Final answer:
The phenomenon demonstrated in the example is self-fulfilling prophecy, where the initial expectation of Dennis's behavior influenced the babysitter's treatment, which in turn caused Dennis to behave in aggressive and mischievous ways.
Step-by-step explanation:
The phenomenon demonstrated in the given example is known as self-fulfilling prophecy. Self-fulfilling prophecy refers to a belief or expectation that influences an individual's behavior in a way that confirms that belief or expectation. In this case, the initial expectation that Dennis is aggressive and mischievous led the babysitter to treat him in a certain way, which in turn caused Dennis to behave in aggressive and mischievous ways.