Mental instability can exist in any animal. Any heifer calf that exhibits even the least amount of this is immediately removed from our prospective breeding program and is sold at market with the rest. Aggressive roosters are eaten. Etc... etc....
We do all (not a whole lot) of the police dogs for the three departments in the county. My wife was a MWD vet in the Army, so they like that. She'll pick an even more aggressive stance than I will on nature vs nurture.
At this point in my life I am heavily motivated to continue my use of mastiff breeds here at the ranch. They have the protective instinct and low energy that I like. My current was out of a breeder that used American Bulldogs as working dogs still. With about two months of initial training, her recall is perfect. I can put her on cattle (not smart enough to herd, but capable of driving), she is my kids' babysitter, and challenger of all things that come down our private driveway. We have not had a remedial training session in 6 years. It's instinct...just like the dogs that get removed from working dog training because they didn't exhibit the necessary natural traits.
If it was all just training (nurture), you'd never have a washout.
This is why I don't trust pits. I actually had two go after each other I our clinic Wednesday with ZERO advance warning. Fortunately my dumbest just happened to be holding onto the one and I managed to snatch the other and pick both off the ground. 60lb pits act just like 6lb cats when you scruff them. Oh...they belonged to the same owner too.