs and wolves don’t aggress. Complicating the fact is that  so-called dominance is really a set of identifiable behaviours that can change. (3) However, this, in no way, should detract from Moyer’s work.
Fear Aggression
As I mentioned earlier, there are only two responses to danger; fight or flight. When a stimulus that sparks fear is present, the animal (including humans) prefers to escape from whatever is sparking the fear response. However, escape is not always an option. If an animal cannot run from the stimulus it will have little recourse but to switch to the fight mechanism. They will try to defend themselves from the Big Scary Thing. A dog can fear a dog, person, snake, monkey, elephant or whatever and still attack if she thinks that is her only option. A fearful dog will first display fear posture  and try to run but then turn aggressive when corneredNever, ever corner an animalFearful dogs may retreat when pursued, but when the person or animal turns to leave, may run up from behind and nip. Usually fearful dogs will bite multiple times, but never put a lot of force in each bite, usually enough to pierce the skin, though. A close cousin to fear aggression is Defensive Aggression. Defensive aggression is also motivated by fear, however a defensive dog will bite first and ask questions later. A defensive dog will very rarely attack a person who calmly walks away from them, but will not hesitate to pursue a dog or person who either advances towards the dog or freezes. 
Possessive Aggression 
Wolves have to compete for food and shelter just to survive in the wild. They have to compete for mates to pass on their genes and avoid becoming an evolutionary failure. Until the Industrial Revolution, dog food didn’t exist, the average dog had to compete for food. Although today’s dogs generally have it better than their wolf cousins and their scavenger ancestors, some dogs still show guarding tendencies. Some dogs will display aggression when another dog or person comes near their toys, bones or food. Some dogs display guarding behaviours towards their crate, bed or furniture.    
Protective Aggression
Wolves are social pack animals who look after their own. If one pack mate is in danger, the rest rush in to protect their brother. Dogs will sometimes display protective aggression when their human pack is in danger, particularly if said human pack member is an infant or child. In extreme cases, a dog may perceive anybody; friend, extended family member or stranger as a threat. Protection aggression won’t manifest until a dog reaches maturity (if at all). 
Although dogs have come in from the wild some 35,000 years ago, instinct isn’t something that goes away overnight. Some dogs will still exhibit predatory response to fast moving people, animals or objects. Some dogs who chase small animals may bite and kill those animals should they catch them. Some dogs will “catch and kill” squeaky toys if left too long with them. Though rare, some dogs will have predatory drive towards infants crying since they high pitched cries can sound like certain prey. However, the odds of this happening are extremely slim. 
Predatory aggression differs from other types of aggression since there is little to no warning before an attack. 
A dog (or any animal) who is experiencing pain may behave aggressively. A dog who is experiencing chronic pain may be more likely to show aggressive response. Aggression brought about by pain may bite with little warning. All first aid kits should contain a muzzle and take precautions when treating a sick or injured dog. 
Training collars (pinch, prong or shock) can sometimes elicit pain aggression in dogs, especially when used incorrectly.  
Intraspecific Aggression. 
This is aggression towards other dogs and can be divided into two camps: dog aggression and inter pack aggression. Dog aggressive dogs are typically happy friendly dogs towards other people and children but can turn on a dime and aggress towards other dogs. Particularly unknown dogs. This can be motivated by fear, protection, territory, predation or other factors but it is unique in that only other dogs are targeted. Inter pack aggression is aggression directed towards other members of the pack. Social pack animals often live by certain rules set to minimize conflict. These rules keep order, but if one dog believes that another dog broke those rules, than a fight may begin. Social relationships are very complex but anything as simple as sleeping in another’s spot, or eating from the wrong bowl or as complex as an ‘alpha roll’ may provoke inter pack aggression.  
Territorial Aggression
Dogs’ closest cousins, the wolves, are very territorial. The live in a clearly defined territory which they defend from invaders. If an animal who is not part of the pack enters their territory the wolves will drive off they intruder. We sometimes see this instinctive behaviour in dogs. They charge at people or animals who they feel is a threat to their territory. Many people value this trait in dogs and purchase dogs with the intent and expectation that the dog will defend the person’s property. This can be a good idea that isn’t. A dog can not always tell the difference between a friend, the mailman, the delivery boy, your grand mom and a robber. Training a dog for the purpose of defending your territory is akin to setting a loaded handgun with a hair-trigger on your coffee table. 
