Structure and Etiology of Co-occurring Internalizing and Externalizing Disorders in Adolescents

Several studies suggest that a two-factor model positing internalizing and externalizing factors explains the interrelationships among psychiatric disorders. However, it is unclear whether the covariation between internalizing and externalizing disorders is due to common genetic or environmental influences. We examined whether a model positing two latent factors, internalizing and externalizing, explained the interrelationships among six psychiatric disorders (major depressive disorder, generalized anxiety disorder, separation anxiety disorder, attention-deficit/hyperactivity disorder, oppositional defiant disorder, and conduct disorder) in adolescents, and whether there are common genetic and environmental influences on internalizing and externalizing latent factors. Multivariate behavior genetic analyses of data from 1162 twin pairs and 426 siblings ascertained from the general population via the Colorado Center for Antisocial Drug Dependence (CADD) were conducted. We found support for a model positing two latent factors (internalizing and externalizing). These factors were moderately heritable and influenced by significant common genetic and nonshared environmental influences. These findings suggest that co-occurrence of internalizing and externalizing psychopathology in adolescents results from both genetic and environmental influences.     

Negative Affect Shares Genetic and Environmental Influences with Symptoms of Childhood Internalizing and Externalizing Disorders

The co-occurrence of internalizing and externalizing disorders suggests that they may have common underlying vulnerability factors. Research has shown that negative affect is moderately positively correlated with both internalizing and externalizing disorders in children. The present study is the first to provide an examination of negative affect in relation to a wide spectrum of childhood internalizing and externalizing problems using a biometric model. This study extends prior findings of more narrowly focused associations by using a factor approach including multiple disorders. The sample for this study included families of 691 same-sex 7- to 13-year old twin pairs. A multifactorial independent pathway model was used to examine the genetic and environmental influences underlying the covariation of parent-reported negative affect, internalizing symptoms, and externalizing symptoms. Results of the current study suggest that negative affect shares genetic and environmental influences with both internalizing and externalizing disorders in childhood. These common influences may partially explain their comorbidity. Understanding that negative affect is at least one contributor to the covariation among these disorders may highlight avenues for early risk assessment, intervention, and perhaps prevention.     

The Association Between Internalizing and Externalizing Behavior in Childhood and Early Adolescence: Genetic or Environmental Common Influences?

This study analyzed the genetic and environmental influences on internalizing and externalizing behavior problems and the nature of their cooccurrence in a national Norwegian twin sample. The sample comprised 526 identical and 389 fraternal same-sexed twin pairs from five birth cohorts, aged 5-6, 8-9, 12-13, 13-14, and 14-15 years. Behavior problems were assessed by parental ratings on the Child Behavior Checklist. A model of additive genetic, shared, and nonshared environmental influences was fitted to both internalizing and externalizing behavior in four sex and age groups. The considerable covariance,r = .51 to .58, between these traits is accounted for mainly by common environmental components; this effect was most marked in the 5 to 9-year olds. Concordance rates for children scoring above 1 standard deviation from the total sample mean on the internalizing scale only, externalizing scale only, or both indicated stronger genetic influences for internalizing or externalizing problems only than for combined problems.     

Understanding the Covariation Among Childhood Externalizing Symptoms: Genetic and Environmental Influences on Conduct Disorder,Attention Deficit Hyperactivity Disorder,and Oppositional Defiant Disorder Symptoms

Conduct disorder (CD), attention deficit hyperactivity disorder (ADHD), and oppositional defiant disorder (ODD) are common childhood externalizing disorders that frequently co-occur. However, the causes of their comorbidity are not well understood. To address that question, we analyzed data from > 600 Finnish twin pairs, who completed standardized interviews at age 14. Behavior genetic methods were used to examine how genetic/environmental factors contribute to each disorders symptoms and to their covariation. We found significant genetic effects on each disorder with only modest evidence of shared environmental influences. Our data suggest the comorbidity among CD, ADHD, and ODD is primarily explained by shared genetic influences; however, each disorder was also under unique genetic influence, supporting the distinction of each disorder.     

