Evolutionary models of psychopathology can shed light on gene-environment interactions. Differential susceptibility to the environment means that heritable traits can have positive or negative effects, depending on environmental context. Thus, traits that increase risk for mental disorders when the environment is negative can be adaptive when the environment is positive. This model can be applied to borderline personality disorder, with predictors such as emotional dysregulation and impulsivity seen as temperamental variations leading to negative effects in an unfavorable environment but to positive effects in a favorable environment. This model may also be useful in conceptualizing the mechanisms of effective therapy for borderline personality disorder.