Eirunn Skaug

• Torgersen, Svenn Omar; Skaug, Eirunn & Waaktaar, Trine (2026). Common and Unique Etiological Influences on Personality and Personality Disorders in Youth. Psychopathology. ISSN 0254-4962. doi: 10.1159/psp/aelag006. Fulltekst i vitenarkiv Vis sammendrag

  Background: Personality and personality disorders (PDs) are closely related constructs, but the nature of their relationship remains uncertain. Dimensional models suggest PDs are extreme manifestations of personality traits. We investigated the etiological links between broad personality factors and dimensional PD factors using quantitative analyses of common and unique genetic and environmental etiological factors affecting both normative and deviant personalities, emphasizing methods to minimize measurement bias. Methods: Seven national twin cohorts participated (N = 2,353 twins from 1,188 families), assessing personality traits with questionnaires across three adolescent waves and PD traits via age-19 clinical interviews. Exploratory factor analyses of personality traits and the three-cluster ABC factors of PDs simplified the factor structure of the measures. Biometric twin analyses with Cholesky modeling identified genetic and environmental links among factors. Results: Factor analysis of personality traits yielded non-resilient, disinhibited, and disaffected factors. PDs fit an emotional/erratic, anxious/fearful, and odd/eccentric factor structure. Phenotypic correlations varied, but the disinhibited factor had minimal relations to specific PD factors. Crucially, approximately one-third of the genetic variance was shared between personality and PDs. However, the majority of genetic variance and all non-shared environmental variance were trait-specific, suggesting predominantly distinct etiological pathways. Conclusion: The significant genetic overlap supports the continuum model; results were bolstered by using broad factors and method triangulation. However, unique genetic and environmental influences point to distinct etiological pathways differentiating personality and PDs. These findings endorse the continuum model yet highlight the need for further research to grasp the complex relationships between personality and PDs.

• Skaug, Eirunn; Waaktaar, Trine & Torgersen, Svenn Omar (2026). Stability and Change in Wellbeing Throughout Adolescence and Its Relationship with Life Events: A Longitudinal Twin Study. Behavior Genetics. ISSN 0001-8244. doi: 10.1007/s10519-026-10262-4. Fulltekst i vitenarkiv
• Waaktaar, Trine; Skaug, Eirunn & Torgersen, Svenn Omar (2025). The etiological relationship between the general factors of psychopathology and personality; a longitudinal twin study from adolescence into young adulthood. Frontiers in Psychology. 16. doi: 10.3389/fpsyg.2025.1564305. Fulltekst i vitenarkiv Vis sammendrag

  Background: Mental disorders and normal personality are interconnected domains. Recent studies highlight the dimensional and hierarchical nature of psychopathology and personality, focusing on their general factor levels. However, their relationship remains unclear. Aims: This study explored the etiological relationship between the general factors of psychopathology and personality from adolescence to young adulthood. Methods: Longitudinal data from seven national twin cohorts (N = 1,538 pairs) were collected across three waves (ages 12–22). Data was analyzed using a genetically informative random intercept cross-lagged panel model and Cholesky decomposition modeling. Results: Negligible cross-lagged effects were observed between the general factors. Both showed substantial stability, with genetic influences explaining most of the time-invariant variance. About one-quarter of genetic stability was shared. Conclusion: Psychopathology and personality are distinct yet parallel domains developing through adolescence into young adulthood. Notably, change in one did not lead to change in the other.

• Skaug, Eirunn; Waaktaar, Trine & Torgersen, Svenn Omar (2024). The relationship between personality throughout adolescence and social anxiety disorder in young adulthood. A longitudinal twin study. PLOS ONE. 19(3). doi: 10.1371/journal.pone.0299766. Fulltekst i vitenarkiv Vis sammendrag

  This study examined the longitudinal relationship between a range of personality related variables measured throughout adolescence, and social anxiety disorder (SAD) in young adulthood. In addition, we examined to what degree the phenotypic associations between personality and SAD could be attributed to shared genetic and environmental factors, respectively. A total of 3394 twins (56% females), consisting of seven national birth cohorts from Norway, participated in the study. Personality was measured with self-report questionnaires at three times throughout adolescence, and SAD was measured with a diagnostic interview in early adulthood (M = 19.1 years, SD = 1.2). Correlation and regression analyses were performed to examine phenotypic associations between personality and SAD. We then created four composite scores of personality, in which the personality variables from four different ages throughout adolescence were weighted relative to their importance for SAD. Finally, a series of Cholesky decomposition models were used to examine the underlying genetic and environmental influences on the phenotypic associations between composite scores of personality and SAD. The results showed that especially higher neuroticism, lower extraversion, higher levels of loneliness, and lower levels of resilience, self-efficacy and sense of coherence, were associated with SAD. The phenotypic correlations between composite scores of personality and SAD increased from 0.42 when personality was measured 6–7 years prior to the assessment of SAD, to 0.52 when personality was measured shortly before the assessment of SAD. These phenotypic associations were mainly due to genetic influences, indicating that personality in adolescence predicts SAD in early adulthood due to shared genetic influences rather than having direct ‘causal’ effects on SAD.

