Evidence that adverse rearing environments exert negative effects particularly on children presumed “vulnerable” for temperamental or genetic reasons may actually reflect something else: heightened susceptibility to the negative effects of risky environments and to the beneficial effects of supportive environments. Building on Belsky’s (1997, 2005) evolutionary-inspired proposition that some children are more affected-both for better and for worse-by their rearing experiences than are others, we consider recent work on child vulnerability, including that involving measured genes, along with evidence showing that putatively vulnerable children are especially susceptible to both positive and negative rearing effects. We also consider methodological issues and unanswered questions in the differential-susceptibility equation.