MAOA and major depressive disorder: Extant evidence also implicates gene-by-environment interactions of a functional variable nucleotide polymorphism in the promoter of MAOA (MAOA-uVNTR) and adversity in modifying risk for negative behavioural outcomes, principally antisocial behaviour, and mental disorders such as depression (Byrd and Manuck 2014; Booij et al. 2015; Nilsson et al. 2018).