Learning behaviours through learning/conditioning
Behaviourists think that the reason we are born with a blank slate (tabula rasa) and we learn all of our phobias through our enviroment. they think the only fears we are born with are falling and loud noises which are innate.
They also believe that we learn in a similar way to animals. This is backed up by Pavlov's experiment with a dog using classical conditioning to make it salivate. Another experiment that shows that we learn in a similar way to animals is Skinner's experiment with a rat where he used operant conditioning to reinforce behaviours that we want to see.
The treatment that behaviouralist's use most widely is systematic desensitisation or flooding to help treat phobias e.g. arachnophobia. Both of which can be deemed as unethical due to the fact both treatments put the participants under more stress than that of which they would experience on a daily basis. However both treatments are seen as effective.