MAOA and major depressive disorder: Converging lines of evidence from gene-by-environment studies and from DNA methylation studies have suggested that MAOA plays a role in antisocial behaviour, substance misuse, depression (Verhoeven et al. 2012; Melas et al. 2013; Byrd and Manuck 2014; Booij et al. 2015; Melas and Forsell 2015; Checknita et al. 2015, 2020; Bendre et al. 2018; Nilsson et al. 2018), and other mental disorders (Reif et al. 2014; Booij et al. 2015; Ziegler et al. 2016, 2018; Checknita et al. 2018).