An obvious genetic hypothesis for sex differences in a heritable trait or disorder is that it is a manifestation of genes on the sex chromosomes. X‐linked inheritance is one of the most important sources of sex differences in disease. In fact, early work in the familial transmission of bipolar disorder were consistent with X‐linkage, 44 that is, a lack of male‐to‐male transmission, and an increased risk of disease in female relatives. 45
