I think true fear aggression and aggresion that stems from domainace assertion are two differnt animals, pardon the pun. In a training situtuation, I would normally deal with them differently.
Dog aggression goes the same way, but many times in animals present with one or the other, and the aggresion does not cross over.
I will have disputers, but it is my opinion that this is a partly genetic predisposion, part learned tendency, Which is why it can often be related to dogs, or people, and only rarely both. My eveidence is in many breeds, where somehow the dog just 'turns off' and fails to redirect, like with a very high drive, well bred malanios, or a very sporty, high drive terrier, that turns to jello when you pull him out of the hole--it goes against common sense that the dog would not redirect--but lo they do not--except with some individuals.
It tells me that somewhere in this dog he is programmed to aggress in this certian situation, and not turn on his human handler....I can see where these tendencies can become skewed from poor breeding and handling at an early age, as well as the development of temperament into adulthood--hence you have dogs that ARE well socialized , but have problems with fear into maturity.
And you must not forget the baiting dogs that have been originally bred to aggress toward other dogs, despite socializartion and training, it would take a lot to get me to trust a pit bull terrier or like breed with toys unattended. Countless trainers have been injured, mauled, or killed for forgetting, or ignoring and animal's nautre.
EDIT: Kasper, don't forget that straining, growling and barking at the end of the lead MAY BE behaviour arising from frustration, and NEITHER fear or true aggression. Although these may be warning signs of problems if not handled appropriately, cannot be constitued as 'human aggression,' or really even aggresssion of any sort being that they are not harming, or neccessarily intending to harm.
PIPER: Aggression can most certainly be learned. Some of the baddest dogs aggress because they have gotten away with it, and have developed power over their owners--kasper's article seems a good example, but that is hard to tell....
I currently have mulitple dogs in training with varying complaints form their owners regarding aggression, both towatd dogs and people. EDIT: many dogs come with 'aggression' because of precusory behaviour, but are not aggressive at all, just frustrated and not taught to deal with it.