some extent, contribute to the higher prevalence of monozygotictwins. Ideally, genotyping will be used in future studies to drawa clearer and more reliable distinction between monozygotic and dizygotic pairs. Future studies should also use genotyping to inves-tigate the genetic or heritable epigenetic components underlyingdiscordance.In line with previous studies, we find suggestive evi-dence of a heritable element of synesthesia. However, consonantwith the findings of Smilek et al. our large sample sizeverifies that synesthesia is not completely conferred by genetics(otherwise monozygotic twins would have had 100% concordance).Collectively, the available evidence to date suggests that a combi-nation of genes, epigenetic events, and environmental influencesunderlie an individual’s development of CSS. Further research isnecessary to determine the relative contributions of environmentalinfluences (e.g., imprinting) on synesthesia, as well as the potentialimpact of other heritable factors such as DNA methylation.