MAOA and major depressive disorder: Aberrant activity of MAOA has been posited as a critical factor involved in system-wide aminergic dysregulation that, in turn, contributes to the development of multiple negative outcomes including increased aggressive behavior, substance misuse, and mental disorders such as depression (Meyer-Lindenberg et al. 2006; Byrd and Manuck 2014; Booij et al. 2015; Nilsson et al. 2018; Ziegler and Domschke 2018).