Familial confounding or measurement error? How to interpret findings from sibling and co-twin control studies.

Epidemiological researchers often examine associations between risk factors and health outcomes in non-experimental designs. Observed associations may be causal or confounded by unmeasured factors. Sibling and co-twin control studies account for familial confounding by comparing exposure levels among siblings (or twins). If the exposure-outcome association is causal, the siblings should also differ regarding the outcome. However, such studies may sometimes introduce more bias than they alleviate. Measurement error in the exposure may bias results and lead to erroneous conclusions that truly causal exposure-outcome associations are confounded by familial factors. The current study used Monte Carlo simulations to examine bias due to measurement error in sibling control models when the observed exposure-outcome association is truly causal. The results showed that decreasing exposure reliability and increasing sibling-correlations in the exposure led to deflated exposure-outcome associations and inflated associations between the family mean of the exposure and the outcome. The risk of falsely concluding that causal associations were confounded was high in many situations. For example, when exposure reliability was 0.7 and the observed sibling-correlation was r = 0.4, about 30-90% of the samples (n = 2,000) provided results supporting a false conclusion of confounding, depending on how p-values were interpreted as evidence for a family effect on the outcome. The current results have practical importance for epidemiological researchers conducting or reviewing sibling and co-twin control studies and may improve our understanding of observed associations between risk factors and health outcomes. We have developed an app (SibSim) providing simulations of many situations not presented in this paper.

Eating Problems Among Adolescent Boys and Girls Before and During the Covid-19 Pandemic.

OBJECTIVE: Studies suggest that adolescents reported more eating problems during the pandemic. Using a population-based sample, we compared eating problems-and how they associate with a range of personal characteristics and genetic factors-among adolescents before (June 2017-April 2020) versus during (April 2020-December 2022) the pandemic. METHOD: Based on a preregistered analysis plan, we used cross-sectional data collected from 22,706 14-16-year-olds over 6 years (55% during the pandemic) in the Norwegian Mother, Father, and Child Cohort. We used measurement invariance analyses to compare the level of eating restraint and body concern before and during the pandemic, and multi-group structural equation models to estimate pre-pandemic and pandemic patterns of associations. RESULTS: Pandemic responders generally reported more eating problems than pre-pandemic responders, specifically on dieting and body dissatisfaction. However, after adjusting for a general linear increase in eating problems across all 6 years of data collection, the pandemic itself seems to be associated with more eating problems only among girls, reporting more eating restraints (meanΔ = 0.14 [CI: 0.07, 0.20]) and body concern (meanΔ = 0.17 [CI: 0.11, 0.23]). Associations between eating problems and a range of other characteristics did not differ across the pandemic and pre-pandemic groups. CONCLUSIONS: There was a general increase in eating problems among 14-16-year-olds over time. Adjusting for this trend, the pandemic seems to exacerbate problems among girls. Although the mechanisms are unclear, our results point to factors susceptible to change that could have been intensified during the pandemic (e.g., screen time, mental distress). Our results highlight the importance of recognizing sex-specific differences in eating problems.

Interparental conflict trajectories across various child residence arrangements when parents live apart.

In Norway, as in most Western countries, a growing proportion of parents living apart choose shared residence for their children. The aim of this study was to investigate trajectories of five interparental conflict dimensions across four child residence arrangement groups (and three combination groups) to improve understanding of different conflict trajectories when parents live apart. We used data from the Dynamics of Family Conflict study. Families (N = 1136) were recruited from 37 family counseling centers across Norway. Parents answered questionnaires in three waves: Wave 1 (December 2017 through August 2019); Wave 2 (November 2019 through January 2021); and Wave 3 (April through May 2021). Mixed effects analyses indicated that (a) for all conflict dimensions, there was less conflict and more cooperation over time across all residence arrangements; (b) except for children's involvement in conflict, the conflict dimensions did not develop differently over time between residence arrangements; (c) families with arrangements in which one parent had minority time (1%-14% and 15%-34%) were more likely to report children being involved in their parents' arguments over time than the 35%-49% and 50/50 residence groups; (d) for families with high relational risk pattern, children's involvement in conflict did not decline in either a high (1-34%) or a low degree (35%-49%) of sharing; and (e) families with a violent risk pattern and low degree of sharing (1%-34%) had the steepest decrease in conflict frequency/intensity over time. Even with an average decrease in destructive conflict dimensions over time, the findings point to the need for providing support for parents with complex needs, particularly for parents with a high relational risk pattern.

