Dogs can have wildly different personalities, which often vary by breed and size. Although individual temperament matters most, larger trends can help predict which pooch will be meek and mild and which is most likely to bite the postal worker.
The Dog Aging Project, a long-term study on canine aging, has collected data on dog behavior from more than 50,000 owners, distilling the results into scores for traits such as trainability, excitability and aggression. Owners of mixed-breed dogs reported higher average scores for fear and aggression than owners of single-breed dogs, for instance. And compared with larger dogs, smaller dogs tended to score higher for fear, aggression and excitability and lower for trainability. Understanding these patterns is key to improving the compatibility of people and their pets. “Dogs are entirely dependent on their owner,” says veterinarian Audrey Ruple of the Virginia-Maryland College of Veterinary Medicine, a collaborator on the Dog Aging Project. A mismatch can be “devastating,” but a good match can ensure dogs receive the care and affection they need.

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Miriam Quick, Lee Lotor and Jen Christiansen; Source: Dog Aging Project (DAP) https://data.dogagingproject.org/Index (data)
SMALL DOG SYNDROME
Dog breeds classified as toy or small have higher owner-rated scores on aggression and fear, as well as lower trainability scores. Large and giant dog breeds, in contrast, are on average the most trainable and the least aggressive and anxious. “I suspect it has to do with the way small dogs are treated as compared with large dogs,” Ruple suggests. Good behavior in a big dog is often judged as more of a necessity, whereas people may let little dogs off the hook.
OLD DOGS, NEW TRICKS
Dogs rated as more trainable tend to learn new skills more readily. They often obey “sit” or “stay” commands immediately, and they aren’t as easily distracted by interesting sights, sounds or smells. Some breeds are known for being easily taught, and mixed-breed dogs tend to get rated as less trainable than purebreds. That pattern, however, might reflect the early life experiences of mixed-breed dogs, who often are adopted from shelters or rescues and have experienced trauma, Ruple says. A purebred dog raised by a reputable breeder, however, will be primed to be comfortable around people and eager to learn.

