Genetics and developmental psychopathology: 2. The main effects of genes and environment on behavioral problems in the Virginia Twin Study of Adolescent Behavioral Development

Early temperamental and family predictors of shyness and anxiety

With a sample of 242 twins (135 girls, 107 boys) studied longitudinally, behavioral inhibition (BI) and inhibitory control (IC) measured at 3 years, as well as early and concurrent family process variables, were examined as predictors of shyness and of anxiety symptoms approximately 4 years later. Structured observational data from laboratory and home contexts were used in conjunction with parent and experimenter ratings. A key goal was to extend previous findings of the positive relationship between early BI and anxiety development by incorporating the consideration of IC and family process variables. Using hierarchical linear modeling with restricted maximum likelihood estimation to adjust for twin dependency, early BI (b = 0.37, p < .01), IC (b = 0.14, p < .05), and concurrent lower family stress (b = -0.22, p < .05) predicted shyness during middle childhood. Findings were similar for parent-rated and laboratory-based shyness measures. Anxiety symptoms were predicted by BI (b = 0.14, p < .05), early negative family affect (b = 0.20, p < .05), and family stress in middle childhood (b = 0.26, p < .05). These findings clarify the relative importance of temperament and family factors in the development of both shyness and anxiety symptoms during childhood.

Depressive symptoms and alcohol use are genetically and environmentally correlated across adolescence

Depressive symptoms and alcohol use are frequently positively associated during adolescence. This study aimed to assess the heritability of each phenotype across adolescence; to assess potential shared liabilities; to examine changes in the nature of shared liabilities across adolescence; and to investigate potential causal relationships between depressive symptoms and alcohol use. We studied a longitudinally assessed sample of adolescent Finnish twins (N = 1,282) to test hypotheses about genetic and environmental influences on these phenotypes within and across ages, using data from assessments at ages 12, 14, and 17.5 years. The heritability of depressive symptoms is consistent across adolescence (~40-50%), with contributions from common and unique environmental factors. The heritability of alcohol use varies across time (a(2) = .25-.44), and age 14 alcohol use is heavily influenced by shared environmental factors. Genetic attenuation and innovation were observed across waves. Modest to moderate genetic (r(A) = .26-.59) and environmental (r(C) = .30-.63) correlations between phenotypes exist at all ages, but decrease over time. Tests for causal relationships between traits differed across ages and sexes. Intrapair MZ difference tests provided evidence for reciprocal causation in girls at ages 14 and 17.5. Formal causal models suggested significant causal relationships between the variables in both boys and girls. The association between depressive symptoms and alcohol use during adolescence is likely due to a combination of shared genetic and environmental influences and causal influences. These influences are also temporally dynamic, complicating efforts to understand factors contributing to the relationship between these outcomes.

Genetics of childhood and adolescent depression: insights into etiological heterogeneity and challenges for future genomic research

There is heterogeneity between depression in childhood, adolescence and adulthood in terms of the gender composition of affected cases, prevalence, rates of recurrence and risk factors. This raises complex questions for refining the phenotype for molecular genetic studies of depression and the selection of appropriate proband groups. This article aims to provide a review of issues arising from family, twin and adoption studies of relevance to molecular genetic studies, and to summarize molecular genetic findings on childhood/adolescent depression. While retrospective studies of adults suggest greater familial aggregation among those with an earlier age of onset, prospective studies do not confirm this association. In fact, taken together, evidence from family and twin studies suggests that prepubertal depression is more strongly associated with psychosocial adversity, is less heritable and shows lower levels of continuity with adult depression than either adolescent or adult depression. Adolescent depressive symptoms and disorder show similar levels of heritability to depression in adult life, although there is only one twin study of adolescent depressive disorder, and heritability estimates of depressive symptoms vary widely between studies. This variability in heritability estimates is partly attributable to age and informant effects. Adoption studies and other intergenerational transmission designs show that the transmission of depression between parents and children involves genetic and environmental processes, with converging evidence that environmental processes are most important. Molecular genetic studies of childhood/adolescent depression have to date used a candidate gene approach and focused on genes already examined in adult studies. Prospective longitudinal studies of community and high-risk samples are needed to clarify issues of etiological heterogeneity in depression, and these should in turn inform the planning of molecular genetic studies.

Familial transmission and heritability of childhood disruptive disorders

OBJECTIVE

There is substantial evidence of a link between parental substance use disorders and antisocial behavior and childhood disruptive disorders in offspring, but it is unclear whether this transmission is specific to particular disorders or if a general liability accounts for familial resemblance. The authors examined whether the association between parental externalizing disorders and childhood disruptive disorders in preadolescent offspring is a result of the transmission of general or disorder-specific liabilities and estimated the genetic and environmental contributions to variation in these general and specific liability indicators.

METHOD

Participants were 1,069 families consisting of 11-year-old twins and their biological mother and father. Structural equation modeling was used to simultaneously estimate the general and specific transmission effects of four parental externalizing disorders (conduct disorder, adult antisocial behavior, alcohol dependence, and drug dependence) on childhood disruptive disorders (attention deficit hyperactivity disorder, conduct disorder, and oppositional defiant disorder).

RESULTS

Parent-child resemblance was accounted for by the transmission of a general liability to externalizing disorders, and this general liability was highly heritable. Specific effects were also detected, but for sibling rather than parental transmission. Specific genetic and nonshared environmental effects were detected for each childhood disruptive disorder, but only conduct disorder exhibited a significant shared environmental effect.

CONCLUSIONS

A highly heritable general liability accounts for the parent-child transmission of externalizing psychopathology from parents to their preadolescent offspring. This general liability should be a focus of research for both etiology and intervention.

Childhood and adolescent anxiety and depression: beyond heritability

OBJECTIVE

To review the methodology of behavior genetics studies addressing research questions that go beyond simple heritability estimation and illustrate these using representative research on childhood and adolescent anxiety and depression.

METHOD

The classic twin design and its extensions may be used to examine age and gender differences in the genetic determinants of complex traits and disorders, the role of genetic factors in explaining comorbidity, the interaction of genes and the environment, and the effect of social interaction among family members. An overview of the methods typically employed to address such questions is illustrated by a review of 34 recent studies on childhood anxiety and depression.

RESULTS

The review provides relatively consistent evidence for small to negligible sex differences in the genetic etiology of childhood anxiety and depression, a substantial role of genetic factors in accounting for the temporal stability of these disorders, a partly genetic basis of the comorbidity between anxiety and depression, a possible role of the interaction between genotype and the environment in affecting liability to these disorders, a role of genotype-environment correlation, and a minor, if any, etiological role of sibling interaction.