Prenatal maternal stress, child internalizing and externalizing symptoms, and the moderating role of parenting: findings from the Norwegian mother, father, and child cohort study.

BACKGROUND: Few studies have examined how parenting influences the associations between prenatal maternal stress and children's mental health. The objectives of this study were to examine the sex-specific associations between prenatal maternal stress and child internalizing and externalizing symptoms, and to assess the moderating effects of parenting behaviors on these associations. METHODS: This study is based on 15 963 mother-child dyads from the Norwegian Mother, Father, and Child Cohort Study (MoBa). A broad measure of prenatal maternal stress was constructed using 41 self-reported items measured during pregnancy. Three parenting behaviors (positive parenting, inconsistent discipline, and positive involvement) were assessed by maternal report at child age 5 years. Child symptoms of internalizing and externalizing disorders (depression, anxiety, attention-deficit hyperactivity disorder, conduct disorder, and oppositional-defiant disorder) were assessed by maternal report at age 8. Analyses were conducted using structural equation modeling techniques. RESULTS: Prenatal maternal stress was associated with child internalizing and externalizing symptoms at age 8; associations with externalizing symptoms differed by sex. Associations between prenatal maternal stress and child depression, and conduct disorder and oppositional-defiant disorder in males, became stronger as levels of inconsistent discipline increased. Associations between prenatal maternal stress and symptoms of attention-deficit hyperactivity disorder in females were attenuated as levels of parental involvement increased. CONCLUSIONS: This study confirms associations between prenatal maternal stress and children's mental health outcomes, and demonstrates that these associations may be modified by parenting behaviors. Parenting may represent an important intervention target for improving mental health outcomes in children exposed to prenatal stress.

Common genetic and environmental risk for personality disorders and psychotic-like experiences in young adult twins.

INTRODUCTION: Psychotic-like experiences (PLE) have been associated with the subsequent emergence of psychotic disorders as well as several other domains of psychopathology. In this twin study, we estimated the genetic and environmental correlations between PLE and 10 personality disorders (PD). METHODS: Diagnoses of 10 PDs according to the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) and PLE from the Composite International Diagnostic Interview (CIDI) were retrieved for 2793 young adult twins from the Norwegian Twin Registry. Risk for having a PD and PLEs was modeled using item response theory. Biometric twin models were fitted to estimate the genetic and environmental correlations between PDs and PLEs. Co-twin control analysis was performed to estimate additional within-family risk for PLEs when having a PD. RESULTS: Phenotypic overlap between PDs and PLEs ranged from 14% to 44% in males and from 11% to 39% in females, with the highest overlap for borderline PD in both sexes. In general, we found higher genetic correlations (r = 0.14-0.72) than environmental correlations (r = 0.06-0.28) between PDs and PLEs. The highest genetic correlations between PLE and PDs were found for borderline (r = 0.72), paranoid (r = 0.56), schizotypal (r = 0.56) and antisocial PD (r = 0.49). CONCLUSION: We found that the co-occurrence between PDs and PLE is the best explained by shared genetic determinants, with minor contributions from environmental factors. Interestingly, borderline PD was highly genetically correlated with PLE, warranting molecular genetic studies of this association.

Incidence rates of treated mental disorders before and during the COVID-19 pandemic-a nationwide study comparing trends in the period 2015 to 2021.

BACKGROUND: There is a concern that exposure to psychosocial stressors during the COVID-19 pandemic may have led to a higher incidence of mental disorders. Thus, this study aimed to compare trends in incidence rates of depressive disorder, anxiety disorders, obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), and eating disorders in primary- and specialist health care before (2015-2019) and during the COVID-19 pandemic (2020-2021). METHODS: We used aggregated population registry data to calculate incidence rates of mental disorders from primary- (The Norwegian Control and Payment of Health Reimbursements Registry (KUHR)) and specialist (The Norwegian Patient Registry (NPR)) health care. The analyses included all Norwegian residents aged 18-65 during the study period. Incident cases were defined as having no previous registration with the same mental disorder in KUHR (from 2006) or NPR (from 2008). We used linear prediction models and mean models to compare incidence rates and test trends before and during the pandemic. RESULTS: During the pandemic, the incidence rates among women were higher or as predicted for OCD in specialist health care and for eating disorders in both primary- and specialist health care. These findings were strongest among women aged 18-24 years. Incidence rates for depression and phobia/OCD among both genders in primary health care and phobic anxiety disorders among both genders in specialist health care were lower or as predicted. CONCLUSION: The COVID-19 pandemic may have led to more women needing treatment for OCD and eating disorders in the Norwegian population. The decreased incidence rates for some disorders might indicate that some individuals either avoided seeking help or had improved mental health during the pandemic.

