A central idea in personality theory is that events in childhood have an effect on the development of personality. The present study applied models of gene–environment interaction that demonstrate how environmental conditions may moderate genetic variability in a population and/or the influence of other environmental effects. Results showed that perceived levels of family conflict and maternal indulgence moderated the genetic influences underlying emotional instability, a central feature of borderline personality disorder. The analyses identified a wide variety of environmental influences that moderate the variability in the liability to emotional instability, such as perceived levels of parental bonding, family functioning, and exposure to nonassaultive traumatic events. Copyright © 2005 John Wiley & Sons, Ltd.