Common Etiological Sources of Anxiety,Depression, and Somatic Complaints in Adolescents: A Multiple Rater twin Study

Somatic complaints in children and adolescents may be considered part of a broader spectrum of internalizing disorders that include anxiety and depression. Previous research on the topic has focused mainly on the relationship between anxiety and depression without investigating how common somatic symptoms relate to an underlying factor and its etiology. Based on the classical twin design with monozygotic and dizygotic twins reared together, our study aimed to explore the extent to which the covariation between three phenotypes in adolescent girls and boys can be represented by a latent internalizing factor, with a focus on both common and specific etiological sources. A population-based sample of twins aged 12–18 years and their mothers and fathers (N?=?1394 families) responded to questionnaire items measuring the three phenotypes. Informants’ ratings were collapsed using full information maximum likelihood estimated factor scores. Multivariate genetic analyses were conducted to examine the etiological structure of concurrent symptoms. The best fitting model was an ACE common pathway model without sex limitation and with one substantially heritable (44 %) latent factor shared by the phenotypes. Concurrent symptoms also resulted from shared (25 %) and non-shared (31 %) environments. The factor loaded most on depression symptoms and least on somatic complaints. Trait-specific influences explained 44 % of depression variance, 59 % of anxiety variance, and 65 % of somatic variance. Our results suggest the presence of a general internalizing factor along which somatic complaints and mental distress can be modeled. However, specific influences make the symptom types distinguishable.     

Do Genetic Factors Explain the Links Between Callous-Unemotional,Attention Hyperactivity and Oppositional Defiant Problems in Toddlers?

Research demonstrates that callous-unemotional (CU) behaviors, Attention Deficit Hyperactivity Disorder (ADHD) and Oppositional Defiant Problems (ODD) are related, but little is known about the sources of covariation among the three externalizing behaviors. The present study looked at genetic and environmental links between all three behavioral domains in twins at ages 2 and 3 years (MZ = 145, DZ = 169), a time when CU behaviors are beginning to emerge. CU, ADHD, and ODD behaviors as assessed using the Child Behavior Checklist 1.5–5 (Achenbach and Rescorla 2000) were strongly interrelated at both ages. Genetic factors primarily explained the covariation among the three behavioral domains via a common externalizing factor; however, there were also genetic factors unique to each behavior. Furthermore, the majority of nonshared environmental influences on each externalizing behavior were behavior-specific. The heritable externalizing factor was highly stable across age, largely due to genetic factors shared across ages 2 and 3 years. Despite their extensive phenotypic and genetic overlap, CU, ADHD, and ODD behaviors have unique genetic and nonshared environmental influences as early as toddlerhood. This supports phenotypic research showing that the three are related but distinct constructs in very young children.     

A behavior-genetic study of parenting quality, infant attachment security, and their covariation in a nationally representative sample

A number of relatively small-sample, genetically sensitive studies of infant attachment security have been published in the past several years that challenge the view that all psychological phenotypes are heritable and that environmental influences on child development--to the extent that they can be detected--serve to make siblings dissimilar. Using the twin subsample (N = 485 same-sex pairs) of the nationally representative Early Childhood Longitudinal Study-Birth Cohort, the authors provide evidence that parenting quality and infant attachment security observed at 24 months, as well as their covariation, are a product of shared and nonshared environmental (but not genetic) variation among children. In contrast, genetic differences between infants played a prominent role in explaining observations of temperamental dependency.     

Genetic and environmental mechanisms underlying stability and change in problem behaviors at ages 3, 7, 10, and 12

Maternal ratings on internalizing (INT) and externalizing (EXT) behaviors were collected in a large, population-based longitudinal sample. The numbers of participating twin pairs at ages 3, 7, 10, and 12 were 5,602, 5,115, 2,956, and 1,481, respectively. Stability in both behaviors was accounted for by genetic and shared environmental influences. The genetic contribution to stability (INT: 43%; EXT: 60%) resulted from the fact that a subset of genes expressed at an earlier age was still active at the next time point. A common set of shared environmental factors operated at all ages (INT: 47%; EXT: 34%). The modest contribution of nonshared environmental factors (INT: 10%; EXT: 6%) could not be captured by a simple model. Significant age-specific influences were found for all components, indicating that genetic and environmental factors also contributed to changes in problem behavior.     

