The Norwegian Institute of Public Health Twin Program of Research: An Update

The underlying architecture of musical sensibility: One general factor, four subdimensions, and strong genetic effects

Current evidence suggests moderate heritability of music phenotypes, such as music listening and achievement. However, other fundamental traits underlying people's interest in music and its relevance for their lives have been largely neglected, and little is known about the genetic and environmental etiology of what we refer to as musical sensibility-the tendency to be emotionally and aesthetically engaged by music. This study investigated the latent structure, as well as the genetic and environmental factors influencing individual variability in multiple domains of musical sensibility, and the etiological architecture of the relationship between the dimensions. To this end, we used phenotypic confirmatory factor analytic and biometric twin modeling to analyze self-reported ratings on four dimensions of musical sensibility in a sample of Norwegian twins (N = 2600). The results indicate a phenotypic higher-order structure, whereby both the resulting general musical sensibility factor and the conceptually narrower domains were strongly heritable (49-65%). Multivariate analyses of the genetic and environmental covariance further revealed substantial overlap in genetic variance across domains.

The structure of well-being: a single underlying factor with genetic and environmental influences

PURPOSE

The structure of well-being has been debated for millennia. Dominant conceptualisations, such as the hedonic and eudaimonic models, emphasise different constituents of the well-being construct. Some previous studies have suggested that the underlying structure of well-being may consist of one or a few general well-being factors. We conducted three studies to advance knowledge on the structure of well-being comprising more than 21,500 individuals, including a genetically informative twin sample.

METHODS

In Study 1, we used hierarchical exploratory factor analysis to identify well-being factors in a population-based sample of Norwegian adults. In Study 2, we used confirmatory factor analysis to examine the model fit of the identified factor model in an independent sample. In Study 3, we used biometric models to examine genetic and environmental influences on general well-being factors.

RESULTS

We identified six well-being factors which all loaded on a single higher-order factor. This higher-order factor may represent a general "happiness factor", i.e. an h-factor, akin to the p-factor in psychopathology research. The identified factor model had excellent fit in an independent sample. All well-being factors showed moderate genetic and substantial non-shared environmental influence, with heritability estimates ranging from 26% to 40%. Heritability was highest for the higher-order general happiness factor.

CONCLUSION

Our findings yield novel insights into the structure of well-being and genetic and environmental influences on general well-being factors, with implications for well-being and mental health research, including genetically informative studies.

Stressful life events increase the risk of major depressive episodes: A population-based twin study

BACKGROUND

Previous studies have found that stressful life events (SLEs) are associated with an increased risk of adult depression. However, many studies are observational in nature and limited by methodological issues, such as potential confounding by genetic factors. Genetically informative research, such as the co-twin control design, can strengthen causal inference in observational studies. Discrete-time survival analysis has several benefits and multilevel survival analysis can incorporate frailty terms (random effects) to estimate the components of the biometric model. In the current study, we investigated associations between SLEs and depression risk in a population-based twin sample (N = 2299).

METHODS

A co-twin control design was used to investigate the influence of the occurrence of SLEs on depression risk. The co-twin control design involves comparing patterns of associations in the full sample and within dizygotic (DZ) and monozygotic twins (MZ). Associations were modelled using discrete-time survival analysis with biometric frailty terms. Data from two time points were used in the analyses. Mean age at Wave 1 was 28 years and mean age at Wave 2 was 38 years.

RESULTS

SLE occurrence was associated with increased depression risk. Co-twin control analyses indicated that this association was at least in part due to the causal influence of SLE exposure on depression risk for event occurrence across all SLEs and for violent SLEs. A minor proportion of the total genetic risk of depression reflected genetic effects related to SLEs.

CONCLUSIONS

The results support previous research in implicating SLEs as important risk factors with probable causal influence on depression risk.

Cancer in twin pairs discordant for smoking: The Nordic Twin Study of Cancer

The discordant twin pair study design is powerful to control for familial confounding. We employed this approach to investigate the associations of smoking with several cancers. The NorTwinCan study combines data from the Danish, Finnish, Norwegian and Swedish twin and cancer registries. Follow‐up started when smoking status was determined and ended at cancer diagnosis confirmed by information in the cancer registry, death or end of follow‐up. We classified the participants as never (n = 59 093), former (n = 21 168) or current (n = 47 314) smokers. We pooled data from twin pairs where one co‐twin was diagnosed with any of the following tobacco‐related cancers: esophagus, kidney, larynx, liver, oral cavity, pancreas, pharynx or urinary bladder, while their co‐twin had none of those. Lung cancer was included in further analysis. We used Cox regression allowing for pair‐specific baseline functions to estimate hazard ratios (HRs) with 95% confidence intervals (CIs). For tobacco‐related cancer sites, we recorded 7379 cases during median 27 years of follow‐up. The analyses based on individual twins showed that former (HR 1.31, 95% CI: 1.17‐1.48) and current (HR 2.14 [1.95‐2.34]) smokers are at increased risk to develop one of cancers listed above, compared to never smokers. Among 109 monozygotic twin pairs discordant for cancer and smoking, the HR was 1.85 (95% CI: 1.15‐2.98) among current smokers and 1.69 (1.00‐2.87) among former smokers when compared to their never smoking co‐twin. Thus, associations of smoking with several cancers were replicated for discordant identical twin pairs. Analyses based on genetically informative data provide evidence consistent with smoking causing multiple cancers.

Introducing two types of psychological resilience with partly unique genetic and environmental sources

Psychological resilience is indicated when individuals demonstrate good mental health despite exposure to significant stress or adversity. Good mental health may involve low levels of illbeing and/or high levels of wellbeing. There is still very limited knowledge about the potential differences between these outcomes in relation to stressors. We propose a distinction between type 1 and type 2 resilience, examine their underlying genetic and environmental architecture, and identify modifiable resilience factors. The data come from a population-based twin sample (N = 1987, mean age = 63) in the Norwegian Twin Registry. Type 1 and type 2 resilience are operationalised as the residual of anxiety/depression symptoms and life satisfaction, respectively, after lifetime cumulative adversity has been regressed out. We used biometric modelling and cotwin-control linear mixed models to estimate underlying factors and identify predictors while controlling for genetic confounding. The results support the notion of two separate, but partly overlapping types of resilience. We find heritabilities of 0.30 (type 1) and 0.24 (type 2) and a genetic correlation of 0.43. Potentially causal resilience factors include, but are not limited to, meaning in life, physical activity, positive affect and relationship satisfaction. Whereas some factors are associated with both resilience types, other factors are unique to each type.

