"Studies suggest that a dog's behavior takes place as a result of genetic material (instinct) and experience (learning)."

A very large percentage of dogs are not properly socialized and trained as puppies. On top of that breeding is not what it used to be, which leave the door open for many temperament issues. Therefore, I find it best to take precaution and not allow certain breeds, due to the high chances of the dog acting upon those instinctual behaviors.