A Common Genetic Factor Explains the Covariation Among ADHD ODD and CD Symptoms in 9–10 Year Old Boys and Girls

Previous studies examining the covariation among Attention Deficit Hyperactivity Disorder (ADHD), Oppositional Defiant Disorder (ODD) and Conduct Disorder (CD) have yielded inconsistent results. Some studies have concluded that the covariation among these symptoms is due to common genetic influences, whereas others have found a common environmental overlap. The present study investigated the genetic and environmental correlations among these three childhood disorders, based on a sample of 1,219 twins, age 9–10 years. A latent externalizing behavior factor was found to explain the covariance among ADHD, ODD and CD symptoms. Genetic influences explained more than half of the variance in this externalizing factor in both boys and girls. There were also unique genetic and environmental influences in each set of symptoms, suggesting some etiological independence of the three disorders. Our findings have implications for molecular genetic studies trying to identify susceptibility genes for these disorders. This study was funded by NIMH (R01 MH58354). Catherine Tuvblad was supported by post-doctoral stipends from the Swedish Council for Working Life and Social Research (Project 2006-1501) and the Sweden-America Foundation. Adrian Raine was supported by NIMH (Independent Scientist Award K02 MH01114-08). We thank the Southern California Twin Project staff for their assistance in collecting data, and the twins and their families for their participation.     

A Genetically Informed Study of the Covariation Between Childhood Anxiety Dimensions and Social Competence

We investigated the nature of latent shared etiological elements in 398 Italian twin pairs aged 8–17, explaining covariation between high levels of anxiety symptoms and low social competence. We found significant negative correlations between Child Behaviour Checklist/6–18 Social Competence Scale and three (Panic Anxiety, Separation Anxiety, Social Anxiety) out of five Screen for Child Anxiety Related Disorders anxiety subscales. Results from causal analysis seem to exclude the hypothesis that co-occurrence between Anxiety Symptoms and Social Competence could be due to a direct phenotypic effect of one trait upon the other. Multivariate analysis suggested that both genetic and shared environmental components contribute to the phenotypic correlation between Social Competence and Anxiety Subscales, whereas unique environmental factors have a negligible influence. This means that both common genetic and shared environmental causal factors contribute simultaneously to increase risk of having low Social Competence and high Anxiety scores. In particular, covariation with Social Competence seems to be influenced by both genetic and shared environmental causal components in Separation Anxiety and Social Anxiety, whereas environmental factors have an irrelevant influence for covariation with Panic/Somatic Anxiety Subscale. Our results support the adoption of a broader view of the relationships between psychopathology and diminished social competences in childhood for both clinicians and educators.     

Neuroticism and self-reported somatic health: a twin study

Using a twin sample (N?=?188; 53 monozygotic and 39 dizygotic twin pairs, and 4 single twins whose co-twin did not participate), this study sought (1) to estimate heritabilities of neuroticism and of somatic complaints rated on the basis of two different time frames ('the last week' vs. 'in general'); (2) to estimate the genetic association between neuroticism and the complaints indices and (3) to examine to what extent somatic complaints are aetiologically distinct from neuroticism. Using models with common additive genetic (A) and individual-specific environmental (E) factors, the heritabilities for neuroticism and complaints 'in general' and during 'the last week' were 0.46 (0.20-0.65), 0.44 (0.22-0.62), and 0.45 (0.22-0.63), respectively. Nearly 60% of the phenotypic correlation between neuroticism and somatic complaints were accounted for by A. Furthermore, a substantial part of the variance in somatic complaints was due to unique genetic and individual-specific environmental influences unrelated to neuroticism. These results suggest that somatic complaints are moderately heritable and that a considerable portion of the covariance between neuroticism and complaints measures is due to genetic factors. Yet, the findings also suggest that these two attributes are distinct entities with overlapping, but not identical, underlying genetic and environmental influences.     