Contact with primary care physicians among adults with pre-existing common mental health problems during the COVID-19 pandemic: a registry-based study from Norway.

BACKGROUND: During the COVID-19 pandemic, individuals with pre-existing mental health problems may have experienced additional stress, which could worsen symptoms or trigger relapse. Thus, this study aimed to investigate if the number of consultations with general practitioners (GPs) among individuals with a pre-existing common mental health problem during the pandemic differed from pre-pandemic years. METHODS: Data on consultations with GPs among 18-65-year-olds registered with common mental health problems in 2017-2021 were retrieved from the Norwegian Control and Payment of Health Reimbursements Database. Based on data from the pre-pandemic years (2017-2019), we predicted the number of consultations per week for depression, anxiety disorder, phobia/obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), and eating disorders during the pandemic (March 2020-December 2021) among individuals with pre-existing mental health problems. The forecasted and observed trends in GP consultations per week during the pandemic were stratified by diagnosis, gender, and age groups. RESULTS: The observed number of consultations for anxiety disorder, PTSD, and eating disorders were significantly higher than forecasted during extended periods of the two pandemic years. The differences were largest for PTSD (on average 37% higher in men and 47% higher in women during the pandemic), and for eating disorders among women (on average 87% higher during the pandemic). There were only minor differences between the predicted and observed number of consultations for depression and phobia/OCD. CONCLUSIONS: During the pandemic, individuals with a recent history of mental health problems were more likely to seek help for anxiety disorder, PTSD, and eating disorders, as compared to pre-pandemic years.

Positive maternal mental health attenuates the associations between prenatal stress and children's internalizing and externalizing symptoms.

Positive maternal mental health can improve perceptions of stressful situations and promote the use of adaptive coping strategies. However, few studies have examined how positive maternal mental health affects children's development. The aims of this study were to examine the associations between positive maternal mental health and children's internalizing and externalizing symptoms, and to ascertain whether positive maternal mental health moderated the associations between prenatal stress and children's internalizing and externalizing symptoms. This study is based on the Norwegian Mother, Father, and Child Cohort Study (MoBa), and comprised 36,584 mother-child dyads. Prenatal stress was assessed using 41 self-reported items measured during pregnancy. Positive maternal mental health (self-efficacy, self-esteem, and enjoyment) was assessed by maternal report during pregnancy and postpartum. Child internalizing and externalizing symptoms were assessed by maternal report at age 5. Structural equation modeling was used for analysis. Maternal self-efficacy, self-esteem, and enjoyment were negatively associated with internalizing and externalizing symptoms in males and females. The association between prenatal stress and internalizing symptoms in males was stronger at low than at high levels of maternal self-esteem and enjoyment, whereas for females, the association was stronger at low than at high levels of maternal self-esteem and self-efficacy. This study provides evidence of associations between positive maternal mental health and children's mental health, and suggests that higher positive maternal mental health may buffer against the impacts of prenatal stress. Positive maternal mental health may represent an important intervention target to improve maternal-child well-being and foster intergenerational resilience.

Educational attainment and mortality in schizophrenia.

