What psychological phenomenon is commonly cited as the cause for the persistence of beliefs regarding 'lunacy' and erratic behavior?

Confirmation bias

The widespread and persistent belief that the full moon drives erratic or aggressive behavior, sometimes referred to as 'lunacy,' is largely attributed to confirmation bias. This psychological phenomenon describes the tendency for individuals to selectively notice, seek out, and remember events that confirm their existing expectations. If a person anticipates strange happenings during a full moon, they are more likely to register and recall unusual incidents that occur during that time, while simultaneously overlooking or forgetting the many uneventful full moons or the strange occurrences that happen during other lunar phases, thus reinforcing the perceived but unsupported link.