Innate, trained, or a mix?
"Aggressive dogs aren't born, they're raised that way."
How many times have you heard this statement? Have you ever wondered how much truth there is to it?
Owners of reactive or aggressive dogs frequently say, "I've had other dogs before and none of them have been like this, so I don't know where I went wrong!"
I meet once a week with a couple of friends who have "dog-aggressive dogs." We work on behaviors like recalls and loose-leash walking with other dogs around. Each of us has two dogs and, without exception, just one of the dogs is reactive to other dogs while the other is completely sociable with other dogs. Why is this?
If owners had a deficiency in their ability to raise a pup, surely the deficiency would be manifest in other dogs that they owned? Is the aggressive behavior a problem with the reactive dog's temperament, the owner's raising and training of the dog, or a bit of both?
Aggressive behavior is reinforced
Let's begin by stating an important fact: aggressive behavior is operant behavior. Yes, yes, I know that aggression is usually a symptom of fear, anxiety, or stress (and, occasionally, instinct), and that most experts recommend classical conditioning to treat the cause of aggression. But the fact remains that the actual aggressive behavior is operant. It is the product of reinforcement.
An example is the dog who, uncomfortable around other dogs, barks out of fear. This may be unconditioned behavior the first time, but if it works and the other dog keeps his distance or leaves, it may serve to reinforce the behavior of barking. The barking then becomes operant behavior. The next time the fearful dog sees another dog, he has a behavior that has worked in the past to keep the other dog away. So, he repeats it.
The dog's owner is likely to be lambasted by well-meaning folk for not socializing a dog adequately, or for not being "alpha," or for not earning the dog's trust—or it may even be implied that the owner leads by a fearful or aggressive example!
Having followed the socialization prescription to the letter with one of my own dogs—a reactive dog—I would bet money that it is the quality of adequate socialization that inoculates the young dog against developing fear-based aggression, not the quantity of socialization. Socialization experiences that are not beneficial are just as likely to cause fear-based aggression as too few socialization experiences.
So, aggression is operant behavior, since every socialization experience that reinforces aggression maintains or increases it. That said, dog owners must provide frequent and varied socialization experiences so that the dog learns that other dogs and other people, new places, new sounds, and new objects are not something to be feared. This level of exposure is what I mean by "adequate"—enough to reinforce appropriate behaviors and have those behaviors generalize to new and varied experiences.
Certainly it is possible for a dog's owner to reinforce fearful or aggressive behaviors in a dog. This reinforcement can come from attempts at soothing, from removing the dog from stressful situations immediately after aggressive behavior has been displayed, from putting the dog into situations where the dog is likely to display reactive behavior in order to cope, or from botched attempts to reprimand aggressive behavior.
Set up for success—and get help when needed
The "Golden Rule" in dog training is to set the dog up for success, then reinforce that success. Experienced dog owners may pick up the signs of fear or anxiety early enough to take decisive and beneficial action, and then reinforce appropriate behaviors. Less experienced dog owners should seek help from a competent instructor as soon as they get a pup, or, if not then, at the first sign of trouble—before accidentally worsening the aggressive or fearful behavior.
A genetic predisposition toward fearful or aggressive behavior can make it very difficult for even experienced dog owners to avoid having fearful or aggressive behaviors reinforced in a dog. Some dogs will never be completely comfortable around whatever their aggression target is, even after being trained so that they don't display aggressive behavior any more. The safety of the aggression target is never assured. I trained one of my dogs not to attack people on bikes under virtually any circumstance, at any speed, even off-leash. Yet one day, while I was tying my shoelace, she lunged at a rider as he passed. I hadn't trained for that! Hope that the training will generalize, but even if it does, do not expect 100% reliability.
Some dogs, and some breeds in particular, are more likely to "fight" than "flee" when faced with a stressful situation. It is no coincidence that my golden retriever runs from a stressful situation, while my German shepherd will stay and face the threat head-on. Both responses are entirely normal for the breeds (to a degree). It would be very difficult to train a police dog who ran away when faced with threat. That doesn't mean that all phenotypically-ideal German shepherds will be aggressive, but it may mean that extra care is required to ensure that behaviors appropriate to the situation are reinforced and trained to fluency.
Separating interconnections is difficult
Dog owners certainly can and do make mistakes that contribute to a dog's aggression or reactivity, but it wouldn't be fair to place the blame solely with a dog's owner. There are many factors and events in a dog's life that contribute to aggressive or reactive behavior. If we could go back in time and erase all the owner's mistakes, in many cases the dog would still learn to use aggressive or reactive behaviors. The good news is that behavior is changeable. We can shape it one click at a time.