One joke psychologists are sure to get is, "Does the name Pavlov ring any bells?" Another psychology joke is, "How many narcissists are necessary to switch out a lightbulb?" The answer is that only one narcissist is needed to hold the bulb in place while the universe revolves around him.
The joke about the name Pavlov ringing bells relates to Ivan Pavlov's discovery that he could condition a dog to salivate upon ringing a bell if he previously rung the bell at the same time he fed the dog. This observation helped lead to the discovery of the psychological concept known as classical conditioning, which psychologists learn about early on in their introductory-level psychology courses. Thus, just about any psychologist or psychology student is sure to understand the joke upon hearing it.
The punchline of the joke asking how many narcissists are required to change a lightbulb is the part of the answer that states the narcissist changing the bulb must simply hold it in place while the universe revolves around him. This joke relates to narcissistic personality disorder, a psychological condition characterized by an exaggerated sense of self-importance and lack of empathy for others. Patients diagnosed with narcissistic personality disorder may come across to others as thinking they are so superior that the world revolves around them.