Weak etiologic links between control and the externalizing behaviors delinquency and substance abuse in adolescence

Impulsive/disinhibitory personality traits have consistently been associated with externalizing symptomatology such as delinquency and substance use problems, often starting in adolescence. Yet the etiological nature of this co-occurrence is not well understood. Using a classic twin study design with self-report data from 717 male and female twin pairs, aged 15–18 years, a hierarchical psychometric model was examined. In this model the shared variance and etiological structure between control, delinquency and substance abuse symptoms, was modeled through a common externalizing factor. Model fitting indicated that the genetic and environmental influences differed in strength between male and female adolescents. The heritability of the externalizing factor was 45% in males and 10% in females, though neither was statistically different from zero. A statistically significant influence of shared environmental factors was seen for both sexes, 21% in males and 54% in females. In both sexes, the externalizing factor accounted for little variance in control, indicating a weak association and little shared etiology with externalizing liability. These results illuminate further that facets of impulsivity are differentially associated with vulnerability for externalizing symptomatology.     

Sources of covariation among attention-deficit/hyperactivity disorder, oppositional defiant disorder, and conduct disorder: the importance of shared environment.

Research has documented high levels of covariation among childhood externalizing disorders, but the etiology of this covariation is unclear. To unravel the sources of covariation among attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder (CD), the authors studied 11-year-old twins (N = 1,506) from the Minnesota Twin Family Study. Symptom counts for each of these disorders were obtained from interviews administered to the twins and their mothers. A model was fit that allowed the parsing of genetic, shared environmental (factors that make family members similar to each other), and nonshared environmental (factors that make family members different from each other) contributions to covariation. The results revealed that although each disorder was influenced by genetic and environmental factors, a single shared environmental factor made the largest contribution to the covariation among ADHD, ODD, and CD.     

Self-Esteem, Negative Emotionality, and Depression as a Common Temperamental Core: A Study of Mid-Adolescent Twin Girls

ABSTRACT We tested the structure and magnitude of genetic and environmental influences on the overlap among self-esteem, negative emotionality, and major depression symptoms in adolescent girls ( N =706) from the Minnesota Twin Family Study. Genetic and environmental influences on all three operated via a general, heritable factor. Genetic influences explained the majority of overlap among the three constructs, as well as most of the variance in self-esteem and negative emotionality. Genetic influences on depression were more modest and largely due to genetic factors specific to depression. These findings support the theory that self-esteem, depression, and neuroticism represent aspects of a common temperamental core. The interrelations among the three constructs in mid-adolescence is consistent with their interrelations in adulthood.     

Neuroticism and Morning Cortisol Secretion: Both Heritable, But No Shared Genetic Influences

ABSTRACT Neuroticism is widely used as an explanatory concept in etiological research of psychopathology. To clarify what neuroticism actually represents, we investigated the phenotypic and genetic relationship between neuroticism and the morning cortisol secretion. In the current classic twin study, 125 female twin pairs (74 monozygotic and 51 dizygotic pairs) participated. For each participant, 4 different neuroticism scores were available to calculate a neuroticism composite score that was used in the statistical analyses. The morning cortisol secretion was assessed by 4 salivary samples in the 1st hour after awakening. Significant genetic influences for the neuroticism composite score (55%), and each of the 4 cortisol samples (52%–69%) were found. There was no phenotypic or genotypic relationship between neuroticism and morning cortisol secretion. Although neuroticism and cortisol were both heritable traits, they did not share any genetic influences.     

Risk for separation anxiety disorder among girls: paternal absence, socioeconomic disadvantage, and genetic vulnerability

The authors examined genetic and environmental influences, including the contributions of 2 measured aspects of the shared environment of twins (paternal absence, socioeconomic disadvantage) on the development of mother-reported separation anxiety disorder (SAD) history in a sample of 1,887 female twin pairs. Four different symptom categories of SAD were considered. Results revealed that all 4 SAD symptom categories were significantly heritable, whereas the contribution of shared environmental influences to the variation in risk was significant for only 2 of the 4 SAD categories. Paternal absence was found to have an important influence in vulnerability for SAD, whereas the effect of socioeconomic disadvantage was less robust. Evidence for race differences in the etiology of SAD was not found.     

