Third, we performed null simulations with a nonzero genetic correlation and differential polygenicity in the non-shared genetic architecture between the two traits. 1% of SNPs were causal for L with effects q1=q2=0.2 on each trait, 2% were causal for trait 1 but not trait 2, and 8% were causal for trait 2 but not trait 1 (Figure 2c, Table S2h-j). Thus, the likelihood that a SNP would be genome-wide significant was higher for causal SNPs affecting trait 1 only than for causal SNPs affecting trait 2 only. As a result of this imbalance, Bidirectional MR (as well as other MR Methods) produced excess false positives, unlike LCV.
