A common misconception about SNP-chip heritability estimates calculated with GCTA and LDSC is that they should be similar to twin study estimates, when in reality twin studies have the advantage of capturing all genetic effects—common, rare and those not genotyped by available methods. Thus, the assumption should be that h2SNP<h2TWIN when using GCTA and LDSC, and this is what we observe for PTSD, as has been observed for many other phenotypes.54 Though somewhat limited by power in the present study, strong evidence of shared genetic effects between PTSD and SCZ, and more modest evidence of shared effects with both MDD and BIP, is consistent with recent reports of partially shared genetic effects across nearly all psychiatric disorders34, 38 and with twin study evidence of shared genetic influences on MDD and PTSD.10, 11, 12 No evidence of overlap was found with attention deficit hyperactivity disorder and autism. However, this could be because of low power and should be reexamined when PGC-PTSD is substantially expanded.