BACKGROUND: Individuals suffering from schizophrenia have a reduced life expectancy with cardiovascular disease (CVD) as a major contributor. Low educational attainment is associated with schizophrenia, as well as with all-cause and CVD mortality. However, it is unknown to what extent low educational attainment can explain the increased mortality in individuals with schizophrenia. AIM: Here, we quantify associations between educational attainment and all-cause and CVD mortality in individuals with schizophrenia, and compare them with the corresponding associations in the general population. METHOD: All Norwegian citizens born between January 1, 1925, and December 31, 1959, were followed up from January 1, 1990, to December 31, 2014. The total sample included 1,852,113 individuals, of which 6548 were registered with schizophrenia. We estimated hazard ratios (HR) for all-cause and CVD mortality with Cox models, in addition to life years lost. Educational attainment for index persons and their parents were included in the models. RESULTS: In the general population individuals with low educational attainment had higher risk of all-cause (HR: 1.48 [95% CI: 1.47-1.49]) and CVD (HR: 1.59 [95% CI: 1.57-1.61]) mortality. In individuals with schizophrenia these estimates were substantially lower (all-cause: HR: 1.13 [95% CI: 1.05-1.21] and CVD: HR: 1.12 [95% CI: 0.98-1.27]). Low educational attainment accounted for 3.28 (3.21-3.35) life years lost in males and 2.48 (2.42-2.55) years in females in the general population, but was not significantly associated with life years lost in individuals with schizophrenia. Results were similar for parental educational attainment. CONCLUSIONS: Our results indicate that while individuals with schizophrenia in general have lower educational attainment and higher mortality rates compared with the general population, the association between educational attainment and mortality is smaller in schizophrenia subjects than in the general population.

Handling unobserved confounding in the relation between prenatal risk factors and child outcomes: a latent variable strategy.

BACKGROUND: Several studies have examined maternal health behavior during pregnancy and child outcomes. Negative control variables have been used to address unobserved confounding in such studies. This approach assumes that confounders affect the exposure and the negative control to the same degree. The current study introduces a novel latent variable approach that relaxes this assumption by accommodating repeated measures of maternal health behavior during pregnancy. METHODS: Monte Carlo simulations were used to examine the performance of the latent variable approach. A real-life example is also provided, using data from the Norwegian Mother, Father, and Child Study (MoBa). RESULTS: Simulations: Regular regression analyses without a negative control variable worked poorly in the presence of unobserved confounding. Including a negative control variable improved result substantially. The latent variable approach provided unbiased results in several situations where the other analysis models worked poorly. Real-life data: Maternal alcohol use in the first trimester was associated with increased ADHD symptoms in the child in the standard regression model. This association was not present in the latent variable approach. CONCLUSION: The current study showed that a latent variable approach with a negative control provided unbiased estimates of causal associations between repeated measures of maternal health behavior during pregnancy and child outcomes, even when the effect of the confounder differed in magnitude between the negative control and the exposures. The real-life example showed that inferences from the latent variable approach were incompatible with those from the standard regression approach. Limitations of the approach are discussed.

Met and unmet need for mental health care before and during the COVID-19 pandemic.

There is a concern that the coronavirus disease 2019 (COVID-19) pandemic will generate large unmet needs for mental health care. Using data from an epidemiological psychiatric diagnostic interview survey (n = 2159) conducted on a probability sample from the general population, the proportions of met and unmet need for mental health care among individuals with and without mental disorders were compared before and during the COVID-19 pandemic. The results showed no statistical difference in met and unmet need for mental health care, but point estimates were suggestive of a higher unmet need for care among those with a current mental disorder after the lock-down period.

Mechanisms linking parental educational attainment with child ADHD, depression, and academic problems: a study of extended families in The Norwegian Mother, Father and Child Cohort Study.

BACKGROUND: Low educational attainment in parents is associated with child psychopathology. It is not clear whether the associations are due to risk factors that family members share or due to effects of maternal or paternal education on the offspring. We investigate whether associations between maternal and paternal educational attainment and child symptoms of attention deficit/hyperactivity disorder (ADHD), depression, and academic problems are due to shared genetic factors, shared family environmental factors, or effects of the parental phenotype educational attainment itself. METHODS: This study is based on the Norwegian Mother, Father and Child Cohort Study (MoBa). The sample comprised 34,958 children (17,128 girls) in 28,372 extended-family units. We used data from related nuclear families linked by siblings in the parent generation. We applied a quasi-experimental extended children-of-twins design that included siblings in both generations and took account of nonrandom mating by including partners. Educational attainment was self-reported by mothers and fathers. Mothers reported children's symptoms of ADHD, symptoms of depression, and academic problems by questionnaire when the children were 8 years old. RESULTS: Children of lowly educated parents scored higher on all outcomes and had an approximate doubling of the risk of high symptom levels. The association between maternal and paternal educational attainment and child symptoms of ADHD and academic problems persisted after controlling for shared genetic and family environmental factors. Phenotypic transmission to depression was weaker and in the best fitting model fully explained by genetic factors shared by the two generations. CONCLUSIONS: Associations between educational attainment of mothers and fathers and child symptoms of ADHD and academic problems could not be ascribed to shared familial risk factors, whereas associations with symptoms of depression could. Parental education or resources and behaviors resulting from low education might be targets of interventions aimed at reducing symptoms of ADHD and academic problems.

