You can never guarantee results of behavior modification if the behaviors you're trying to modify have an emotional cause, as all extreme behaviors do. How silly would it be for a psychologist to guarantee results for a patient suffering from severe anxiety? I think we could all safely say that psychologist is a quack. The reason is that you can never know the boundaries and all the complex factors in a being's emotional makeup. So how can we say we can pick out the ones that are effecting a certain behavior and address them effectively? We can do our best and often have great success, and at the very least we hope to provide some comfort. But we can never guarantee anything. Professionalism prohibits us from being disingenuous, and guarantees are most certainly that.
So, no trainer who aims to address the actual emotional causes of a behavior (the only way to get lasting and healthy change), can guarantee results. But here's the thing - some trainers CAN guarantee a change in behavior, because they DON'T address the root cause. Instead, they use pain, fear or intimidation to change a surface behavior, exacerbating the underlying emotion that causes it. Sure, a psychologist might be able to get a person to kick a drug habit by threatening his life. But will they be able to stop that person from jumping off a bridge spurred by the underlying emotion which has probably been made worse because of the fear of pain or death at the hands of the psychologist? This may sound extreme - but the fear of death is ultimately what trainers who use pain are tapping into with dogs. Physical pain is life-threatening to them, and causes deep fear. If your dog is stressed and pulling on the leash, put a prong collar on your dog and she'll stop pulling because she's afraid of the pain caused by the sharp metal edges digging into her neck. She'll learn to suppress that behavior. But she'll still be stressed. And that stress WILL find an outlet in ANOTHER behavior. And it will be a less predictable, more extreme behavior - often a much more dangerous one than pulling on the leash. That's why even though it is tempting to go with a trainer that guarantees results, it's a huge, bright, waving red flag. Opt for honesty and compassion and you'll be doing right by your dog and yourself.