CONCLUSION

The results clearly demonstrate a role for genetic factors in the etiology and temporal stability of individual differences in childhood anxiety and depression. Clinical implications of the findings are discussed.

Genetic influences on the cognitive biases associated with anxiety and depression symptoms in adolescents

BACKGROUND

There is a substantial overlap between genes affecting anxiety and depression. Both anxiety and depression are associated with cognitive biases such as anxiety sensitivity and attributional style. Little, however, is known about the relationship between these variables and whether these too are genetically correlated.

METHODS

Self-reports of anxiety sensitivity, anxiety symptoms, attributional style and depression symptoms were obtained for over 1300 adolescent twin and sibling pairs at two time points. The magnitude of genetic and environmental influences on the measures was examined.

RESULTS

Strongest associations were found between anxiety sensitivity and anxiety ratings at both measurement times (r=.70, .72) and between anxiety and depression (r=.62 at both time points). Correlations between the cognitive biases were modest at time 1 (r=-.12) and slightly larger at time 2 (r=-.31). All measures showed moderate genetic influence. Generally genetic correlations reflected phenotypic correlations. Thus the highest genetic correlations were between anxiety sensitivity and anxiety ratings (.86, .87) and between anxiety and depression ratings (.77, .71). Interestingly, depression ratings also showed a high genetic correlation with anxiety sensitivity (.70, .76). Genetic correlations between the cognitive bias measures were moderate (-.31, -.46).

LIMITATIONS

The sample consists primarily of twins, there are limitations associated with the twin design.

CONCLUSIONS

Cognitive biases associated with depression and anxiety are not as genetically correlated as anxiety and depression ratings themselves. Further research into the cognitive processes related to anxiety and depression will facilitate understanding of the relationship between bias and symptoms.

Genetic and environmental influences on the transmission of parental depression to children's depression and conduct disturbance: an extended Children of Twins study

BACKGROUND

Despite the increased risk of depression and conduct problems in children of depressed parents, the mechanism by which parental depression affects their children's behavioral and emotional functioning is not well understood. The present study was undertaken to determine whether parental depression represents a genuine environmental risk factor in children's psychopathology, or whether children's depression/conduct can be explained as a secondary consequence of the genetic liability transmitted from parents to their offspring.

METHODS

Children of Twins (COT) data collected on 2,674 adult female and male twins, their spouses, and 2,940 of their children were used to address whether genetic and/or family environmental factors best account for the association between depression in parents and depression and conduct problems in their children. Data collected on juvenile twins from the Virginia Twin Study of Adolescent Behavioral Development (VTSABD) were also included to estimate child-specific genetic and environmental influences apart from those effects arising from the transmission of the parental depression itself. The fit of alternative Children of Twin models were evaluated using the statistical program Mx.

RESULTS

The most compelling model for the association between parental and juvenile depression was a model of direct environmental risk. Both family environmental and genetic factors accounted for the association between parental depression and child conduct disturbance.

CONCLUSIONS

These findings illustrate how a genetically mediated behavior such as parental depression can have both an environmental and genetic impact on children's behavior. We find developmentally specific genetic factors underlying risk to juvenile and adult depression. A shared genetic liability influences both parental depression and juvenile conduct disturbance, implicating child conduct disturbance (CD) as an early indicator of genetic risk for depression in adulthood. In summary, our analyses demonstrate differences in the impact of parental depression on different forms of child psychopathology, and at various stages of development.

Environmental contributions to the stability of antisocial behavior over time: are they shared or non-shared?

It has recently been argued that shared environmental influences are moderate, identifiable, and persistent sources of individual differences in most forms of child and adolescent psychopathology, including antisocial behavior. Unfortunately, prior studies examining the stability of shared environmental influences over time were limited by possible passive gene-environment correlations, shared informants effects, and/or common experiences of trauma. The current study sought to address each of these limitations. We examined adolescent self-reported antisocial behavior in a 3.5 year longitudinal sample of 610 biological and adoptive sibling pairs from the Sibling Interaction and Behavior Study (SIBS). Results revealed that 74-81% of shared environmental influences present at time 1 were also present at time 2, whereas most non-shared environmental influences (88-89%) were specific to a particular assessment period. Such results provide an important constructive replication of prior research, strongly suggesting that shared environmental contributions to antisocial behavior are systematic in nature.

Mechanisms underlying the lifetime co-occurrence of tobacco and cannabis use in adolescent and young adult twins

Using twins assessed during adolescence (Virginia Twin Study of Adolescent Behavioral Development: 8-17 years) and followed up in early adulthood (Young Adult Follow-Up, 18-27 years), we tested 13 genetically informative models of co-occurrence, adapted for the inclusion of covariates. Models were fit, in Mx, to data at both assessments allowing for a comparison of the mechanisms that underlie the lifetime co-occurrence of cannabis and tobacco use in adolescence and early adulthood. Both cannabis and tobacco use were influenced by additive genetic (38-81%) and non-shared environmental factors with the possible role of non-shared environment in the adolescent assessment only. Causation models, where liability to use cannabis exerted a causal influence on the liability to use tobacco fit the adolescent data best, while the reverse causation model (tobacco causes cannabis) fit the early adult data best. Both causation models (cannabis to tobacco and tobacco to cannabis) and the correlated liabilities model fit data from the adolescent and young adult assessments well. Genetic correlations (0.59-0.74) were moderate. Therefore, the relationship between cannabis and tobacco use is fairly similar during adolescence and early adulthood with reciprocal influences across the two psychoactive substances. However, our study could not exclude the possibility that 'gateways' and 'reverse gateways', particularly within a genetic context, exist, such that predisposition to using one substance (cannabis or tobacco) modifies predisposition to using the other. Given the high addictive potential of nicotine and the ubiquitous nature of cannabis use, this is a public health concern worthy of considerable attention.

