Causes, consequences, and life-course trajectories of personality pathology - a longitudinal extended children-of-twins study

Personality disorders affect the way a person thinks, feels, and acts, and can be destructive in many areas of life. They may manifest as extreme mistrust, intense mood swings, impulsive behavior, or paralyzing perfectionism. Such disorders affect not only the individual but also families and close relationships. Although around 10% of the population meet the criteria for a personality disorder, we still know less about their causes, life course, and societal consequences compared to many other mental disorders. In 2025, the project investigated how personality disorders influence important aspects of everyday life. We find that they can have major consequences for close relationships, such as an increased risk of divorce, and we study how children’s school performance and educational pathways are affected when parents have a personality disorder. We have also worked on the basic epidemiology of personality disorders and have recently completed analyses of their prevalence and of changes in diagnostic practices in Norway in recent years. At the same time, we are working to develop new statistical methods that will make it possible to use data from national registries in a more precise and flexible way. The goal is to establish analytical tools that can provide more reliable answers to questions about heredity, environment, and personality disorders. Preliminary results were presented at the conference of the Behavior Genetics Association in Atlanta in the summer of 2025, and in November we will contribute with several presentations at the International Congress for the Study of Personality Disorders. The project has also established closer ties with research groups in Sweden and Finland working with registry-based mental health research, and we are now collaborating on joint analyses across countries. The next phase is to invite all Norwegian twins born between 1967 and 1979 to a new data collection, in which siblings, partners, and children over the age of 16 will also take part. This will make it possible to follow the development of personality disorders and other mental disorders over 25 years and provide new insights into how genetic and environmental factors interact.

Individuals with personality pathology are at high risk of suffering, disability, and a range of negative health and psychosocial outcomes. Since personality pathology can have a detrimental impact on virtually all social interactions, the risk of distress and negative outcomes also extends to people in close relationships with those afflicted, in particular their spouse and children. In the proposed study we aim to assess personality pathology and clinical disorders in all twins born 1967-1979, as well as their non-twin siblings, spouses, and children over 16 years of age. To this stem we will link data on a subset of the same cohort, from the only large and genetically informative population representative study assessing all PDs in DSM-IV (AI/AII study) , the 10 year follow-up (AI/AII-FU), as well as data from national health and social registries unique to the Scandinavian countries. This will permit us to advance the international research front with respect to personality pathology on three key areas. First, we will investigate the etiology of personality pathology across a 25 year span from early to middle adulthood, as well as the causes underlying their longitudinal relationships with clinical disorders. Second, we will determine the cumulative health, functional and psychosocial outcomes of personality pathology. Third, we will provide unique insight into the processes by which personality pathology and clinical disorders are transmitted across generations, and the impact and mechanisms by which children are influenced by personality pathology in parents, and visa versa. The children-of-twins-design will permit us to accurately estimate the genetic influence on PP, investigate the causal effects of parental mental health problems on children, and determine the risk or protective effects of a range of measured environmental exposures, all while controlling for genetic confounding and circumventing the limitations inherent in the classical twin design.