Background. Most of our knowledge about borderline personality disorder features has been obtained through the study of clinical samples. Although these studies are important in their own right, they are limited in their ability to address certain important epidemiological and aetiological questions such as the degree to which there is a genetic influence on the manifestation of borderline personality disorder features. Though family history studies of borderline personality disorder indicate genetic influences, there have been very few twin studies and the degree of genetic influence on borderline personality disorder remains unclear. Method. Data were drawn from twin samples from The Netherlands (n=3918), Belgium (n=904) and Australia (n=674). In total, data were available on 5496 twins between the ages of 18 and 86 years from 3644 families who participated in the study by completion of a mailed self-report questionnaire on borderline personality disorder features. Results. In all countries, females scored higher than males and there was a general tendency for younger adults to endorse more borderline personality disorder features than older adults. Model-fitting results showed that additive genetic influences explain 42% of the variation in borderline personality disorder features in both men and women and that this heritability estimate is similar across The Netherlands, Belgium and Australia. Unique environmental influences explain the remaining 58% of the variance. Conclusion. Genetic factors play a role in individual differences in borderline personality disorder features in Western society. © 2007 Cambridge University Press.