The mediating effect of parental neglect on adolescent and young adult anti-sociality: a longitudinal study of twins and their parents

The causes of correlation between parental treatment and offspring behavior are ambiguous since genetic and social factors are correlated in typical family studies. The problem is complicated by the need to characterize the effects of genes and environment on both juvenile and adult behavioral outcomes. A model is developed for the resemblance between juvenile and adult twins and their parents that allows some of these effects to be resolved. Data on childhood adversity, parental anti-social behavior, and longitudinal adult and juvenile anti-social behavior were obtained from 1,412 families of adolescent and young adult twins. A structural model is fitted that allows for the effects of genetic and social transmission of information from parents to children. Environmental effects of parents may be mediated through measured features of the home environment. Parameters were estimated by diagonal weighted least squares applied to the 33 distinct polychoric correlations between relatives and between variables within and between ages. Sub-hypotheses were tested. Results confirmed that effects of genes and environment were both highly significant. Genetic effects were large in juveniles and largely age and sex-specific. Approximately 30% of the variation due to the shared environment was due to the effect of childhood adversity. The remaining shared environmental effects are unexplained. Adversity is affected significantly by maternal anti-social behavior. The correlation between paternal ASP and adversity may be explained by antisocial fathers selecting (or creating) antisocial mothers. All significant environmental effects of parental ASP are mediated through the measure of adversity. Though transmission of ASP is both genetic and social, passive genotype-environment correlation is very small. Assortative mating for ASP has barely detectable consequence for the genetic correlation between siblings. The longitudinal study of twins and their parents makes it possible to demonstrate there is a direct causal effect of childhood adversity on child conduct disorder over and above any indirect genetic correlation.

Inhibition, reinforcement sensitivity and temporal information processing in ADHD and ADHD+ODD: evidence of a separate entity?

This study compared children with ADHD-only, ADHD+ODD and normal controls (age 8–12) on three key neurocognitive functions: response inhibition, reinforcement sensitivity, and temporal information processing. The goal was twofold: (a) to investigate neurocognitive impairments in children with ADHD-only and children with ADHD+ODD, and (b) to test whether ADHD+ODD is a more severe from of ADHD in terms of neurocognitive performance. In Experiment 1, inhibition abilities were measured using the Stop Task. In Experiment 2, reinforcement sensitivity and temporal information processing abilities were measured using a Timing Task with both a reward and penalty condition. Compared to controls, children with ADHD-only demonstrated impaired inhibitory control, showed more time underestimations, and showed performance deterioration in the face of reward and penalty. Children with ADHD+ODD performed in-between children with ADHD-only and controls in terms of inhibitory controls and the tendency to underestimate time, but were more impaired than controls and children with ADHD-only in terms of timing variability. In the face of reward and penalty children with ADHD+ODD improved their performance compared to a neutral condition, in contrast to children with ADHD-only. In the face of reward, the performance improvement in the ADHD+ODD group was disproportionally larger than that of controls. Taken together the findings suggest that, in terms of neurocognitive functioning, comorbid ADHD+ODD is a substantial different entity than ADHD-only.

Perinatal and familial risk factors are associated with full syndrome and subthreshold attention-deficit hyperactivity disorder in a korean community sample

OBJECTIVE

The objective of this study was to examine the effect of perinatal and familial risk factors on full syndrome and subthreshold attention-deficit/hyperactivity disorder (ADHD) among Korean children and adolescents.

METHODS

A sample of 2,673 students was randomly selected from 19 representative schools in Seoul, Korea. The parents of the students completed the Diagnostic Interview Schedule for Children-version IV (DISC-IV), as well as questionnaires on perinatal and familial risk factors.

RESULTS

Maternal stress and alcohol use during pregnancy, parental marital discord, parental separation or divorce, changes in primary caregivers, and notbreastfeeding were significantly associated with full syndrome ADHD; however, maternal stress during pregnancy was the only variable that differentiated subthreshold ADHD from non-ADHD.

CONCLUSION

Our results provide evidence the perinatal and familial risk factors contribute to the development of ADHD in Korea children and adolescents, and suggest that these perinatal and familial risk factors are more closely related to full syndrome than to subthreshold ADHD.

The development of anxiety disorders in childhood: an integrative review

We present an integrative review of the development of child anxiety, drawing on a number of strands of research. Family aggregation and genetic studies indicate raised vulnerability to anxiety in offspring of adults with the disorder (e.g. the temperamental style of behavioural inhibition, or information processing biases). Environmental factors are also important; these include adverse life events and exposure to negative information or modelling. Parents are likely to be key, although not unique, sources of such influences, particularly if they are anxious themselves. Some parenting behaviours associated with child anxiety, such as overprotection, may be elicited by child characteristics, especially in the context of parental anxiety, and these may serve to maintain child disorder. Emerging evidence emphasizes the importance of taking the nature of child and parental anxiety into account, of constructing assessments and interventions that are both disorder specific, and of considering bidirectional influences.

Association between catechol-O-methyltransferase gene polymorphism and attention-deficit hyperactivity disorder in Korean population

Recently, the relationship between allele frequency distribution and attention-deficit hyperactivity disorder (ADHD) has been actively studied. In Korea, the relationship between the genetic type and alleles for catechol-O-methyltransferase (COMT) gene has been studied in ADHD patients. ADHD was diagnosed in 60 patients according to the Diagnostic and Statistical Manual of Mental Disorders Version IV (DSM-IV) diagnostic criteria and Schedule for Affective Disorders and Schizophrenia for School-Age Children--Present and Lifetime Version (K-SADS-PL), and they were selected for the study. For the control group, normal volunteers were chosen. Blood samples were taken from the 160 subjects. DNA was extracted from blood lymphocytes, and PCR was performed for COMT NlaIII VNTR polymorphism. For the case-control analyses, allele and genotype frequencies were compared using the chi(2) method. When the ADHD group and the normal control group were compared, significant difference was seen on the COMT genetic type, but was not seen on the allele distribution. As a result, it is viewed that there is no relationship between ADHD and the COMT gene, but final decision is indefinite.

Genome scan in sibling pairs with juvenile-onset mood disorders: Evidence for linkage to 13q and Xq

Mood disorders (bipolar and depressive disorders) in children and adolescents are associated with significant morbidity and mortality. Twin and family studies, for the most part, indicate higher familiality and heritability for mood disorders that onset in childhood/adolescence than those that onset in adulthood. To identify the genetic contribution to mood disorders that onset in childhood/adolescence, we performed a genome scan on 146 nuclear families from Hungary containing an affected proband and affected siblings. In total, the pedigrees contained 303 affected children: 146 probands, 137 siblings with a first episode of mood disorder before 14.9 years of age, and 20 siblings with onset of their first episode after 14.9 years of age but before the age of 18. The results of the genome scan using 405 microsatellite markers did not provide evidence for linkage at the recommended genome wide significance level for any novel loci. However, markers on two chromosomes, 13q and Xq, provided evidence for linkage in regions previously identified as linked to bipolar disorder in multiple studies. For the marker on chromosome 13q the peak non-parametric multipoint LOD score was at the marker D13S779 (LOD = 1.5, P = 0.004). On chromosome Xq, evidence for linkage was observed across a large region spanning two regions previously linked to bipolar disorder; Xq24 to Xq28, with a peak at marker TTTA062 (LOD 2.10, P = 0.0009) in Xq28. Results for these regions exceed the recommended P-value for a replication study of P < 0.01 and thus provide evidence for these two loci as contributing to mood disorders with juvenile onset.