Education differences in sickness absence and the role of health behaviors: a prospective twin study

BACKGROUND

Long-term sickness absences burden the economy in many industrialized countries. Both educational attainment and health behaviors are well-known predictors of sickness absence. It remains, however, unclear whether these associations are causal or due to confounding factors. The co-twin control method allows examining causal hypotheses by controlling for familial confounding (shared genes and environment). In this study, we applied this design to study the role of education and health behaviors in sickness absence, taking sex and cohort differences into account.

METHODS

Participants were two cohorts of in total 8806 Norwegian twins born 1948 to 1960 (older cohort, mean age at questionnaire = 40.3, 55.8% women), and 1967 to 1979 (younger cohort, mean age at questionnaire = 25.6, 58.9% women). Both cohorts had reported their health behaviors (smoking, physical activity and body mass index (BMI)) through a questionnaire during the 1990s. Data on the twins' educational attainment and long-term sickness absences between 2000 and 2014 were retrieved from Norwegian national registries. Random (individual-level) and fixed (within-twin pair) effects regression models were used to measure the associations between educational attainment, health behaviours and sickness absence and to test the effects of possible familial confounding.

RESULTS

Low education and poor health behaviors were associated with a higher proportion of sickness absence at the individual level. There were stronger effects of health behaviors on sickness absence in women, and in the older cohort, whereas the effect of educational attainment was similar across sex and cohorts. After adjustment for unobserved familial factors (genetic and environmental factors shared by twin pairs), the associations were strongly attenuated and non-significant, with the exception of health behaviors and sickness absence among men in the older cohort.

CONCLUSIONS

The associations between educational attainment, health behaviors, and sickness absence seem to be confounded by unobserved familial factors shared by co-twins. However, the association between health behaviors and sickness absence was consistent with a causal effect among men in the older cohort. Future studies should consider familial confounding, as well as sex and age/cohort differences, when assessing associations between education, health behaviors and sickness absence.

The Norwegian Twin Registry

The Norwegian Twin Registry (NTR) is maintained as a research resource that was compiled by merging several panels of twin data that were established for research into physical and mental health, wellbeing and development. NTR is a consent-based registry. Where possible, data that were collected in previous studies are curated for secondary research use. A particularly valuable potential benefit associated with the Norwegian twin data lies in the opportunities to expand and enhance the data through record linkage to nationwide registries that cover a wide array of health data and other information, including socioeconomic factors. This article provides a brief description of the current NTR sample and data collections, information about data access procedures and an overview of the national registries that can be linked to the NTR for research projects.

The structure of genetic and environmental influences on normative personality, abnormal personality traits, and personality disorder symptoms

BACKGROUND

Can the structure of genetic and environmental influences on normative personality traits (NPTs), abnormal personality traits (APTs), and DSM-IV criteria for personality disorders (PD) fit a high or low congruence model positing, respectively, close or more limited etiologic continuity?

METHOD

Exploratory factor analysis was applied to transformed correlation matrices from Cholesky twin decompositions obtained in OpenMx. In 2801 adult twins from the Norwegian Institute of Public Health Twin Panel, NPTs and APTs were assessed by self-report using the Big Five Inventory (BFI) and PID-5-Norwegian Brief Form (PID-5-NBF), respectively. PDs were assessed at interview using the Structured Interview for DSM-IV Personality (SIDP-IV).

RESULTS

The best model yielded three genetic and three unique environmental factors. Genetic factors were dominated, respectively, by (i) high loadings on nearly all PDs and NPT/APT neuroticism and compulsivity, (ii) negative loadings on NPT agreeableness/conscientiousness and positive loadings on APT/PD measures of antisocial traits, and (iii) negative loadings on NPT extraversion and histrionic PD, and positive loadings on APT detachment and schizoid/avoidant PD. Unique environmental factors were dominated, by (i) high loadings on all PDs, (ii) high loadings on all APT dimensions and NPT neuroticism, and (iii) negative loadings on NPT extraversion and positive loadings on NPT detachment/avoidant PD.

CONCLUSIONS

Two genetic and one environmental common factor were consistent with a high congruence model while one genetic and two environmental factors were more supportive of a low congruence model. The relationship between genetic and environmental influences on personality assessed by NPTs, APTs, and PDs is complex and does not fit easily into a low or high congruence model.

Relationships Among Avoidant Personality Disorder, Social Anxiety Disorder, and Normative Personality Traits: A Twin Study

Avoidant personality disorder (AvPD) and social anxiety disorder (SAD) share risk factors to a substantial degree, and both are characterized by the experience of anxiety in social situations. The authors investigated whether these disorders are differentially related to the Big Five personality traits. They also examined the underlying genetic and environmental influences on these associations. A population-based sample of 1,761 female twins was interviewed at baseline, and 1,471 of these were re-interviewed 10 years later. Associations between AvPD, SAD, and personality traits were investigated with multivariate biometric analyses. The authors found that AvPD and SAD are differentially related to several personality traits at the phenotypic, genetic, and environmental level. The genetic and environmental liability to AvPD could be fully accounted for by the genetic and environmental factors influencing SAD and personality. The findings may increase current etiological understanding of these disorders and inform future classification and treatment efforts.

