A number of behaviours associated with borderline personality disorder (including attempted suicide, suicide, substance abuse, and antisocial behaviour) are on the increase among the young. The common factor in these disorders is impulsiveness. Evidence is reviewed suggesting that social disintegration reduces the threshold of impulsive behaviours. It is proposed that this is the mechanism through which social risk factors effect the prevalence and morbidity of borderline personality. A number of ways of testing this hypothesis are suggested.