Adult OCD patients showed smaller hippocampus (Cohen's d = −0.13), most pronounced in patients with an adult onset of OCD (Cohen's d = −0.18), patients with a comorbid lifetime diagnosis of depression (Cohen's d = −0.27), and patients using medication (Cohen's d = −0.29). Adult OCD patients also showed larger pallidum (Cohen's d = 0.16), most pronounced in patients with a childhood onset of OCD (Cohen's d = 0.25), and patients using medication (Cohen's d = 0.29). The hippocampal findings do not seem to be specific to OCD, as similar results have been reported by the ENIGMA working groups for schizophrenia (Van Erp et al., 2016), bipolar disorder (Hibar et al., 2016), major depressive disorder (Schmaal et al., 2016), and posttraumatic stress disorder (Logue et al., 2018), and so may be related to trans‐diagnostic aspects of chronic distress. The pallidum findings fit well with all prior meta‐ and mega‐analyses (De Wit et al., 2014; Peng et al., 2012; Radua et al., 2010; Radua & Mataix‐Cols, 2009; Rotge et al., 2010) and appear to be more specific to OCD. Larger pallidum
