We applied a polygenic risk score (PRS) in EUR with MVP as the base and PGC as the target. The MVP case-control and MVP PCL-Total PRS explained approximately 0.4% (P = 2.4 × 10−74) and 0.7–0.8% of the variance (P = 2.2 × 10−134), respectively, in the PGC case-control phenotype at P-value thresholds (PT) ≤ 0.05 (Extended Data Fig. 2). The low phenotypic variance explained is likely due to different characteristics of the MVP and PGC-PTSD cohorts: across three MVP hold-out PRS analyses, we observed phenotypic variance explained ranging from 4% to 5.3% (P < 6 × 10−92; Supplementary Table 7). Evaluating the extent to which cross-ancestral PRS were useful, we found PRS biased by ancestry, with density plots of EUR and AFR PRS being substantially different (Extended Data Fig. 3).
