Fueled by substantially larger sample sizes, resting heart rate provides a picture in stark contrast. It is the only candidate endophenotype to fully meet our threshold for verified discovery. Although a putative endophenotype for antisocial behavior and perhaps PTSD (see Table 5), resting heart rate has not received as much attention as an endophenotype as other candidates in Table 5. Yet, as we discussed in section 6.1, GWAS meta-analyses of heart rate and related measures, such as Q-T interval, have identified several loci with genome-wide significant associations with heart rate in samples comprising subjects from different racial or ethnic backgrounds (European, Icelandic, Asian, African-American) (Cho et al., 2009; Deo et al., 2013; Eijgelsheim et al., 2010; Holm et al., 2010). There have been replicated findings, and several of the variants discovered are nonsynonymous and biologically plausible. The effect sizes are small, accounting for no more than 1 beat per minute. For instance, one variant accounted for 0.4% of the variance in heart rate (shortening the R-R interval by 12.6 ms) (Deo et al., 2013). Other studies report somewhat smaller effects (see Figure 1 and Table 4).
