Maybe, because of our preconceived notions about the full moon, we just seem to notice crazy behavior when it happens during the full moon. Psychologists refer to this as selective attention , which is a type of subjective validation. To test this possibility, some psychologists did a study of nurses, asking them ahead of time if they believed in the lunar effect or not. Then they asked all the nurses, during the next full moon, to record whether or not they observed any unusual behavior in the patients. Sure enough, the nurses that previously said that they believed in the lunar effect noted more unusual behavior during the full moon while nurses working in the very same environment noticed nothing unusual.
Selective attention is a HUGE problem for everyone in making sense of the world.
For example, if you are a person convinced by environmental alarmists who think global warming will soon boil the oceans, you will tend to notice the stinking hot days and the unusually warm days in winter but you will tend to conveniently forget about a 3-week cold spell during winter and the few lovely days of cool weather during an otherwise hot summer. On the other hand, if you are a person convinced by listening to rotund talk radio hosts who think that the global warming idea is some kind of conspiracy to produce a New World Order, then you will tend to take special note of the cool spells and conveniently forget about a Christmas when people in Minnesota went for a stroll in t-shirts. We turn to objective methods - that is, instruments that record the temperature every day and since instruments have no prior notions about global warming, they will tell us what the temperature was over the whole year with no psychological bias. These trends are now very well established and the correlation between them is consistent with what we know about the effects of gases like CO2 on the heat exchange processes of Earth.