It’s a defect if it causes a person’s brain to have an illogical stress response. For instance, the vast majority of pet dogs are harmless. But, my daughter, who was nipped by a dog when she was a toddler, will probably always be somewhat fearful of dogs (as I am, for the same reasons). Some people have a greater risk for trauma from these types of events. And, while useful at one point, it is now problematic for modern living.