Previous research has provided observational evidence for an association between IL-6, depression and psychosis using population-based longitudinal (Gimeno et al., 2009, Khandaker et al., 2014) and cross-sectional studies (Dowlati et al., 2010, Goldsmith et al., 2016, Haapakoski et al., 2015, Howren et al., 2009, Miller et al., 2011, Potvin et al., 2008, Upthegrove et al., 2014). This evidence concerns the gene IL6 and depressive symptom measurement.