As it is expected that approximately 70% of participants will express the S/Lg allele, the magnitude of activity within the amygdala is estimated from a meta-analysis by Munafo et al (2008). to be 0.10 with a standard deviation of 0.11 Assuming a difference in mean change (baseline to 8 weeks) between groups to be 0.05 with a change standard deviation of 0.07, a total of 32 patients per group is required to detect this difference with 80% power at a two-sided 5% level of significance. A 50% difference in mean change is reasonable based upon the results by Sheline et al (2001).