environment. For instance, Gilbertson and colleagues found that PTSD severity in combat-exposed veterans negatively correlated with their hippocampal volume as well as volumes of their non-combat-exposed MZ co-twins, implicating predispositional rather than causal mechanisms12. In contrast, Lessov-Schlaggar et al. found functional differences in VS response to reward and punishment in regular tobacco smokers when compared to their non-regular-smoking MZ co-twin, implying potential causal mechanisms13. Importantly, although data from MZ twins are essential to demonstrate causality in this manner, absence of neural differences within discordant dizygotic (DZ) twin or non-twin sibling pairs is still compelling evidence against a causal hypothesis. If volume differences are not observed among discordant pairs who share only 50% of their genes, then finding an association with further genetic matching (i.e. MZ pairs) would be unlikely10.
