The results that we observe probably reflect several mechanisms. Most important is probably the fact that neuroticism reflects an underlying liability to MDD (Boyce et al., 1991; Fanous et al., 2007; Kendler et al., 1993; Nystrom and Lindegrard, 1975). At least two other mechanisms likely play a role: a direct effect of MDD on the level of neuroticism for those currently in an episode (“state” effect) (Coppen and Metcalfe, 1965; Fanous et al., 2007; Farmer et al., 2002; Hirschfeld et al., 1983; Kendler et al., 1993; Kerr et al., 1970) and a long-term effect of prior depressive episodes on neuroticism (“scar” effect) (Fanous et al., 2007; Farmer et al., 2002; Kendler et al., 1993; Reich et al., 1987).