Effects of stressful life events, maternal depression and 5-HTTLPR genotype on emotional symptoms in pre-adolescent children

There has been a large but inconsistent literature on interactions between the 5-HTTLPR polymorphism of the serotonin transporter gene and adversity on emotional disorders. We investigated these interactions in 4,334 children from a birth longitudinal cohort: the Avon Longitudinal Study of Parents and Children (ALSPAC). We measured emotional symptoms at 7 years with the Strengths and Difficulties Questionnaire. Mothers rated stressful life events between ages 5 and 7 years. Maternal depression was defined as a score > or =12 on the Edinburgh Postnatal Depression Scale at 2 or 8 months postnatally. Triallelic genoptyping of the 5-HTTLPR polymorphism was performed. We found strong associations between stressful life events (OR 1.19; 1.12-1.26; P < 0.01) and maternal postnatal depression (OR 1.91; 1.63-2.24; P < 0.01) with emotional symptoms in the children. There were no main 5-HTTLPR genotype effects or significant interactions between genotype and life events or maternal postnatal depression on emotional symptoms. There was marginal evidence (P = 0.08) for an interaction between stressful life events and genotype in boys only, with those in the low and high 5-HTTLPR expression groups showing stronger associations. In these 7-year-old children, we did not replicate previously reported G x E interactions between 5-HTTLPR and life events for emotional symptoms. Gene by environment interactions may be developmentally dependent and show variation depending on the type and levels of exposure and sex. Young cohorts are essential to improve our understanding of the impact of development on gene and environment interactions.

Aggressive versus nonaggressive antisocial behavior: distinctive etiological moderation by age

Research has supported the existence of distinct behavioral patterns, demographic correlates, and etiologic mechanisms for aggressive (AGG) versus nonaggressive but delinquent (DEL) antisocial behavior. Though behavioral genetic studies have the potential to further crystallize these dimensions, inconsistent results have limited their contribution. These inconsistencies may stem in part from the limited attention paid to the impact of age. In the current study, the authors thus examined age-related etiological moderation of AGG and DEL antisocial behavior in a sample of 720 sibling pairs (ranging in age from 10 to 18 years) with varying degrees of genetic relatedness. Results reveal that the magnitude of genetic and environmental influences on AGG remained stable across adolescence. By contrast, genetic influences on DEL increased dramatically with age, whereas shared environmental influences decreased. Subsequent longitudinal analyses fully replicated these results. Such findings highlight etiological distinctions between aggression and delinquency, and offer insights into the expression of genetic influences during development.

Contextualizing the neurobiology of conduct disorder in an emotion dysregulation framework

Conduct disorder (CD) represents the most common childhood psychiatric disorder found in community and mental health clinics. This paper provides a comprehensive review of the neurobiology of CD; specifically, neurological and neurochemical correlates. Converging evidence suggests that neurological profiles of individuals with CD, compared to peers, are characterized by reduced P300 brain wave amplitude, deactivation of the anterior cingulated cortex and reduced activation in the left amygdala in response to negative stimuli, and reduced right temporal lobe volume. The neurochemical profiles of individuals with CD are characterized by reduced serotonin and cortisol levels (i.e., decreased HPA axis function), as well as attenuated autonomic nervous system functioning. Popular theoretical frameworks cited within the CD literature are limited in their ability to explain and consolidate the neurological and neurochemical findings. We believe that emotion dysregulation theory, though not often used within CD research, may provide the most comprehensive and inclusive framework for understanding neurobiological aspects of this disorder. Limitations within the literature, future directions for research, and implications of the findings will be discussed.

Shared aetiology of risky sexual behaviour and adolescent misconduct: genetic and environmental influences

Risky sexual behaviour (RSB) is a major risk factor for serious diseases as well as unplanned pregnancy. It is not known if RSB has a genetic basis or if it is only influenced by social and cultural conditions. Adolescent conduct disorder has previously been linked to RSB and has been found to be influenced by both genetic and environmental factors. In this study, we look at normal variation in a broad measure of RSB and in retrospectively reported adolescent misconduct in a large community sample of twins (n = 4904) to partition the variance and covariance between the traits into genetic and environmental components. We found that RSB is influenced to the same extent by genes, shared environment and unshared environment. Adolescent misconduct is moderately influenced by genetic factors and only modestly by shared environmental factors. Moreover, RSB is associated with adolescent misconduct (r = 0.5), primarily because of genetic correlation between the variables. The implications of our findings as well as possible sex differences are discussed.

The genetics of depression in childhood and adolescence

This article reviews family, twin, and adoption studies of childhood and adolescent depression. Results from several family and twin studies suggest that an etiologic heterogeneity exists in depression in childhood and adolescence. Twin studies show that genetic influences on depression in young people may be indirect and work via effects on environmental risk exposure (gene-environment correlation) or genetic sensitivity to environmental risks (gene-environment interaction). Recent research on gene-environment interaction has examined the effect of specific functional genetic polymorphisms in conjunction with environmental stressors. Future research needs to work toward identifying which environmental and genetic risk factors are crucial to the development of depression in youth, as well as mechanisms involved in the familial transmission of depression. This will not only improve understanding of the etiology of childhood and adolescent depression but also inform the development of therapeutic and preventive interventions.

Personality traits among ADHD adults: implications of late-onset and subthreshold diagnoses

BACKGROUND

Diagnosing attention deficit hyperactivity disorder (ADHD) in adults is difficult when diagnosticians cannot establish onset prior to the DSM-IV criterion of age 7 or if the number of symptoms does not achieve the DSM threshold for diagnosis. Previous work has assessed the validity of such diagnoses based on psychiatric co-morbidity, family history and neuropsychological functions but none of these studies have used personality as a validation criterion.

METHOD

We compared four groups of adults: (1) full ADHD subjects who met all DSM-IV criteria for childhood-onset ADHD; (2) late-onset subjects who met all criteria except the age at onset criterion, (3) subthreshold subjects who did not meet full symptom criteria and (4) non-ADHD subjects who did not meet any of the above criteria. Diagnoses were made by using the Structured Clinical Interview for DSM-IV (SCID) and the Temperament and Character Inventory (TCI) was used to assess personality traits.

RESULTS

We found that full ADHD and late-onset ADHD showed similar personality profiles with significant deviations on all TCI scales except reward dependence and self-transcendence. By contrast, subthreshold cases only showed deviations on novelty seeking and self-directiveness.

