This study aims to reveal the mechanism of the influences of the full
moon and high temperatures on investors’ Monday irrationality. In
contrast to other studies, this research was built under a retroductive
approach by using a time series quasi experimental study. Investors were
directly assessed for their irrationality by using an adapted
psychometric test on 4 occasions. The results indicate that there were
indeed effects of the full moon and high temperatures on investors’
irrationality. Because the full moon and high temperatures frequently
occurred on Mondays, it is most probably those two variables that were
the drivers of the Monday irrationality. In the end, we concluded that
the rational behaviour assumption can no longer be held. Instead of
rationality, the investors were quasi rational. The utility function of
Von Neumann-Morgenstern in decision making has to be replaced by the
hedonic utility.