Final answer:
Moodiness attributed to a certain gender without substantial evidence is an example of an illusion of covariation, a cognitive bias of perceived nonexistent relationships.
Step-by-step explanation:
Attributing moodiness to a certain gender is an example of an illusion of covariation, which is a cognitive bias where individuals perceive a relationship between two variables even when no such relationship exists. This is similar to the well-known illusory correlation that people believe in, such as the idea that the moon's phases affect human behavior. When one gender is labeled as moodier than another without substantial evidence, it is an assumption based on stereotyping rather than factual covariation.