CONCLUSIONS

These data call into question the stringent age of onset of ADHD symptom criteria for adults when making retrospective diagnoses of ADHD. Subthreshold ADHD seems to be a milder form of the disorder that is consistent with dimensional views of the disorder.

Monoamine oxidase A and childhood adversity as risk factors for conduct disorder in females

BACKGROUND

Recent studies among males have reported a genotype-environment interaction (GxE) in which low-activity alleles at the monoamine oxidase A (MAOA) locus conferred greater sensitivity to the effects of childhood adversity on risk for conduct disorder (CD). So far, few studies of females have controlled for gene-environment correlation or used females heterozygous for this X-linked gene.

METHOD

Logistic regression analysis of a sample of 721 females ages 8-17 years from the longitudinal Virginia Twin Study of Adolescent Behavioral Development (VTSABD) assessed the additive effects of MAOA genotypes on risk for CD, together with the main effect of childhood adversity and parental antisocial personality disorder (ASP), as well as the interaction of MAOA with childhood adversity on risk for CD.

RESULTS

A significant main effect of genotype on risk for CD was detected, where low-activity MAOA imparted the greatest risk to CD in girls while controlling for the significant effects of maternal ASP and childhood adversity. Significant GxE with weak effect was detected when environmental exposure was untransformed, indicating a higher sensitivity to childhood adversity in the presence of the high-activity MAOA allele. The interaction was no longer statistically significant after applying a ridit transformation to reflect the sample sizes exposed at each level of childhood adversity.

CONCLUSIONS

The main effect of MAOA on risk for CD in females, its absence in males and directional difference of interaction is suggestive of genotype-sex interaction. As the effect of GxE on risk for CD was weak, its inclusion is not justified.

Development and Initial Validation of the Disinhibition Inventory

The broad personality trait of disinhibition reflects the tendency to behave in an underconstrained versus overconstrained manner and is associated with externalizing psychopathology and risk-taking behaviors. This article describes the development and initial validation of the Disinhibition Inventory (DIS-I), a multifaceted measure of disinhibition that helps explicate the nature of this important higher-order dimension more fully. Factor analyses of an initial item pool resulted in five content-distinct, yet correlated scales measuring both high (Manipulativeness, Distractibility, Risk Taking) and low (Prosociality, Orderliness) levels of disinhibition that cross-validated in an independent sample. Evidence for the construct validity of the DIS-I is presented, including convergent and discriminant relations with Big-Three and Big-Five/five-factor model measures of personality. Results indicate that the DIS-I scales are associated most strongly with other measures of disinhibition, but that the DIS-I additionally contains content absent in extant adult measures of disinhibition that may prove useful in the assessment of externalizing psychopathology.

Mood disorders in children and adolescents

Childhood mood disorders such as major depression, dysthymia, and bipolar disorder have been found to be highly prevalent among children and adolescents. The emotional and behavioral dysfunction associated with these mood disorders can cause impairments across areas of functioning, including academic and social arenas. This article reviews the course, possible causes, assessment, and treatment of this group of disorders in youth and concludes by examining the implications for nurses and other health care providers of youth with mood disorders.

Are there meaningful etiological differences within antisocial behavior? Results of a meta-analysis

There is mounting evidence of etiologically driven distinctions between aggressive (AGG) and non-aggressive rule-breaking (RB) forms of antisocial behavior. To date, however, these differences remain somewhat speculative. The current meta-analysis of twin and adoption studies sought to clarify these distinctions by comparing meta-analytic estimates of genetic, shared environmental, and non-shared environmental influences across AGG and RB to more clearly ascertain whether they evidence differential patterns of genetic and environmental influence. A comprehensive literature search resulted in the collection of 103 twin and adoption studies, of which 15 RB samples and 19 AGG samples were ultimately included in the analyses. Results reveal clear evidence of etiological distinctions between AGG and RB. Namely, AGG appears to be a highly heritable condition (genetic factors account for 65% of the variance), with little role for the shared or common environment, particularly after childhood. By contrast, while genetic influences also contribute to RB (48% of the variance), there is an important role for shared environmental effects as well (18% of the variance). Such findings are indicative of meaningful etiologic distinctions between aggressive and rule-breaking forms of antisocial behavior, and underscore the advantage of differentiating between these behavioral subtypes when studying the causal processes that underlie antisocial behavior.

Maternal depressive symptoms in infancy: unique contribution to children's depressive symptoms in childhood and adolescence?

This prospective 20-year study assessed associations between maternal depressive symptoms in infancy, childhood, and adolescence, and child and adolescent depressive symptoms in a sample of families at high psychosocial risk. Maternal symptomatology was assessed with the Center for Epidemiological Studies Depression Scale (CES-D) when children were infants (12 months), school-aged (age 8), and adolescents (age 19). Children's depressive symptoms were measured at age 8 (Dimensions of Depression Profile for Children and Adolescents) and age 19 (CES-D). Maternal depressive symptoms during infancy contributed to the prediction of child depressive symptoms at age 8, after controlling for concurrent maternal depressive symptoms, clinical risk in infancy, and gender. Clinical risk in infancy marginally contributed to the prediction model. Disorganization of attachment in infancy and maternal hostility were independent predictors of depressive symptoms at age 8 and did not mediate the relation between maternal and child depressive symptoms. Depressive symptoms in adolescence were predicted by gender, children's depressive symptoms at age 8, maternal depressive symptoms in adolescence, and maternal depressive symptoms in infancy. There was no moderating effect of gender. Adding to previous evidence on the importance of early maternal depression, maternal depressive symptoms during infancy were related to the development of depressive symptoms in childhood and adolescence even when other variables of potential relevance were controlled.

DSM-IV combined type ADHD shows familial association with sibling trait scores: a sampling strategy for QTL linkage

Attention deficit hyperactivity disorder (ADHD) is a discrete clinical syndrome characterized by the triad of inattention, hyperactivity, and impulsivity in the context of marked impairments. Molecular genetic studies have been successful in identifying genetic variants associated with ADHD, particularly with DSM-IV inattentive and combined subtypes. Quantitative trait locus (QTL) approaches to linkage and association mapping have yet to be widely used in ADHD research, although twin studies investigating individual differences suggest that genetic liability for ADHD is continuously distributed throughout the population, underscoring the applicability of quantitative dimensional approaches. To investigate the appropriateness of QTL approaches, we tested the familial association between 894 probands with a research diagnosis of DSM-IV ADHD combined type and continuous trait measures among 1,135 of their siblings unselected for phenotype. The sibling recurrence rate for ADHD combined subtype was 12.7%, yielding a sibling recurrence risk ratio (lambda(sib)) of 9.0. Estimated sibling correlations around 0.2-0.3 are similar to those estimated from the analysis of fraternal twins in population twin samples. We further show that there are no threshold effects on the sibling risk for ADHD among the ADHD probands; and that both affected and unaffected siblings contributed to the association with ADHD trait scores. In conclusion, these data confirm the main requirement for QTL mapping of ADHD by demonstrating that narrowly defined DSM-IV combined type probands show familial association with dimensional ADHD symptom scores amongst their siblings.

