BDNF and depressive disorder: Based on the findings regarding the relationship between low BDNF and depression, and the role of early and/or adult stressful events in promoting depressive symptoms, it had been suggested that gene polymorphisms should not be considered as a “vulnerability factor,” but should instead be viewed as a “plasticity factor” that may favor either positive or negative outcomes depending upon the individuals specific experiences (Belsky and Pluess, 2009; Belsky et al., 2009).