• Skaug, Eirunn; Czajkowski, Nikolai Olavi; Waaktaar, Trine & Torgersen, Svenn Omar (2024). The Longitudinal Relationship Between Life Events and Loneliness in Adolescence: A Twin Study. Developmental Psychology. ISSN 0012-1649. 60(5), s. 966–977. doi: 10.1037/dev0001692. Fulltekst i vitenarkiv Vis sammendrag

  The aim of the study was to examine associations between life events and self-assessed loneliness in adolescence. We used data from a Norwegian population-based twin sample including seven birth cohorts (N = 2,879, 56% females). The participants completed self-report questionnaires three times throughout adolescence, with 2 years in between (i.e., 12–18 years old at Wave 1). By using a random intercept cross-lagged panel model (RI-CLPM), we were able to separate stable influences in the measured constructs from the within-person changes at each measurement occasion. In addition, using data from twins allowed us to examine to what degree the associations between life events and loneliness were genetic and/or environmental in nature. The results showed moderate stability of both loneliness and aggregation scores of life events throughout adolescence. The life events were assigned to clusters based on their independence (i.e., events considered dependent or independent on a person’s behavior) and desirability (i.e., positive or negative). The time-stable between-person variance in all three measures, loneliness and person-dependent positive and negative life events, could almost exclusively be accounted for by genetic factors. However, as expected, also shared environmental factors influenced person-independent life events. The associations between time-stable between-person variance in loneliness and life events were small, and nearly exclusively due to shared genetic influences. Furthermore, life events do not seem to predict changes in loneliness or vice versa. In conclusion, the self-reported levels of loneliness throughout adolescence seems to be independent of life events.

• Skaug, Eirunn; Czajkowski, Nikolai Olavi; Waaktaar, Trine & Torgersen, Svenn (2022). The role of sense of coherence and loneliness in borderline personality disorder traits: a longitudinal twin study. Borderline Personality Disorder and Emotion Dysregulation. 9. doi: 10.1186/s40479-022-00190-0. Fulltekst i vitenarkiv Vis sammendrag

  Background Borderline personality disorder (BPD) implies having problems with identity and relations with other people. However, not much is known about whether these indications of BPD are present in adolescence, i.e., before personality disorders usually are diagnosed. In this study, we examined the prediction of an aspect of identity (i.e., sense of coherence [SOC]) and social relations (i.e., perceived loneliness) throughout adolescence on BPD traits in young adulthood. In addition, we examined to what degree the predictive ability could be attributed to genetic and environmental factors. We also examined whether life events in adolescence were related to BPD traits. Methods Three thousand three hundred ninety-one twins, consisting of seven national birth cohorts from Norway, participated in the study. SOC, loneliness and life events were measured three times throughout adolescence with self-report questionnaires, with 2 years in between measurements. BPD traits were measured at the end of adolescence around the age of 19 with a structured interview. Regression analyses were performed to examine the prediction of SOC, loneliness and life events on BPD traits. Cholesky decomposition models were then used to determine to what degree the associations were due to genetic and environmental influences. Results The prediction of SOC and loneliness on BPD traits increased from R = .25 (when measured 6 years prior to the assessment of BPD traits) to R = .45 (when measured shortly before the assessment of BPD traits). In addition, negative life events considered dependent on a person’s behavior were related to BPD traits. Negative independent and positive dependent life events did not contribute to the prediction of BPD traits. Cholesky decomposition models showed that SOC and loneliness were associated with BPD traits mainly due to shared genetic influences (i.e., the proportion due to genetic influences ranged from 71 to 86%). Adding negative dependent life events to the prediction of BPD traits did not change these percentages. Conclusions These findings indicate that the weaker SOC, the stronger feelings of loneliness, and the negative life events associated with BPD traits are mainly consequences of the genetic aspects of BPD traits, rather than having direct effects on levels of BPD symptoms.

• Skaug, Eirunn; Czajkowski, Nikolai Olavi; Waaktaar, Trine & Torgersen, Svenn (2022). Childhood Trauma and Borderline Personality Disorder Traits: A Discordant Twin Study. Journal of Psychopathology and Clinical Science. ISSN 2769-7541. 131(4), s. 365–374. doi: 10.1037/abn0000755. Fulltekst i vitenarkiv
• Skaug, Eirunn; Czajkowski, Nikolai Olavi; Waaktaar, Trine & Torgersen, Svenn (2022). The relationship between life events and sense of coherence in adolescence. A longitudinal twin study. Journal of Research in Personality. ISSN 0092-6566. 99. doi: 10.1016/j.jrp.2022.104259. Fulltekst i vitenarkiv Vis sammendrag

  This three-wave study examined the relationship between life events and sense of coherence (SOC) throughout adolescence by using genetically informative random intercept cross-lagged panel models. We also examined the genetic and environmental contribution to variance in the measured constructs. The data come from a Norwegian population-based twin sample (N = 2,878). Life events and SOC were associated, and both showed substantial genetic variance. Negative longitudinal effects were observed from negative dependent life events to SOC, from SOC to negative dependent life events and from SOC to positive dependent life events. However, these longitudinal effects were negligible in magnitude. In summary, the associations between all three clusters of life events and SOC were almost completely accounted for by shared genetic influences.

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