Often unnoticed in Caspi et al.'s2 groundbreaking G × E research showing that males with the less active version of the MAOA gene proved most antisocial in young adulthood when they experienced maltreatment in childhood, is that individuals with the same MAOA allele scored lowest in anti-social behavior when not exposed to child maltreatment, even if not by much. A re-interpretation of this study's results in terms of plasticity and differential susceptibility rather than vulnerability and diathesis stress would seem viable given results of a significant number of efforts to replicate the findings. For example, Kim-Cohen et al.3 studied 975 boys to determine whether the MAOA polymorphism moderated effects of mother-reported physical abuse in early childhood on later mental health problems. At age 7 years, boys with the low-MAOA-activity variant were rated by mothers and teachers as having more mental health problems—and specifically attention-deficit hyperactivity disorder (ADHD) symptoms—if they had been victims of abuse, but fewer problems if they had not, compared with boys with the high-MAOA-activity genotype. In another longitudinal study, this one of 514 adolescent twin boys aged
