Smaller, older and male dogs are more likely to be aggressive and growl, snap and bark at humans, a study has found. Some breeds are also more likely than others to exhibit aggressive behaviour, with Long-Haired Collies, like Lassie, the most aggressive of all breeds. In contrast, Labradors and Golden Retrievers, beloved for their docile temperament and gentle nature, were found by scientists to be the least aggressive breeds. When comparing the Rough Collie (left) with the Labrador (right) the researchers found the former is 5.44 times more likely to be aggressive What dog breeds are the most aggressive? The below list was compiled by researchers from helsinki who studied the behaviour of more than 9,000 et dogs.