A couple of emblematic examples illustrate the challenges faced. Hodgkinson et al. (2010) performed a genome wide association study (GWAS) on alpha, beta, and theta power in a Native American cohort of 322 individuals and found that several SNPs in SGIP1 accounted for 8.8% of the variance in theta power. This association was replicated in a European American sample of 185, with one of the SNPs accounting for 3.5% of the variance in theta (although this finding did not survive correction for multiple testing). Effect sizes this large are exactly what we should hope endophenotypes to yield if they are to prove their value. Our study (Vrieze et al., 2014b), with almost ten times the sample size, was definitively powered to detect an effect this large. However, we obtained a nonsignificant p-value of .199 for the association between this gene and theta power using a gene-based test and we did not replicate findings at the level of individual SNPs within the gene. Greenwood et al. (2013) carried out a linkage study for antisaccade error in approximately 1000 individuals drawn from
