BDNF and major depressive disorder: Prior studies related to BDNF and depression have produced conflicting results that fail to show conclusively whether BDNF is directly or indirectly proportional to depression (Castrén & Kojima, 2017; Druzhkova et al., 2019; Miao et al., 2020; Miranda et al., 2019; Papakostas et al., 2013; Zhou et al., 2017).