There has been a lot of progress in the field of canine behavioural science in the last twenty years which has dispelled many long-held theories and practices. Despite this fact, many of those traditional theories and practices stubbornly persist.
One example is how so many people continue to mistakenly link dog aggression with specific breeds, and then propose and/or support breed specific legislation (aka: BSL) with the notion that it will safeguard the public.
Simply put, aggression is a complex issue which is comprised of many different factors but, unfortunately for dogs, we humans have a tendency to want easy answers and one-size-fits-all solutions. I say "unfortunately for dogs" because the antecedents that lead to incidents of dog aggression are often human-made, yet it's the dogs that suffer the consequences.
It has already been determined that the majority of dog-bit incidents towards humans aren't proactive or antagonistic - instead the underpinnings of bite incidents area most commonly driven by stress, anxiety and fear.
Why is that? It doesn't matter which species we're talking about, if you put an animal into a position, situation or environment where (a) it doesn't have the necessary coping skills, and/or (b) it perceives a threat, and/or (c) it cannot flee from a perceived threat, it will then protect itself. Protecting oneself is normal behaviour in such circumstances: Humans punch, horses kick, birds peck, dogs bite, etc.
Depending on the situation and/or environment, aggression can be deemed a normal behavioural response.
The "coping skills" mentioned above cannot be emphasized strongly enough because (and this is assuming that there isn't a neurological disorder or other 'condition' that is driving abnormal aggressive behaviour) a dog is a product of more than just it's hereditry - it's also a product of its upbringing and environment - and this is why the topic of socialization has come to the forefront in the last few years.
Socialization: "The process of learning interpersonal and interactional skills that are in conformity with the values of the society one lives in."
On this same topic, an interesting study from the School of Veterinary Science at the University of Bristol was published in the journal "Applied Animal Behaviour Science" which reinforces aspects of what other modern studies have shown.
The University researchers sent out a survey to approximately 15,000 dog owners in the UK regarding dog aggression directed towards humans, and of the 15,000 surverys distributed approximately 4,000 were returned.
In part, the statistical analysis of the study suggests:
Dogs owned by people younger than than the age of twenty-five are almost two-times more likely to be aggressive than dogs owned by people who are over the age of forty.
Neutered male dogs were twice as likely to show aggression than would neutered female dogs.
There was little difference in the risk for aggression between neutered male dogs and non-neutered male dogs.
Puppy-training classes create dogs that are 1.5 times less likely to show aggression to strangers.
Dogs that have been trained or maintained using aversives are twice as likely to demonstrate aggression to strangers and are three times more likely to demonstrate aggression those those in their human family.
Dogs that were rescued or adopted, or obtained from "other sources" were significantly more likely to be aggressive than those dogs bought from a breeder.
Personally speaking, I don't think it's helpful to read too much into a single study, but when one well-conducted research project can then be combined with the findings from a variety of other valid research projects, an overall picture starts to appear that is more reliable.
In this particular study there were two aspects that immediately gave me pause when considering the results. The first of which was that the data came from dog owners themselves, who can't be expected to be scientifically objective about their dog's behaviour. The other aspect was that 'barking' was included in the study as an aggressive behaviour.
Needless to say, as long as human beings are intimately involved with dogs, canine aggression will continue to be a topic of study, debate and even misunderstanding for the foreseeable future.