Cancer Incidence and Mortality in 260,000 Nordic Twins With 30,000 Prospective Cancers

The Nordic countries have comprehensive, population-based health and medical registries linkable on individually unique personal identity codes, enabling complete long-term follow-up. The aims of this study were to describe the NorTwinCan cohort established in 2010 and assess whether the cancer mortality and incidence rates among Nordic twins are similar to those in the general population. We analyzed approximately 260,000 same-sexed twins in the nationwide twin registers in Denmark, Finland, Norway and Sweden. Cancer incidence was determined using follow-up through the national cancer registries. We estimated standardized incidence (SIR) and mortality (SMR) ratios with 95% confidence intervals (CI) across country, age, period, follow-up time, sex and zygosity. More than 30,000 malignant neoplasms have occurred among the twins through 2010. Mortality rates among twins were slightly lower than in the general population (SMR 0.96; CI 95% [0.95, 0.97]), but this depends on information about zygosity. Twins have slightly lower cancer incidence rates than the general population, with SIRs of 0.97 (95% CI [0.96, 0.99]) in men and 0.96 (95% CI [0.94, 0.97]) in women. Testicular cancer occurs more often among male twins than singletons (SIR 1.15; 95% CI [1.02, 1.30]), while cancers of the kidney (SIR 0.82; 95% CI [0.76, 0.89]), lung (SIR 0.89; 95% CI [0.85, 0.92]) and colon (SIR 0.90; 95% CI [0.87, 0.94]) occur less often in twins than in the background population. Our findings indicate that the risk of cancer among twins is so similar to the general population that cancer risk factors and estimates of heritability derived from the Nordic twin registers are generalizable to the background populations.

Genetics, personality and wellbeing. A twin study of traits, facets and life satisfaction

Human wellbeing is influenced by personality traits, in particular neuroticism and extraversion. Little is known about which facets that drive these associations, and the role of genes and environments. Our aim was to identify personality facets that are important for life satisfaction, and to estimate the contribution of genetic and environmental factors in the association between personality and life satisfaction. Norwegian twins (N = 1,516, age 50-65, response rate 71%) responded to a personality instrument (NEO-PI-R) and the Satisfaction With Life Scale (SWLS). Regression analyses and biometric modeling were used to examine influences from personality traits and facets, and to estimate genetic and environmental contributions. Neuroticism and extraversion explained 24%, and personality facets accounted for 32% of the variance in life satisfaction. Four facets were particularly important; anxiety and depression in the neuroticism domain, and activity and positive emotions within extraversion. Heritability of life satisfaction was 0.31 (0.22-0.40), of which 65% was explained by personality-related genetic influences. The remaining genetic variance was unique to life satisfaction. The association between personality and life satisfaction is driven mainly by four, predominantly emotional, personality facets. Genetic factors play an important role in these associations, but influence life satisfaction also beyond the effects of personality.

Prevalence and stability of mental disorders among young adults: findings from a longitudinal study

BACKGROUND

Mental disorders often have onset early in life, contribute substantially to the global disease burden, and may interfere with young people's ability to complete age-relevant tasks in important developmental periods. However, knowledge about prevalence and course of mental disorders in young adulthood is sparse. The aim of the current study was to estimate prevalence and stability of mental disorders from the twenties to the thirties/forties.

METHODS

DSM-IV mental disorders were assessed with the Composite International Diagnostic Interview in two waves (1999-2004 and 2010-2011) in 1623 young adult Norwegian twins (63.2% women, aged 19-29 years in wave 1).

RESULTS

In wave 1, the 12-month prevalence of any mental disorder among people in the twenties was 19.8% (men) and 32.4% (women), anxiety disorders: 9.6% (men) and 26.7% (women), anxiety disorders excluding specific phobias: 2.5% (men) and 6.9% (women), major depressive disorder (MDD): 4.4% (men) and 7.2% (women), and alcohol use disorder (AUD): 8.7% (men) and 4.4% (women). The prevalence of any mental disorder decreased from the twenties to the thirties/forties. This was due to a decrease in AUD and specific phobias. Anxiety disorders in the twenties predicted anxiety disorders and MDD ten years later, even when controlling for the association between these disorders in the twenties. MDD in the twenties predicted MDD ten years later. At both ages, two-week and 12-month prevalence estimates differed markedly for MDD - indicating an episodic course.

CONCLUSIONS

Common mental disorders are highly prevalent among young adults in the twenties, and somewhat less prevalent in the thirties/forties. Those who suffer from one mental disorder in the twenties are at considerably increased risk for suffering from a disorder ten years later as well. This may have significant implications for young people's ability to attain education, establish a family, and participate in occupational life.

Do DSM-5 Section II personality disorders and Section III personality trait domains reflect the same genetic and environmental risk factors?

BACKGROUND

DSM-5 includes two conceptualizations of personality disorders (PDs). The classification in Section II is identical to the one found in DSM-IV, and includes 10 categorical PDs. The Alternative Model (Section III) includes criteria for dimensional measures of maladaptive personality traits organized into five domains. The degree to which the two conceptualizations reflect the same etiological factors is not known.

METHODS

We use data from a large population-based sample of adult twins from the Norwegian Institute of Public Health Twin Panel on interview-based DSM-IV PDs and a short self-report inventory that indexes the five domains of the DSM-5 Alternative Model plus a domain explicitly targeting compulsivity. Schizotypal, Paranoid, Antisocial, Borderline, Avoidant, and Obsessive-compulsive PDs were assessed at the same time as the maladaptive personality traits and 10 years previously. Schizoid, Histrionic, Narcissistic, and Dependent PDs were only assessed at the first interview. Biometric models were used to estimate overlap in genetic and environmental risk factors.

RESULTS

When measured concurrently, there was 100% genetic overlap between the maladaptive trait domains and Paranoid, Schizotypal, Antisocial, Borderline, and Avoidant PDs. For OCPD, 43% of the genetic variance was shared with the domains. Genetic correlations between the individual domains and PDs ranged from +0.21 to +0.91.

CONCLUSION

The pathological personality trait domains, which are part of the Alternative Model for classification of PDs in DSM-5 Section III, appears to tap, at an aggregate level, the same genetic risk factors as the DSM-5 Section II classification for most of the PDs.

Pre-pregnancy mental distress and musculoskeletal pain and sickness absence during pregnancy - a twin cohort study

Background

Sickness absence (SA) among pregnant women is high. The aim of this study was to examine whether factors known to predict SA in general also predict SA during pregnancy by estimating the association between prior mental distress and musculoskeletal pain and SA during pregnancy, and to assess the influence of familial (genetic and shared environmental) factors. In this prospective cohort study, data from 2076 female twins born 1967-79 who participated in a questionnaire study in 1998 were linked to register data on SA and childbirth during the years 1998-2008. Baseline measures included mental distress (symptoms of anxiety and depression; SCL-5) and musculoskeletal pain (lumbar spine, neck/shoulder and/or persisting muscular pain). SA was measured as a ratio of days on SA divided by potential working days. Negative binomial regression was performed for individual and within-pair effects. Musculoskeletal pain, but not mental distress, was prospectively associated with overall SA during pregnancy in the adjusted individual-level analyses. With each standard deviation increase in musculoskeletal pain, SA granted for any disorder increased with 12% (IRR 1.12, 95% CI = 1.07-1.17) and SA granted for pregnancy related disorders increased with 9% (IRR 1.09, 95% CI = 1.02-1.17). Within-pair estimates were similar, suggesting little or no familial confounding. Women with previous musculoskeletal pain are at increased risk of SA during pregnancy, whereas no increased risk in women with previous symptoms of mental distress could be demonstrated. SA during pregnancy seems partly to be associated with different factors than SA in general.