Continuity of genetic and environmental influences on clinically assessed major depression from ages 18 to 45.

BACKGROUND: Studies on the stability of genetic risk for depression have relied on self-reported symptoms rather than diagnoses and/or short follow-up time. Our aim is to determine to what degree genetic and environmental influences on clinically assessed major depressive disorder (MDD) are stable between age 18 and 45. METHODS: A population-based sample of 11 727 twins (6875 women) born between 1967 and 1991 was followed from 2006 to 2015 in health registry data from primary care that included diagnoses provided by treating physicians. Individuals with schizophrenia or bipolar disorder (n = 163) were excluded. We modelled genetic and environmental risk factors for MDD in an accelerated longitudinal design. RESULTS: The best-fitting model indicated that genetic influences on MDD were completely stable from ages 18 to 45 and explained 38% of the variance. At each age, the environmental risk of MDD was determined by the risk at the preceding observation, plus new environmental risk, with an environmental correlation of +0.60 over 2 years. The model indicated no effects of shared environment and no environmental effects stable throughout the observational period. All long-term stability was therefore explained by genetic factors. CONCLUSIONS: Different processes unfolded in the genetic and environmental risk for MDD. The genetic component is stable from later adolescence to middle adulthood and accounted for nearly all long-term stability. Therefore, molecular genetic studies can use age-heterogenous samples when investigating genetic risk variants of MDD. Environmental risk factors were stable over a short span of years with associations rapidly decreasing and no evidence of permanent environmental scarring.

Preventing bias from selective non-response in population-based survey studies: findings from a Monte Carlo simulation study.

BACKGROUND: Health researchers often use survey studies to examine associations between risk factors at one time point and health outcomes later in life. Previous studies have shown that missing not at random (MNAR) may produce biased estimates in such studies. Medical researchers typically do not employ statistical methods for treating MNAR. Hence, there is a need to increase knowledge about how to prevent occurrence of such bias in the first place. METHODS: Monte Carlo simulations were used to examine the degree to which selective non-response leads to biased estimates of associations between risk factors and health outcomes when persons with the highest levels of health problems are under-represented or totally missing from the sample. This was examined under different response rates and different degrees of dependency between non-response and study variables. RESULTS: Response rate per se had little effect on bias. When extreme values on the health outcome were completely missing, rather than under-represented, results were heavily biased even at a 70% response rate. In most situations, 50-100% of this bias could be prevented by including some persons with extreme scores on the health outcome in the sample, even when these persons were under-represented. When some extreme scores were present, estimates of associations were unbiased in several situations, only mildly biased in other situations, and became biased only when non-response was related to both risk factor and health outcome to substantial degrees. CONCLUSIONS: The potential for preventing bias by including some extreme scorers in the sample is high (50-100% in many scenarios). Estimates may then be relatively unbiased in many situations, also at low response rates. Hence, researchers should prioritize to spend their resources on recruiting and retaining at least some individuals with extreme levels of health problems, rather than to obtain very high response rates from people who typically respond to survey studies. This may contribute to preventing bias due to selective non-response in longitudinal studies of risk factors and health outcomes.

Explaining the association between anxiety disorders and alcohol use disorder: A twin study.

BACKGROUND: It is unknown whether social anxiety disorder (SAD) has a unique association with alcohol use disorder (AUD) over and beyond that of other anxiety disorders, how the associations develop over time, and whether the associations are likely to be causal. METHODS: Diagnoses of AUD, SAD, generalized anxiety disorder, panic disorder, agoraphobia, and specific phobias were assessed twice using the Composite International Diagnostic Interview among 2,801 adult Norwegian twins. The data were analyzed using logistic regression analyses and multivariate biometric structural equation modeling. RESULTS: SAD had the strongest association with AUD, and SAD predicted AUD over and above the effect of other anxiety disorders. In addition, SAD was prospectively associated with AUD, whereas other anxiety disorders were not. AUD was associated with a slightly elevated risk of later anxiety disorders other than SAD. Biometric modeling favored a model where SAD influenced AUD compared to models where the relationship was reversed or due to correlated risk factors. Positive associations between AUD and other anxiety disorders were fully explained by shared genetic risk factors. CONCLUSIONS: Unlike other anxiety disorders, SAD plausibly has a direct effect on AUD. Interventions aimed at prevention or treatment of SAD may have an additional beneficial effect of preventing AUD, whereas interventions aimed at other anxiety disorders are unlikely to have a similar sequential effect on AUD.