Individual resiliency factors from a genetic perspective: results from a twin study

This article is part of the Twin Mother's Study, a study that examines influences on maternal adjustment. A number of studies have investigated the importance of genetic factors for mental health, but few of these examine how genes and the environment influence resiliency/salutogenic factors. This article investigates the relative importance of genetic and environmental influences on resiliency/salutogenic factors. This study includes 326 twin pairs (150 monozygotic and 176 dizygotic) who are mothers, who are living with their spouse, and who are part of the Swedish twin register. Using self-report structured questionnaires, we assessed salutogenic factors, depression, and quality of life; however, we analyzed the questionnaires completed by the mothers. Statistical analyses were conducted using structural equation modeling. We conclude that nonshared environmental components were of principal importance in individual resiliency/salutogenic factors in a genetically informative design, but we also noted that genetic influences were important. The shared environment had mainly no effect.

Strengthening the case: prenatal alcohol exposure is associated with increased risk for conduct disorder

OBJECTIVE

The purpose of this study was to examine the relationship between alcohol exposure in pregnancy and offspring conduct disorder symptoms in adolescence and to examine how much this increasingly known association may be mediated by maternal and paternal externalizing diagnoses, including lifetime maternal and paternal alcohol and drug abuse/dependence diagnoses as well as antisocial disorders. Few other studies have examined the contribution of these diagnoses across both parents.

METHOD

A population sample of 1252 adolescents (53.8% female; drawn from the Minnesota Twin Family Study) as well as both of their parents completed structured diagnostic interviews to generate lifetime psychiatric diagnoses; mothers were also retrospectively interviewed about alcohol and nicotine use during pregnancy. Linear regression models were used to test the effects of prenatal alcohol exposure on adolescents' conduct-disorder symptoms.

RESULTS

Prenatal exposure to alcohol was associated with higher levels of conduct-disorder symptoms in offspring, even after statistically controlling for the effects of parental externalizing disorders (illicit substance use disorders, alcohol dependence, and antisocial/behavioral disorders), prenatal nicotine exposure, monozygosity, gestational age, and birth weight.

CONCLUSIONS

Prenatal alcohol exposure contributes to increased risk for conduct disorder in offspring.

Parental divorce and adolescent delinquency: ruling out the impact of common genes

Although the well-documented association between parental divorce and adolescent delinquency is generally assumed to be environmental (i.e., causal) in origin, genetic mediation is also possible. Namely, the behavior problems often found in children of divorce could derive from similar pathology in the parents, pathology that is both heritable and increases the risk that the parent will experience divorce. To test these alternative hypotheses, the authors made use of a novel design that incorporated timing of divorce in a sample of 610 adoptive and biological families. They reasoned that if genes common to parent and child mediate this association, nonadopted youth should manifest increased delinquency in the presence of parental divorce even if the divorce preceded their birth (i.e., was from a prior parental relationship). However, should the association be environmental in origin, the authors reasoned that adolescents should manifest increased delinquency only in response to divorce exposure, and this association should not vary by adoption status. Results firmly supported the latter, suggesting that it is the experience of parental divorce, and not common genes, that drives the association between divorce and adolescent delinquency.

Developmental-genetic effects on level and change in childhood fears of twins during adolescence

BACKGROUND

If the adaptive significance of specific fears changes with age, the genetic contribution to individual differences may be lowest at the age of greatest salience. The roles of genes and environment in the developmental-genetic trajectory of five common childhood fears are explored in 1094 like-sex pairs of male and female monozygotic and dizygotic twins assessed on up to three occasions during adolescence (ages 8-18 years).

METHODS

Dichotomous self-ratings of a cluster of five correlated fears from Ollendick's schedule of fears (FSSC-R) were extracted for subjects at each occasion of assessment. The effects of genes and environment on overall level of fears and rates of adolescent decline were explored by fitting an item-response theory ('IRT') model that allowed for individual genetic and environmental differences in initial fear level ('intercept') and rates of adolescent change ('slope') across the repeated waves of measurement. Different forms of the model were explored using Markov Chain Monte Carlo (MCMC) methods to derive the posterior distribution of subject and item parameters from the raw responses.

RESULTS

Additive genetic differences affect the common factor underlying the five fear-items. The same genes also affect rates of change with age, especially in boys. Male adolescents with higher overall genetic predisposition to childhood fears tended to show slower recovery with age than subjects with relatively low initial values. Thus, the genetic variance apparently increases with age.

CONCLUSIONS

This finding is consistent with a prediction that the regulation of genetic differences will be strongest, and thus the additive genetic variance will be smallest, at the age when the particular stimulus is most salient. Items differed in the extent to which they were sensitive to underlying random differences in the rate of developmental change. Individual differences in rates of change with age were more marked in boys than girls.

Dopamine system genes and ADHD: a review of the evidence

The search for genes influencing the development of attention-deficit/hyperactivity disorder (ADHD) has identified a number of associated genes within, or influencing, the dopamine neurotransmitter system. The focus on this system as the site of genetic susceptibility was prompted by information from animal models, particularly transgenics, as well as the mechanism of action of the psychostimulants, the primary pharmacological treatment for ADHD. Thus far, genes in the dopamine system reported as associated with ADHD, by at least one study, include the dopamine transporter, the dopamine receptors D1, D4 and D5, as well as genes encoding proteins that control the synthesis, degradation and release of dopamine. For some of these genes, replication across studies provides evidence supporting the relationship; however, for others, the data is far from conclusive and further work is needed. The quick progress in the genetic findings was initially surprising given the complexity of the phenotype and the relatively small sample sizes used in the initial studies. However, the high heritability of ADHD, as indicated by twin studies, may have contributed to the success. The genes studied so far are estimated to contribute only weakly or moderately to the risk for the development of ADHD. This may be because these genes, in fact, make only a small contribution. However, few studies have comprehensively examined the genetic information across the gene. This will lead to underestimates of risk if the polymorphism(s) tested is/are not the functional change(s) actually contributing to the genetic susceptibility and if linkage disequilibrium between tested marker(s) and causal variant(s) is weak, or if there is substantial allelic heterogeneity. While the studies thus far are very promising, virtually nothing is known on precisely how genetic variation in these genes actually contributes to risk; thus, functional studies are now required.

