The third type, spurious pleiotropy, captures variants that are falsely associated with multiple traits due to different forms of bias. One such source of bias can arise due to linkage disequilibrium, which refers to the correlational structure in the genome induced by physically proximal variants being inherited together (Slatkin, 2008). This has the consequence that a variant identified as associated with two traits may be a surrogate for two, separate causal variants. Fine mapping strategies that aim to identify the likely causal variant in a genomic region (Schaid, Chen, & Larson, 2018) can be used in future work to determine if the actual causal variant is shared across disorders. Diagnostic misclassification will also inflate levels of spurious pleiotropy, a particular challenge for psychiatric genomics given findings such as 15% misdiagnosis rates across bipolar disorder and schizophrenia (Bromet et al., 2011). However, it should be noted that it can also be unclear what the “correct” diagnosis is for so-called mixed presentations that do not neatly fall into a single disorder class. In addition, longitudinal studies using standardized assessment batteries indicate changing