Genetic and Environmental Contributions to the Association Between Cannabis Use and Psychotic-Like Experiences in Young Adult Twins

To investigate contributions of genetic and environmental risk factors and possible direction of causation for the relationship between symptoms of cannabis use disorders (CUD) and psychotic-like experiences (PLEs), a population-based sample of 2793 young adult twins (63.5% female, mean [range] age 28.2 [19-36] y) were assessed for symptoms of CUD and PLEs using the Composite International Diagnostic Interview. Latent risk of having symptoms of CUD or PLEs was modeled using Item Response Theory. Co-twin control analysis was performed to investigate effect of familiar confounding for the association between symptoms of CUD and PLEs. Biometric twin models were fitted to estimate the heritability, genetic and environmental correlations, and direction for the association. Lifetime use of cannabis was reported by 10.4 % of the twins, and prevalence of PLEs ranged from 0.1% to 2.2%. The incidence rate ratio of PLEs due to symptoms of CUD was 6.3 (95% CI, 3.9, 10.2) in the total sample and 3.5 (95% CI, 1.5, 8.2) within twin pairs. Heritability estimates for symptoms of CUD were 88% in men and women, and for PLEs 77% in men and 43% in women. The genetic and environmental correlations between symptoms of CUD and PLEs were 0.55 and 0.52, respectively. The model allowing symptoms of CUD to cause PLEs had a better fit than models specifying opposite or reciprocal directions of causation. The association between symptoms of CUD and PLEs is explained by shared genetic and environmental factors and direct effects from CUD to risk for PLEs.

The Genetic and Environmental Sources of Resemblance Between Normative Personality and Personality Disorder Traits

Recent work has suggested a high level of congruence between normative personality, most typically represented by the "big five" factors, and abnormal personality traits. In 2,293 Norwegian adult twins ascertained from a population-based registry, the authors evaluated the degree of sharing of genetic and environmental influences on normative personality, assessed by the Big Five Inventory (BFI), and personality disorder traits (PDTs), assessed by the Personality Inventory for DSM-5-Norwegian Brief Form (PID-5-NBF). For four of the five BFI dimensions, the strongest genetic correlation was observed with the expected PID-5-NBF dimension (e.g., neuroticism with negative affectivity [+], conscientiousness with disinhibition [-]). However, neuroticism, conscientiousness, and agreeableness had substantial genetic correlations with other PID-5-NBF dimensions (e.g., neuroticism with compulsivity [+], agreeableness with detachment [-]). Openness had no substantial genetic correlations with any PID-5-NBF dimension. The proportion of genetic risk factors shared in aggregate between the BFI traits and the PID-5-NBF dimensions was quite high for conscientiousness and neuroticism, relatively robust for extraversion and agreeableness, but quite low for openness. Of the six PID-5-NBF dimensions, three (negative affectivity, detachment, and disinhibition) shared, in aggregate, most of their genetic risk factors with normative personality traits. Genetic factors underlying psychoticism, antagonism, and compulsivity were shared to a lesser extent, suggesting that they are influenced by etiological factors not well indexed by the BFI.

A longitudinal, population-based twin study of avoidant and obsessive-compulsive personality disorder traits from early to middle adulthood

BACKGROUND

The phenotypic stability of avoidant personality disorder (AVPD) and obsessive-compulsive personality disorder (OCPD) has previously been found to be moderate. However, little is known about the longitudinal structure of genetic and environmental factors for these disorders separately and jointly, and to what extent genetic and environmental factors contribute to their stability.

METHOD

AVPD and OCPD criteria were assessed using the Structured Interview for DSM-IV Personality in 2793 young adult twins (1385 pairs, 23 singletons) from the Norwegian Institute of Public Health Twin Panel at wave 1 and 2282 (986 pairs, 310 singletons) of these on average 10 years later at wave 2. Longitudinal biometric models were fitted to AVPD and OCPD traits.

RESULTS

For twins who participated at both time-points, the number of endorsed sub-threshold criteria for both personality disorders (PDs) decreased 31% from wave 1 to wave 2. Phenotypic correlations between waves were 0.54 and 0.37 for AVPD and OCPD, respectively. The heritability estimates of the stable PD liabilities were 0.67 for AVPD and 0.53 for OCPD. The genetic correlations were 1.00 for AVPD and 0.72 for OCPD, while the unique environmental influences correlated 0.26 and 0.23, respectively. The correlation between the stable AVPD and OCPD liabilities was 0.39 of which 63% was attributable to genetic influences. Shared environmental factors did not significantly contribute to PD variance at either waves 1 or 2.

CONCLUSION

Phenotypic stability was moderate for AVPD and OCPD traits, and genetic factors contributed more than unique environmental factors to the stability both within and across phenotypes.

A longitudinal twin study of borderline and antisocial personality disorder traits in early to middle adulthood

BACKGROUND

Antisocial personality disorder (ASPD) and borderline personality disorder (BPD) share genetic and environmental risk factors. Little is known about the temporal stability of these etiological factors in adulthood.

METHOD

DSM-IV criteria for ASPD and BPD were assessed using structured interviews in 2282 Norwegian twins in early adulthood and again approximately 10 years later. Longitudinal biometric models were used to analyze the number of endorsed criteria.

