a significance threshold of 0·05 with just 18 subjects. This finding could be due to chance statistical fluctuation. Indeed, a subsequent study of 92 individuals (including 19 from the first study) carried out by the same group again showed a significant effect, but with a reduction in the effect size. The reported mean values were this time 0·16 and 0·03 for the two groups (Hariri et al. 2005), equivalent to an effect size of just over 10% of the phenotypic variance. Additional studies are needed to confirm whether the effect is indeed this large.