Neurobiology and the development of violence: common assumptions and controversies

This paper addresses four common assumptions and related controversies regarding neurobiological factors explaining violence: (i) scholars often assume stability of individual differences in neurobiological factors pertaining to violence, yet much change occurs in aggression/violence during the life course, (ii) individual differences in aggression/violence reflect one or more underlying mechanisms that are believed to have neurobiological origins, yet there is little agreement about which underlying mechanisms apply best, (iii) the development of aggression/violence to some degree can be explained by social, individual, economic and environmental factors, yet it is unclear to what extent neurobiological factors can explain the escalation to, and desistance from, violence over and above social, individual, economic and environmental factors, and (iv) violence waxes and wanes in society over time, yet the explanation of secular differences in violence by means of neurobiological and other factors is not clear. Longitudinal analyses from the Pittsburgh Youth Study are used to illustrate several of these issues.

Incorporating the family as a critical context in genetic studies of children: implications for understanding pathways to risky behavior and substance use

The availability of candidate gene markers for biobehavioral traits will undoubtedly result in increasing attention to genetic influences in studies of childhood risk factors for health behaviors. However, a strict emphasis on genomics without consideration of the social contexts that give rise to risky behaviors will miss opportunities to understand more fully the powerful effect of the family on childhood development. This article discusses the rationale for using the family as a critical context for studying the translation of genetic propensity for risky behavior into developmental pathways that span childhood and adolescence. Attention is given to the importance of family environmental factors; the emerging literature on genetic influences on potential intermediate phenotypes; the need for rich and detailed characterizations of both phenotypes and environmental risk factors embedded within genomic studies of children; and implications for interventions and preventions aimed at risky behaviors. Via discussion of these issues, pragmatic considerations of how studying families as a context may facilitate the thoughtful inclusion of children into genetic paradigms are emphasized.

Depression and internally directed aggression: genetic and environmental contributions

This study uses behavior genetic (BG) methodology to investigate Freud's theory of depression as aggression directed toward the self (1930) and the extent to which genetically and environmentally influenced aggressive tendencies contribute to depressive symptoms. Data from the Twin and Offspring Study in Sweden (TOSS) is used to demonstrate how, in estimating shared and unique environmental influences, BG methods can inform psychoanalytic theory and practice, particularly because of their shared emphasis on the importance of individual experience in development. The TOSS sample consists of 909 pairs of adult twins, their partners, and one adolescent child per couple. The Center for Epidemiologic Studies Depression Scale (Radloff 1977) was used to measure depressive symptoms and the Karolinska Scales of Personality (Schalling and Edman 1993) to measure internally directed aggression. Genetic analyses indicated that for both men and women, their unique experiences as well as genetic factors contributed equally to the association between internally directed aggression and depressive symptoms. These findings support Freud's theory that constitutionally based differences in aggression, along with individual experiences, contribute to a person's depressive symptoms. Establishing that an individual's unique, not shared, experiences and perceptions contribute to depressive symptoms and internally directed aggression reinforces the use of patient-specific treatment approaches implemented in psychoanalytic psychotherapy or psychoanalysis.

The relationship between separation anxiety and impairment

The goal of this study was to characterize the contemporaneous and prognostic relationship between symptoms of separation anxiety disorder (SAD) and associated functional impairment. The sample comprised n=2067 8-16-year-old twins from a community-based registry. Juvenile subjects and their parents completed a personal interview on two occasions, separated by an average follow-up period of 18 months, about the subject's current history of SAD and associated functional impairment. Results showed that SAD symptoms typically caused very little impairment but demonstrated significant continuity over time. Older youth had significantly more persistent symptoms than younger children. Prior symptom level independently predicted future symptom level and diagnostic symptom threshold, with and without impairment. Neither diagnostic threshold nor severity of impairment independently predicted outcomes after taking account of prior symptom levels. The results indicate that impairment may index current treatment need but symptom levels provide the best information about severity and prognosis.

High heritability for a composite index of children's activity level measures

Despite the high heritability of children's activity level, which forms part of the core symptom domain of hyperactivity-impulsivity within attention deficit hyperactivity disorder (ADHD), there has only been a limited success with identifying candidate genes involved in its etiology. This may reflect a lack of understanding about the different measures used to define activity level across studies. We aimed to study the genetic and environmental etiology across three measures of activity level: parent and teacher ratings of hyperactivity-impulsivity and actigraph measurements, within a population-based sample of 463 7-9 year old twin pairs. We further examined ways in which the three measures could be combined for future molecular studies. Phenotypic correlations across measures were modest, but a common underlying phenotypic factor was highly heritable (92%); as was a simple aggregation of all three measurements (77%). This suggests that distilling what is common to all three measures may be a good method for generating a quantitative trait suitable for molecular studies of activity level in children. The high heritabilities found are encouraging in this respect.

The etiology of behavioral problems and competencies in very young twins

Although genetic and environmental influences on behavior problems in middle childhood and adolescence have been well-studied, little is known about the etiology of behavior problems in very early childhood. The present study explores genetic and environmental contributions to individual differences in behavior problems and competences in an infant-toddler sample of twins. There were 1,950 twin pairs (mean age=23.8 months) who were rated by parents on the Infant-Toddler Social and Emotional Assessment. All four domains (Externalizing, Internalizing, Dysregulation, Competence) and 20 subscales-indices on the Infant-Toddler Social and Emotional Assessment displayed significant heritability. There were also substantial shared environmental influences operating on most of the domains and subscales. Compared with behavior problems, behavioral competencies were less heritable and more influenced by shared environments.

A multivariate genetic analysis of specific phobia, separation anxiety and social phobia in early childhood

BACKGROUND

Comorbidity amongst anxiety disorders is very common in children as in adults and leads to considerable distress and impairment, yet is poorly understood. Multivariate genetic analyses can shed light on the origins of this comorbidity by revealing whether genetic or environmental risks for one disorder also influence another. We examined the genetic and environmental influences on the comorbidity between three common childhood anxiety disorders: Specific Phobia, Separation Anxiety and Social Phobia.

METHODS

Using a two-phase design 4,662 twin-pairs were screened in the first phase and 854 pairs were assessed in the second phase by maternal-informant diagnostic interview using DSM-IV criteria.