RESULTS

The mean criterion count for ASPD and BPD decreased 40% and 28%, respectively, from early to middle adulthood. Rank-order stability was 0.58 for ASPD and 0.45 for BPD. The best-fitting longitudinal twin model included only genetic and individual-specific environmental factors. Genetic effects, both those shared by ASPD and BPD, and those specific to each disorder remained completely stable. The unique environmental effects, however, changed substantially, with a correlation across time of 0.19 for the shared effects, and 0.39 and 0.15, respectively, for those specific to ASPD and BPD. Genetic effects accounted for 71% and 72% of the stability over time for ASPD and BPD, respectively. The genetic and environmental correlations between ASPD and BPD were 0.73, and 0.43, respectively, at both time points.

CONCLUSION

ASPD and BPD traits were moderately stable from early to middle adulthood, mostly due to genetic risk factors which did not change over the 10-year assessment period. Environmental risk factors were mostly transient, and appear to be the main source of phenotypic change. Genetic liability factors were, to a large extent, shared by ASPD and BPD.

A longitudinal twin study of cluster A personality disorders

BACKGROUND

While cluster A personality disorders (PDs) have been shown to be moderately heritable, we know little about the temporal stability of these genetic risk factors.

METHOD

Paranoid PD (PPD) and schizotypal PD (STPD) were assessed using the Structured Interview for DSM-IV Personality in 2793 young adult twins from the Norwegian Institute of Public Health Twin Panel at wave 1 and 2282 twins on average 10 years later at wave 2. Using the program Mx, we fitted a longitudinal latent factor model using the number of endorsed criteria for PPD and STPD.

RESULTS

The stability over time of the criteria counts for PPD and STPD, estimated as polychoric correlations, were +0.34 and +0.40, respectively. The best-fit longitudinal model included only additive genetic and individual-specific environmental factors with parameter estimates constrained to equality across the two waves. The cross-wave genetic and individual-specific environmental correlations for a latent cluster A factor were estimated to equal +1.00 and +0.13, respectively. The cross-time correlations for genetic and environmental effects specific to the individual PDs were estimated at +1.00 and +0.16-0.20, respectively. We found that 68% and 71% of the temporal stability of PPD and STPD derived, respectively, from the effect of genetic factors.

CONCLUSION

Shared genetic risk factors for two of the cluster A PDs are highly stable in adults over a 10-year period while environmental risk factors are relatively transient. Over two-thirds of the long-term stability of the common cluster A PD liability can be attributed to genetic influences.

Socioeconomic status and sick leave granted for mental and somatic disorders: a prospective study of young adult twins

Background

Low socioeconomic status (SES), indicated by low income and education, has consistently been found to be a strong predictor of sick leave. Several possible pathways from SES to sick leave have been described in previous literature, but there are also evidence indicating that the association can be confounded by common underlying factors. This study utilizes a population-based sample of employed young adult twins to estimate (i) the degree to which education and income are prospectively related to sick leave granted for mental, somatic, and any disorder, and (ii) whether these associations are confounded by familial factors.

Methods

Registry data on educational attainment and income at age 30 and subsequent sick leave were available for 6,103 employed young adult twins, among which there were 2,024 complete twin pairs. The average follow-up time was 6.57 years. Individual-level associations and fixed effects within twin pairs were estimated.

Results

Low education and income were associated with sick leave granted for both mental and somatic disorders, and with sick leave granted for any disorder. Associations were attenuated within dizygotic twin pairs and reduced to non-significance within monozygotic twin pairs, suggesting influence of familial factors on the associations between SES and sick leave.

Conclusions

Low SES is associated with a higher level of sick leave granted for both mental and somatic disorders among young adults, but these associations are confounded by factors that are common to co-twins. Education and income are therefore not likely to strongly affect sick leave in young adulthood.

Co-occurrence of IBS and symptoms of anxiety or depression, among Norwegian twins, is influenced by both heredity and intrauterine growth

Background

Environmental and genetic factors contribute to variation in irritable bowel syndrome (IBS), anxiety and depression. Comorbidity between these disorders is high. A previous investigation of our population-based twin cohort revealed that low birth weight increased the risk for development of IBS, with environmental influences in utero as the most relevant contributing factor. We hypothesise that both intrauterine and genetic factors influence the co-occurrence of IBS and symptoms of anxiety and depression.

Methods

A postal questionnaire sent to 12700 Norwegian twins born between 1967 and 1979 comprised a checklist of 31 illnesses and symptoms, including IBS and symptoms of anxiety and depression. The influence of genetic factors and intrauterine growth on comorbidity between these disorders were analysed in the full sample and compared to those based on only monozygotic (MZ) twin pairs discordant for IBS (95 pairs) in birth weight group < 2500 g and ≥ 2500 g.

Results

In the co-twin analyses restricted growth (birth weight < 2500 g) was significantly associated with anxiety and depression (average birth weight difference of 181.0 g (p <0.0001) and 249.9 g (p < 0.0001), respectively).

The analysis of the full sample revealed that IBS was significantly associated with symptoms of anxiety (adjusted OR = 2.5, 95% CI: 1.9, 3.3) and depression (adjusted OR = 2.3. 95% CI: 1.8, 3.0). Analyses of MZ pairs discordant for IBS indicated significant associations between IBS and symptoms of anxiety (OR = 3.7, 95% CI: 1.3, 10.5) and between IBS and symptoms of depression (OR = 4.2, 95% CI: 1.7, 9.9) only in the birth weight group below 2500 g.

Conclusion

Our findings suggest that genetic factors partly explain the association between IBS and symptoms of anxiety and depression. In the low range of birth weight (<2500 g), restricted fetal growth seems to be a common contributing factor to the co-occurrence between these disorders.

Sex Differences in Genetic and Environmental Influences on Educational Attainment and Income

In many Western countries, women now reach educational levels comparable to men, although their income remains considerably lower. For the past decades, it has become increasingly clear that these measures of socio-economic status are influenced by genetic as well as environmental factors. Less is known about the relationship between education and income, and sex differences. The aim of this study was to explore genetic and environmental factors influencing education and income in a large cohort of young Norwegian twins, with special emphasis on gender differences. National register data on educational level and income were obtained for 7,710 twins (aged 29–41 years). Bivariate Cholesky models were applied to estimate qualitative and quantitative gender differences in genetic and environmental influences, the relative contribution of genetic and environmental factors to the correlation between education and income, and genetic correlations within and between sexes and phenotypes. The phenotypic correlation between educational level and income was 0.34 (0.32–0.39) for men and 0.45 (0.43–0.48) for women. An ACE model with both qualitative and quantitative sex differences fitted the data best. The genetic correlation between men and women (rg) was 0.66 (0.22–1.00) for educational attainment and 0.38 (0.01–0.75) for income, and between the two phenotypes 0.31 (0.08–0.52) for men and 0.72 (0.64–0.85) for women. Our results imply that, in relatively egalitarian societies with state-supported access to higher education and political awareness of gender equality, genetic factors may play an important role in explaining sex differences in the relationship between education and income.