Longitudinal genetic analysis of anxiety sensitivity

Anxiety sensitivity is associated with both anxiety and depression and has been shown to be heritable. Little, however, is known about the role of genetic influence on continuity and change of symptoms over time. The authors' aim was to examine the stability of anxiety sensitivity during adolescence. By using a genetically sensitive design, the authors were also able to investigate the extent to which genetic and environmental factors influence anxiety sensitivity over time. Self-reports of anxiety sensitivity were obtained for over 1,300 twin and sibling pairs at 3 time points. Data were analyzed using multivariate genetic models. Anxiety sensitivity was moderately heritable at all time points with substantial nonshared environmental contributions. Time 1 genetic factors accounted for continuity of symptoms at Times 2 and 3. New genetic factors at Time 2 also influenced Time 3 symptoms. New nonshared environmental influences emerged at each time point. Analysis of a latent factor of trait anxiety sensitivity revealed some stable nonshared environmental influences. Genetic effects were generally stable over time, with new genetic influences emerging in late adolescence. Environmental influences on anxiety sensitivity were, on the whole, more time specific; however, some stable environmental influences were also found.     

Comorbidity between and within childhood externalizing and internalizing disorders: reflections and directions

The term and concept of lcomorbidity has been mired in controversy, although there is little question that the existence of covariation among psychiatric diagnoses poses significant challenges to current models of psychiatric classification and diagnosis. The papers in this Special Section underscore a number of important issues relevant to the comorbidity between and within childhood externalizing and internalizing disorders, and illustrate both methodological and substantive reasons for such comorbidity. Weiss, Susser, and Catron's distinction among common, broad-band specific, and narrow-band specific features provides a helpful framework for understanding the comorbidity of childhood externalizing and internalizing disorders (B, Weiss, K. Susser, & T. Catron, 1998). Hierarchical models of psychopathology help to dissolve the distinction between splitters and lumpers and point to variables that may elucidate the etiology of externalizing and internalizing disorders.     

Genetic and Environmental Contributions to Common Psychopathologies of Childhood and Adolescence: A Study of Twins and Their Siblings

We report findings based on analyses of self-reports of six common adolescent psychopathologies (attention deficit/hyperactivity disorder, ADHD; conduct disorder, CD; oppositional defiant disorder, ODD; generalized anxiety disorder, GAD; separation anxiety disorder, SAD; and major depressive disorder, MDD) in a sample of 1,162 male and female adolescent (12–19 years) twin pairs and 426 siblings. Prevalence statistics for past year and lifetime reports confirm differences between genders for CD, GAD, SAD, and MDD, and a lack of differences between twins and their non-twin siblings. Biometrical modeling was conducted to ascertain the relative influences of genes, and shared and non-shared environments contributing to these disorders. A more robust estimate of these parameters was obtained by including non-twin siblings. Age-specific thresholds were integrated into the analyses to appropriately model the developmental patterns of behavior. We found evidence for both genetic and non-shared environmental influences for all disorders. Shared environmental influences also seem to be important for MDD and lifetime GAD.     

Genetic and environmental influences on positive and negative affect: support for a two-factor theory.

Genetic and environmental etiologies of positive and negative affect, as measured by the Bradburn Affect Balance Scale, were studied to gain understanding of a two-factor theory of well-being. It was hypothesized that negative affect would demonstrate significant genetic and environmental variance and positive affect would be explained primarily by environmental influences. Data were combined from 105 pairs of twins (ages 18-72) and 220 multigenerational families (ages 16-98). Simultaneous model fitting indicated significant heritable effects for negative affect and a significant effect of common environment for twins. Significant effects for positive affect included common environment (for parents and offspring and for twin pairs) and assortative mating. These results, documenting differential genetic and environmental influences on positive and negative affect, provide further support for their being separate components of well-being.