Controlled pitbull aggression (Worlds best off Breed Street Protection K9s)
Older dog owners (the humans being older, rather than the dogs) reported less aggression toward family members and toward strangers entering the house, while older dogs were more likely to be aggressive outside of the home. Spayed female dogs were less aggressive in all three categories.
Dogs purchased from breeders were less aggressive than those gotten from pet shops or rescue groups.
Attending puppy class correlated with less aggression toward strangers both inside and outside the whole — but attending a training class for four or more weeks was related to more aggression toward family members.
(Remember: the researchers found correlations here, not causation, which means it’s possible that dogs were going to obedience class for a long time because of their aggressive tendencies.)