Early age of alcohol initiation is not the cause of alcohol use disorders in adulthood, but is a major indicator of genetic risk. A population-based twin study

BACKGROUND AND AIMS

An early age of alcohol initiation (AAI) is associated with and has been hypothesized to be a cause of alcohol use disorders (AUD) in adulthood. Results from twin studies, however, indicate that AAI is an indicator of risk for AUD. We aimed to test a causal hypothesis versus a risk indicator hypothesis for the relationship between early AAI and AUD.

DESIGN

A population-based twin study using biometric twin modelling.

SETTING

Norway.

PARTICIPANTS

A population-based sample of 1336 Norwegian twins.

MEASUREMENTS

Life-time DSM-IV AUDs were assessed by structured clinical interview and AAI by questionnaire.

FINDINGS

The risk indicator model in which the association between AAI and AUD was explained by common vulnerability was the best fitted to the data. The heritability was 37% [95% confidence interval (CI) = 21%, 53%] for AAI and 62% (95% CI = 51%, 73%) for AUD. Genetic risk for AAI accounted for 44% (95% CI = 17%, 71%) of the total genetic risk for AUD and the correlation between genetic factors for AAI and AUD was -0.66 (95%CI -0.87, -0.46). Individual-specific environmental risk for AAI explained only 1% (95% CI = 0%, 3%) of the risk for AUD. Shared environmental factors did not influence AUD, but accounted for 25% (95% CI = 7%, 35%) of the variance in AAI.

CONCLUSIONS

The association between early age of alcohol initiation and alcohol use disorders in later life does not reflect a causal relationship, but is due almost entirely to common genetic risk factors.

Maternal eating disorder and infant diet. A latent class analysis based on the Norwegian Mother and Child Cohort Study (MoBa)

Knowledge of infant diet and feeding practices among children of mothers with eating disorders is essential to promote healthy eating in these children. This study compared the dietary patterns of 6-month-old children of mothers with anorexia nervosa, bulimia nervosa, binge eating disorder, and eating disorder not otherwise specified-purging subtype, to the diet of children of mothers with no eating disorders (reference group). The study was based on 53,879 mothers in the Norwegian Mother and Child Cohort Study (MoBa). Latent class analysis (LCA) was used to identify discrete latent classes of infant diet based on the mothers' responses to questions about 16 food items. LCA identified five classes, characterized by primarily homemade vegetarian food (4% of infants), homemade traditional food (8%), commercial cereals (35%), commercial jarred baby food (39%), and a mix of all food groups (11%). The association between latent dietary classes and maternal eating disorders were estimated by multinomial logistic regression. Infants of mothers with bulimia nervosa had a lower probability of being in the homemade traditional food class compared to the commercial jarred baby food class, than the referent (O.R. 0.59; 95% CI 0.36-0.99). Infants of mothers with binge eating disorder had a lower probability of being in the homemade vegetarian class compared to the commercial jarred baby food class (O.R. 0.77; 95% CI 0.60-0.99), but only before adjusting for relevant confounders. Anorexia nervosa and eating disorder not otherwise specified-purging subtype were not statistically significantly associated with any of the dietary classes. These results suggest that maternal eating disorders may to some extent influence the child's diet at 6 months; however, the extent to which these differences influence child health and development remains an area for further inquiry.

Genetic and environmental contributions to long-term sick leave and disability pension: a population-based study of young adult Norwegian twins

Although exclusion from the workforce due to long-term sick leave (LTSL) and disability pension (DP) is a major problem in many Western countries, the etiology of LTSL and DP is not well understood. These phenomena have a strong association as most patients receiving DP have first been on LTSL. However, only a few of those on LTSL end up with DP. The present study aimed to investigate the common and specific genetic and environmental risk factors for LTSL and DP. The present study utilizes a population-based sample of 7,710 young adult twins from the Norwegian Institute of Public Health Twin Panel, which has been linked to the Historical-Event Database (FD-Trygd; 1998-2008). Univariate and bivariate twin models were fitted to determine to what degree genetic and environmental factors contribute to variation in LTSL and DP. The estimated heritabilities of LTSL and DP were 0.49 and 0.66, respectively. There was no evidence for shared environmental or sex-specific factors. The phenotypic-, genetic-, and non-familial environmental correlations between the variables were 0.86, 0.82, and 0.94, respectively. Our results indicate that familial transmission of LTSL and DP is due to genetic and not environmental factors. The risk factors contributing to LTSL and DP were mainly shared, suggesting that what increases risk for LTSL also increases risk for DP. However, a non-negligible part of the genetic variance was not shared between the variables, which may contribute to explaining why some progress from LTSL to DP, whereas others return to work.

Major depression and life satisfaction: a population-based twin study

BACKGROUND

The extent to which positive and negative indicators of mental health share etiological influences has been studied to a limited degree only. This study examines the genetic and environmental influences on association between liability to lifetime DSM-IV Major Depressive Disorder (MDD) and dispositional life satisfaction (LS).

METHODS

Two-wave questionnaire data on LS (assessed 6 years apart) and lifetime MDD obtained by structured clinical interviews in a population-based sample of adult twins were analysed using structural equation modelling in Mx.

RESULTS

The prevalence of lifetime MDD was estimated to be 11.1% and 15.8% in males and females, respectively. Individuals fulfilling the criteria for MDD reported significantly lower levels of LS. The co-variation in MDD and dispositional LS was found to be accounted for by genetic and unique environmental influences only. The phenotypic correlation was estimated to be 0.36, of which genetic influences accounted for 74% and environmental factors the remaining 26%. The correlation between genetic factors for MDD and LS was estimated to be -0.55 and the correlation between unique environmental factors to be -0.22. Heritability was estimated to 0.34 and 0.72 for MDD and LS, respectively.