Bias from self selection and loss to follow-up in prospective cohort studies.

Self-selection into prospective cohort studies and loss to follow-up can cause biased exposure-outcome association estimates. Previous investigations illustrated that such biases can be small in large prospective cohort studies. The structural approach to selection bias shows that general statements about bias are not possible for studies that investigate multiple exposures and outcomes, and that inverse probability of participation weighting (IPPW) but not adjustment for participation predictors generally reduces bias from self-selection and loss to follow-up. We propose to substantiate assumptions in structural models of selection bias through calculation of genetic correlations coefficients between participation predictors, outcome, and exposure, and to estimate a lower bound for bias due to self-selection and loss to follow-up by comparing effect estimates from IPP weighted and unweighted analyses. This study used data from the Norwegian Mother and Child Cohort Study and the Medical Birth Registry of Norway. Using the example of risk factors for ADHD, we find that genetic correlations between participation predictors, exposures, and outcome suggest the presence of bias. The comparison of exposure-outcome associations from regressions with and without IPPW revealed meaningful deviations. Assessment of selection bias for entire multi-exposure multi-outcome cohort studies is not possible. Instead, it has to be assessed and controlled on a case-by-case basis.

Prenatal and perinatal risk factors for eating disorders in women: A population cohort study.

OBJECTIVE: The fetal programming model hypothesizes that developmental programming in utero and in early life induces adaptations that predetermine the adult phenotype. This study investigated whether prenatal/perinatal complications are associated with lifetime eating disorders in women. METHOD: Participants included 46,373 adult women enrolled in the Norwegian Mother and Child Cohort Study (den norske Mor & barn-undersøkelsen [MoBa]). MoBa mothers and their mothers (MoBa grandmothers) were the focus of the current study. MoBa mothers with lifetime eating disorders were compared to a referent group. RESULTS: MoBa mothers who weighed more at birth (birth weight, adjusted odds ratio [OR] = 1.14; 95% confidence interval [CI]: 1.10-1.19) or were born large-for-gestational-age (adjusted OR = 1.39; 95% CI: 1.27-1.52) were more likely to develop binge-eating disorder in later life. MoBa mothers who weighed less at birth were more likely to develop anorexia nervosa (birth weight, adjusted OR = 0.88; 95% CI: 0.81-0.95). Bulimia nervosa and purging disorder (PD) were not significantly predicted by the prenatal and perinatal factors examined. DISCUSSION: Results of this study, which include the first known investigation of prenatal and perinatal factors in binge-eating disorder and PD, suggest that fetal programming may be relevant to the development of anorexia nervosa and binge-eating disorder. Future genetically informative research is needed to help disentangle whether these associations are a function of genetic influences or a true environmental fetal programming effect.

Development of ADHD symptoms in preschool children: Genetic and environmental contributions.

We examined genetic and environmental contributions to the development of symptoms of attention-deficit/hyperactivity disorder (ADHD) in preschool children. ADHD symptoms in siblings at 1.5, 3, and 5 years of age were investigated in a population-based sample from the prospective Norwegian Mother and Child Cohort Study. The longitudinal contributions of additive genetic, shared, twin-specific, and unique environmental influences were estimated using biometric structural equation models. Heritability of ADHD symptoms ranged from 54% to 70%. There was evidence of partially new genetic influences at successive ages, with genetic correlations ranging from .58 to .89. Contributions from shared environmental factors and twin-specific factors were minor. The importance of unique environmental effects appeared to increase across ages, and was mostly specific to a given age. There was no evidence suggesting that this pattern differs across males and females. Symptoms of ADHD are highly heritability in young children from as early as 1.5 years of age. Longitudinal stability of ADHD symptoms is mainly attributable to genetic influences, but there is also some evidence for age-specific genetic influences. These findings contribute to our understanding of development of ADHD early in life, and can guide future molecular genetics studies.

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.