Some dogs defend the boundaries of the person’s property, some defend the home itself and some defend the actual person (though often this is categorized as protective aggression or possession aggression). As your dog reaches maturity he may begin to to develop territorial aggression if the behaviour is not checked at a young age. 
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If your dog is displaying aggressive behaviours it is wise to consult your friendly, neighbourhood veterinarian, if for no other reason than to rule out a medical condition. Dogs in chronic pain, cognitive disorders, seizures, sensory deficits, orthopedic problems or other medical conditions are more likely to undergo personality changes, short tempers or aggression. If a medical condition is the cause you will need to cooperate with your vet to improve the outcome. 
Do not attempt to treat an aggressive dog yourself. Seek professional help.  A qualified professional can help you customize a treatment plan to meet your individual dog’s unique needs. Ask your vet for recommendations on a certified animal behaviourist. Not all trainers are qualified to help you so make sure that she has the education and experience necessary to treat canine aggression.
How to prevent aggression
Train your dogA dog should be able to sit, lie down, stay and walk politely on lead. A dog who is under your verbal  and/or visual command is less likely to be a threat to himself or others. 
Socialize your dog. Between the ages of 8 and 16 weeks and again between the ages of 2 and 3, a dog should come in contact with 100 different people (of different sizes, disabilities, ages and races) and 100 different dogs, cats and other animals. As they age, continue to expose them to different people and animals. This will teach them to learn important communication and gain confidence. Well socialized dogs get into fewer fights than under-socialized dogs. 
Spay and Neuter your dog. Un-neutered males are more likely to bite than neutered males. Female dogs in heat, pregnant females and lactating females are more likely to bite and behave in an unpredictable manner. 
Never leave your dog unattended.  If you can’t monitor your dog you won’t be able to see what provokes him,  and you won’t be able to stop someone from teasing or injuring him.
Don’t chain a dog. Dog’s who spend their lives chained up experience more stress, frustration, vulnerability, and territoriality than dogs who are indoors or at least behind fences. They are over twice as likely to bite as dogs who do not spend their lives tethered. 
Breed Specific Legislation (BSL) 
Some communities have enacted breed specific legislation that makes ownership of certain breeds illegal. Research indicates that BSL does little to change dog bite statistics. This is likely because any dog can bite when provoked and BSL does not address the actual problems that cause dog bites to be on the rise. (Factors which include poor training or complete lack of training, isolation, restraint (being chained up or on lead), fences (both invisible and visible) and lack of socialization, among others.)
Breed Specific Legislation is very difficult to enforce and often the counties that enact them end up spending hundreds of thousands of dollars trying to enforce the bans. Responsible breeding would go much farther and be much cheaper than breed specific legislation, especially when there is no science, research or evidence to support that one breed of dog is inherently  more aggressive than another. It is knee-jerk legislation and  anti-intellectualism at its worst…and innocent dogs suffer and die from it. 
Public education and enforcement of existing laws would are the most effective means of reducing dog bites. 
Don’t be a Victim
If you spot a dog who is unrestrained and poised to attack, you can still protect yourself. 
Don’t cut and run.  If you run, you will ignite the dog’s chase instinct and he will catch you if he wants to. An average dog can run 20 miles per hour. You can’t. 
Remain motionless. Keep your hands at your side, and avoid making eye contact with an angry dog. But keep an eye on him with your peripheral vision. 
Never turn your back on an angry dog. You want to be able to try and predict a dog’s moves and you can’t do that if you can’t see him. 
Slowly back away from the dog until he loses interest or is safely out of sight. 
If the dog does attack; Curl into a ball on the ground, with your face tucked into your chest, while covering the back of your head and neck with your hands and remain motionless. This will protect your vital organs, face and neck and reduce the likelihood of the attack becoming fatal. 
(1) American Humane Society
(2) Moyer KE: Kinds of Aggression and their physiological basis Part A. 
(3) Morgan Spector: Moving Beyond the Dominance Myth.