LIMITATIONS

The sample consists of twins only and there are limitations associated with the twin design.

CONCLUSIONS

Whereas genetic influences on vulnerability to lifetime MDD are considerably shared with liability to (low) LS, environmental influences are more distinct. Thus, environmental factors associated with risk of MDD do not strongly impact on dispositional LS, and conversely, environmental factors influencing dispositional LS do not strongly buffer against MDD.

The Norwegian Twin Registry from a Public Health Perspective: A Research Update

We describe the importance of the Norwegian Twin Registry (NTR) for research in public health and provide examples from several programs of twin research at the Norwegian Institute of Public Health (NIPH), including the Nordic Twin Study of Cancer, our epigenetics platform, and our large program of research in mental health. The NTR has become an integral component of a national strategy for maximizing the research potential from Norwegian registries and biobank-based studies. The information provided herein builds upon and complements our recent report describing the establishment of the NTR and the cohorts comprising it. Although Norway has a long tradition in twin research, the centralization and administration of the twin data through a single register structure is fairly recent. The NTR was established in 2009 and currently includes 47,989 twins covering birth years 1895–1960 and 1967–1979; 31,440 of these twins have consented to participate in medical research (comprising 5,439 monozygotic pairs, 6,702 dizygotic same-sexed pairs, and 1,655 dizygotic opposite-sexed pairs). DNA from approximately 4,800 twins is banked at the NIPH biobank and new studies continuously add new data to the registry. The value of NTR data is greatly enhanced through record linkage possibilities offered by Norway's many nation-wide registries (medical, demographic, and socio-economic) and several studies are already taking advantage of these linkage opportunities for research.

The Norwegian Twin Registry

Norway has a long-standing tradition in twin research, but the data collected in several population-based twin studies were not coordinated centrally or easily accessible to the scientific community. In 2009, the Norwegian Twin Registry was established at the Norwegian Institute of Public Health (NIPH) in Oslo with the purpose of creating a single research resource for Norwegian twin data. As of today, the Norwegian Twin Registry contains 47,989 twins covering birth years 1895-1960 and 1967-1979; 31,440 of these twins consented to participate in health-related research. In addition, DNA from approximately 4,800 of the twins is banked at the NIPH biobank and new studies are continually adding new data to the registry. The value of the Norwegian twin data is greatly enhanced by the linkage opportunities offered by Norway's many nationwide registries, spanning a broad array of medical, demographic, and socioeconomic information.

Genetic and environmental contributions to the relationship between education and anxiety disorders - a twin study

OBJECTIVE

To examine the negative statistical relationship between educational level and risk of anxiety disorders, and to estimate to what extent this relationship may be explained by genes or environmental factors influencing both phenotypes.

METHOD

Registry data on educational level for 3339 young adult Norwegian twin pairs and diagnostic data on anxiety disorders for 1385 of these pairs were analysed, specifying structural equations models using MX software.

RESULTS

In the best-fitting model, genes accounted for 59% of the variance in education. 18% of the variance was due to environmental factors shared by co-twins, and the remaining 23% due to non-shared environment; 46% of the variance in liability to anxiety disorders was genetic, the remaining variance was due to non-shared environment. A phenotypic polychoric correlation of -0.30 between educational level and 'any anxiety disorder' was estimated to be primarily (83% in the best-fitting model) caused by genes common to the two traits.

CONCLUSION

The relationship between low education and risk of anxiety disorders appears to be primarily determined by genetic effect common to educational level and anxiety disorders.

Mates and Marriage Matter: Genetic and Environmental Influences on Subjective Wellbeing Across Marital Status

Specific environments and social relationships may alter the impact of genes. Previous studies have shown marriage to moderate heritability for depressive symptoms in females, suggesting that marriage provides protection or compensation against genetic risks. Similar mechanisms may be relevant for subjective wellbeing (SWB), which is considerably influenced by genes and almost universally associated with marital status. Questionnaire data on SWB from a population-based sample of 1250 monozygotic (MZ) and 981 dizygotic (DZ) male and female twin pairs (n= 4462) were analyzed using structural equation modeling by means of Mx to investigate genetic and environmental influences on SWB across marital status. Resemblance for SWB in MZ twins exceeded that of DZ twins, but the magnitude of this difference varied across marital status. Genetic factors explained 51% and 54% of the variance in SWB among unmarried males and females, and 41% and 39% in married or cohabitating respondents. Remaining variance was attributable to the nonshared environment. The genetic influences were partly different (rg= 0.64) across marital status in females, but overlapping in married and single males. Our findings show that marriage moderates the magnitude of genetic influences on SWB in both males and females, with a smaller estimate of genetic influences for those with a marital or equivalent partner. The genetic influences on SWB are thus clearly contingent on the environmental context.

DNA methylation and gene expression changes in monozygotic twins discordant for psoriasis: identification of epigenetically dysregulated genes

Monozygotic (MZ) twins do not show complete concordance for many complex diseases; for example, discordance rates for autoimmune diseases are 20%-80%. MZ discordance indicates a role for epigenetic or environmental factors in disease. We used MZ twins discordant for psoriasis to search for genome-wide differences in DNA methylation and gene expression in CD4(+) and CD8(+) cells using Illumina's HumanMethylation27 and HT-12 expression assays, respectively. Analysis of these data revealed no differentially methylated or expressed genes between co-twins when analyzed separately, although we observed a substantial amount of small differences. However, combined analysis of DNA methylation and gene expression identified genes where differences in DNA methylation between unaffected and affected twins were correlated with differences in gene expression. Several of the top-ranked genes according to significance of the correlation in CD4(+) cells are known to be associated with psoriasis. Further, gene ontology (GO) analysis revealed enrichment of biological processes associated with the immune response and clustering of genes in a biological pathway comprising cytokines and chemokines. These data suggest that DNA methylation is involved in an epigenetic dysregulation of biological pathways involved in the pathogenesis of psoriasis. This is the first study based on data from MZ twins discordant for psoriasis to detect epigenetic alterations that potentially contribute to development of the disease.

Specific advantages of twin registries and biobanks

This chapter briefly reviews the role of twin studies and study designs based on using twins in different settings. In the Nordic countries, twin registers and cohorts have existed already for many decades. These are a unique resource for scientific studies; a major strength being their unselected and representative nature. In the past years, biological samples are also being collected within the studies conducted on the Nordic twins.

