Far less is known about the environmental and genetic risk factors for bipolar disorder (BPD) than for schizophrenia (Sz). However, at the recent meeting of the Sz International Research Society in Florence, Alan Brown of Columbia Univ presented data from an unpublished study of BPD using the same prospective birth cohort that he had used to show that maternal infection is a risk factor for Sz. In fact, his new data show a 4 to 5-fold increase in risk for BPD in the offspring of mothers who had an influenza infection during pregnancy. This is about the same magnitude of effect size as the risk for Sz (increases of 3 to 7-fold). How does maternal infection increase risk for two rather different disorders? Brown indicated that the risk for BPD is higher for infection in the third trimester of pregnancy, while the risk for SZ in the offspring is higher for infections earlier in pregnancy. These new findings are consistent with a study showing that BPD subjects are more likely than controls to be born in the winter, when influenza infection peaks. A similar point about season of birth has been repeatedly made for Sz. Recall also, as discussed in my book (and earlier posts on this blog), that maternal infection is a risk factor for autism. Full disclosure: I have published several articles with Alan Brown (picture below), and we recently published a book together: “The Origins of Schizophrenia” (Columbia Univ Press).