A study published in the journal Applied Animal Behavior Science in 2008 concluded that dachshunds, chihuahuas and Jack Russell terriers were the breeds with the overall highest level of aggression directed towards owners, strangers and other dogs. Australian cattle dogs were next on the list with high levels of aggression towards human strangers. Akitas and pit bull terriers exhibited high levels of aggression only towards other dogs .
A more recent study published in the journal Applied Animal Behavior Science in 2014 reported that breed was not a good predictor of aggressiveness towards humans. Dogs purchased from breeders were less aggressive towards humans than dogs purchased from pet shops or obtained from rescues. Dogs that had attended puppy training classes were less aggressive towards humans. Spayed female dogs were less likely to be aggressive towards humans in all contexts. The 2014 study did not evaluate aggression towards other dogs.
In the 2008 study, American cocker spaniels and beagles exhibited high levels of aggression towards their owners only and not towards strangers or other dogs. The dog breeds that were least aggressive towards dogs, human owners and strangers were golden retrievers, Labrador retrievers, Bernese mountain dogs, Brittany spaniels, greyhounds and whippets.