What I’ve seen in my fearful (and reactive) dogs is a clear connection between genetics and fearful behavior. I adopted a lab/border collie mix who appeared to be confident as a puppy until she reached about 5 months old. Then she started lunging and barking at new people and dogs.I had kept contact with one of her siblings’ owners. Turns out that her sister started displaying similar fearful behavior at the same age. Another dog I adopted as a puppy from a rescue had a fearful mother who had to be live trapped so that she and her pups could be rescued. His mother was fearful, as well as all the puppies, according to the rescuers.We were told that the pups were particularly fearful of men. And my dog, who is 12 now, is still fearful of strange men and is unpredictable, at times lunging and snapping at them.With both dogs we did lots of work on counter-conditioning,worked with behaviorists, took them to classes especially designed for fear-aggressive dogs. The classes helped us manage their behaviors, but we never could trust them.