LD score regression has identified multiple diseases that have shared genetic factors with schizophrenia (Bulik-Sullivan et al., 2015a). Of these, bipolar disorder (rg=0.79) and depression (rg=0.51) have the highest genetic correlations. The next tier of genetic correlations include other psychiatric disorders (Attention Deficit Hyperactivity Disorder rg=0.23, Anorexia rg=0.19, Autism Spectrum Disorders rg=0.14) and autoimmune disorders (Crohn's Disease rg=0.13, Ulcerative Colitis rg=0.13). Our observed genetic correlations between schizophrenia and smoking behaviors fit into this second tier (ranging from 0.14 for nicotine dependence to 0.10 for ever/never smoking). This suggests that smoking behaviors should be included when leveraging other phenotypes to investigate the genetic etiology of schizophrenia.