Concordance for IBD among twins compared to ordinary siblings--a Norwegian population-based study

AIMS

Comparing the risk to develop concordant disease among twins with inflammatory bowel disease (IBD) to ordinary siblings. Moreover, clinical characteristics of IBD and the association between perinatal factors and IBD, were evaluated.

METHODS

Patients with IBD, enrolled from an incidence study between 1990 and 1994, and the twins were identified from the Norwegian national birth registry, which was established in 1967.

RESULTS

Eight monozygotic and 16 dizygotic pairs, in which at least one twin reported a positive history of IBD were compared to 84 patients with Crohn's disease (CD) and 87 patients with ulcerative colitis (UC) from the incidence study. The relative risks for concordant disease in monozygotic pairs were estimated to 95.4 (95% CI: 76.3, 114.6) and 49.5 (95% CI: 35.7, 63.3) for CD and UC, respectively. The corresponding risks in dizygotic pairs were 42.4 (95% CI: 29.6, 55.2) and 0.0. Among ordinary siblings of CD and UC the risks for concordance were 22.7 (95% CI: 13.3, 32.1) and 4.6 (95% CI: 0.4, 8.7), respectively. Stricturing disease was significantly higher in twins with CD compared to incidental cases. The first-born twin in pairs discordant for disease, 12 out of 19 (63.2%), tended to be affected by IBD (p=0.10).

CONCLUSION

Genetic factors influence the development of IBD and fibrostenotic disease in CD. The increased risk for concordant disease among dizygotic twins compared to ordinary siblings, at least in CD, might underscore the importance of shared environment in utero or in childhood.

The Norwegian Institute of Public Health twin study of mental health: examining recruitment and attrition bias

All Norwegian twin pairs born 1967-1974 and still living in Norway in 1992 were invited to a health questionnaire study (Q1). 2,570 pairs (65%) participated. These cohorts and the twin cohorts born 1967-1979 were invited to a new questionnaire study (Q2) in 1998. This time 3,334 pairs (53%) participated. Almost all pairs having participated in the 1998 study were invited to an interview study of mental health (MHS), taking place 1999-2004. 1,391 complete pairs (44%) participated. The questionnaire studies included extensive data on somatic health with fewer items on mental health and demography. Health-related and demographic information available from the Medical Birth Registry on all invited twins was applied to predict participation to the first study. A few registry variables indicating poor health predicted nonparticipation in Q1. Health information and demography from Q1 were tested as predictors of participation in the follow-up study (Q2). Monozygosity, female sex, being unmarried, having no children, and high education predicted participation, whereas few indicators of poor mental and somatic health and unhealthy lifestyle moderately predicted nonparticipation in Q2. No health indicators reported in Q2 predicted further participation. Standard genetic twin analyses of indicators of various mental disorders from Q2, validated by diagnostic data from the MHS, did not indicate differences in genetic/environmental covariance structures between participants and nonparticipants in MHS. In general the results show a moderate selection towards good mental and somatic health. Attrition from Q2 to the MHS does not appear to affect twin analyses of mental health related variables.

Assessing the heritability of anorexia nervosa symptoms using a marginal maximal likelihood approach

BACKGROUND

Assessment of eating disorders at the symptom level can facilitate the refinement of phenotypes. We examined genetic and environmental contributions to liability to anorexia nervosa (AN) symptoms in a population-based twin sample using a genetic common pathway model.

METHOD

Participants were from the Norwegian Institute of Public Health Twin Panel (NIPHTP) and included all female monozygotic (MZ; 448 complete pairs and four singletons) and dizygotic (DZ; 263 complete pairs and four singletons) twins who completed the Composite International Diagnostic Interview (CIDI) assessing DSM-IV Axis I and ICD-10 criteria. Responses to items assessing AN symptoms were included in a model fitted using the marginal maximum likelihood (MML) approach.

RESULTS

Heritability of the overall AN diagnosis was moderate [a2=0.22, 95% confidence interval (CI) 0.0-0.50] whereas heritabilities of the specific items varied. Heritability estimates for weight loss items were moderate (a2=0.31-0.34) and items assessing weight concern when at a low weight were smaller (0.18-0.29). Additive genetic factors contributed little to the variance of amenorrhea, which was most strongly influenced by unshared environment (a2=0.16, e2=0.71).

CONCLUSIONS

AN symptoms are differentially heritable. Specific criteria such as those related to body weight and weight loss history represent more biologically driven potential endophenotypes or liability indices. The results regarding weight concern differ somewhat from those of previous studies, highlighting the importance of assessing genetic and environmental influences on variance of traits within specific subgroups of interest.

A twin study of the common vulnerability between heightened sensitivity to hypercapnia and panic disorder

For unknown reasons the inhalation of CO(2)-enriched air mixtures evokes acute panic-like symptoms in people with panic disorder and in their unaffected relatives. This study was set to determine whether, and to what extent, CO(2)-induced acute anxiety and panic disorder share the same genetic and environmental determinants. Cholesky structural equation models were used to decompose into genetic and environmental elements the correlation between self-assessed anxiety post-35%CO(2)-65%O(2) inhalation and interview-based DSM-IV lifetime diagnoses of panic disorder in 346 young adult twin pairs of the Norwegian Institute of Health Panel, 12% of whom had been invited to take part into the CO(2) study on the basis of self-reported symptoms of anxiety gathered 4-7 years before the provocation challenge. A full model corrected for the partially selective ascertainment showed that the phenotypic correlation between post-CO(2) anxiety and DSM-IV panic was largely due to additive genetic influences, while shared and unique environmental agents concurred to explain a relatively minor proportion of the correlation between these two traits. According to the best-fitting model the genetic correlation between post-CO(2) anxiety and panic was 0.81 (0.50-0.98); a common genetic factor was sufficient to explain the traits' covariation and a further, specific genetic factor was necessary to account for the residual phenotypic variance. The genetic determinants that lead to overreact to a hypercapnic stimulus coincide at a considerable extent with those that influence liability to naturally occurring panic. Environmental factors provide a modest--or no--contribution to the covariation of CO(2)-provoked anxiety with naturally occurring panic.