I think that in many cases you will find that the problem started with the owner…not the breed. Some breeds do have a predisposition to act aggressively, if not trained properly by the owner, but again the fault lies with the owner not the dog. In fact, most of the research published in scientific journals shows that breeds like the springer spaniel, golden retriever, labrador and beagle are seen more often by veterinarians and behaviorists for aggression related problems.
My cousins have this shih tsu that bites everyone. No one is safe. I'm just glad he doesn't have the jaws of a pit bull or we'd all be in trouble!:) On the other hand, we have a unneutered pit bull that lives in the apartment downstairs that Manning plays with and absolutely loves. When I take Manning out to the courtyard he sits in front of my neighbor's glass door and waits for the pit bull to come out and play.