RESULTS

Multivariate genetic analysis revealed significant shared environmental over-lap between Specific Phobia and Separation Anxiety and significant familial and non-shared environmental over-lap between Specific Phobia and Social Phobia.

CONCLUSIONS

Familial influences, especially shared environment, are central to the comorbidity between Specific Phobia and both Separation Anxiety and Social Phobia.

Familial and temperamental predictors of resilience in children at risk for conduct disorder and depression

In this study, we evaluated predictors of resilience among 8- to 12-year-old children recruited from primarily low socioeconomic status neighborhoods, 117 of whom suffered from clinical levels of conduct problems and/or depression, and 63 of whom suffered from no significant symptoms. Tests of interactions were conducted between (a) paternal antisocial behavior and maternal depression and (b) several physiological indices of child temperament and emotionality in predicting (c) children's conduct problems and depression. Both internalizing and externalizing outcomes among children were associated specifically with maternal melancholic depression, and not with nonmelancholic depression. In addition, low levels of respiratory sinus arrhythmia (RSA) among children conferred significant risk for depression, regardless of maternal melancholia, whereas high RSA offered partial protection. Furthermore, high levels of maternal melancholia conferred significant risk for child depression, regardless of paternal antisocial behavior, whereas low levels of maternal melancholia offered partial protection. Finally, low levels of electrodermal responding (EDR) conferred significant risk for conduct problems, regardless of paternal antisocial behavior, whereas high EDR offered partial protection. None of the identified protective factors offered complete immunity from psychopathology. These findings underscore the complexity of resilience and resilience-related processes, and suggest several potential avenues for future longitudinal research.

Parental punitive discipline, negative life events and gene-environment interplay in the development of externalizing behavior

BACKGROUND

To investigate the extent to which three putative 'environmental' risk factors, maternal punitive discipline (MPD), paternal punitive discipline (PPD) and negative life events (NLEs), share genetic influences with, and moderate the heritability of, externalizing behavior.

METHOD

The sample consisted of 2647 participants, aged 12-19 years, from the G1219 and G1219Twins longitudinal studies. Externalizing behavior was measured using the Youth Self-Report, MPD, PPD and exposure to NLEs were assessed using the Negative Sanctions Scale and the Life Event Scale for Adolescents respectively.

RESULT

Genetic influences overlapped for externalizing behavior and each 'environmental' risk, indicating gene-environment correlation. When controlling for the gene-environment correlation, genetic variance decreased, and both shared and non-shared environmental influences increased, as a function of MPD. Genetic variance increased as a function of PPD, and for NLEs the only interaction effect was on the level of non-shared environment influence unique to externalizing behavior.

CONCLUSION

The magnitude of the influence of genetic risk on externalizing behavior is contextually dependent, even after controlling for gene-environment correlation.

Genetic influences on anxiety in children: what we've learned and where we're heading

Anxiety is a common problem, typically beginning early in life. This article explores reasons for individual differences in levels of anxiety among children, by reviewing the genetic literature. The plethora of research to date has demonstrated clearly that both genes and environmental influences play important roles in explaining differences in levels of anxiety of various types among children. This has encouraged researchers to search for specific genes and environmental influences upon anxiety. Despite important progress in identifying links between anxiety and specific genes--including associations between serotonin and dopamine genes and different symptoms of anxiety--overall, progress has been slow because multiple genes of small effect size are likely to influence anxiety. This article explains how the hunt for genes involved in anxiety is likely to benefit from genetically sensitive research, which examines the co-occurrence of symptoms; includes measures of the environment; and examines endophenotypes and risk pathways.

Genetic and environmental influences on the relation between attention problems and attention deficit hyperactivity disorder

Objective The assessment of symptoms of ADHD in children is usually based on a clinical interview or a behavior checklist. The aim of the present study is to investigate the extent to which these instruments measure an underlying construct and to estimate the genetic and environmental influences on individual differences in ADHD. Methods Maternal ratings were collected on 10,916 twins from 5,458 families. Child Behavior Checklist (CBCL) ratings were available for 10,018, 6,565, and 5,780 twins at the ages 7, 10, and 12, respectively. The Conners Rating Scale (4,887 twins) and the DSM interview (1,006 twins) were completed at age 12. The magnitude of genetic and environmental influences on the variance of the three measures of ADHD and the covariance among the three measures of ADHD was obtained. Results Phenotypic correlations range between .45 and .77. Variances and covariances of the measurements were explained mainly by genetic influences. The model that provided the best account of the data included an independent pathway for additive and dominant genetic effects. The genetic correlations among the measures collected at age 12 varied between .63 and 1.00. Conclusions The genetic overlap between questionnaire ratings and the DSM-IV diagnosis of ADHD is high. Clinical and research implications of these findings are presented.

Across the continuum of attention skills: a twin study of the SWAN ADHD rating scale

INTRODUCTION

Most behavior checklists for attention problems or attention deficit/hyperactivity disorder (ADHD) such as the Child Behavior Checklist (CBCL) have a narrow range of scores, focusing on the extent to which problems are present. It has been proposed that measuring attention on a continuum, from positive attention skills to attention problems, will add value to our understanding of ADHD and related problems. The Strengths and Weaknesses of ADHD symptoms and Normal behavior scale (SWAN) is such a scale. Items of the SWAN are scored on a seven-point scale, with in the middle 'average behavior' and on the extremes 'far below average' and 'far above average'.

METHOD

The SWAN and the CBCL were completed by mothers of respectively 560 and 469 12-year-old twin pairs. The SWAN consists of nine DSM-IV items for Attention Deficit (AD) and nine DSM-IV items for Hyperactivity/Impulsivity (HI). The CBCL Attention Problem (AP) scale consists of 11 items, which are rated on a three-point scale.

RESULTS

Children who had a score of zero on the CBCL AP scale can be further differentiated using the SWAN, with variation seen between the average behavior and far above average range. In addition, SWAN scores were normally distributed, rather than kurtotic or skewed as is often seen with other behavioral checklists. The CBCL AP scale and the SWAN-HI and AD scale were strongly influenced by genetic factors (73%, 90% and 82%, respectively). However, there were striking differences in genetic architecture: variation in CBCL AP scores is in large part explained by non-additive genetic influences. Variation in SWAN scores is explained by additive genetic influences only.

CONCLUSION

Ratings on the SWAN cover the continuum from positive attention skills to attention and hyperactivity problems that define ADHD. Instruments such as the SWAN offer clinicians and researchers the opportunity to examine variation in both strengths and weaknesses in attention skills.

Why are hyperactivity and academic achievement related?

Although a negative association between hyperactivity and academic achievement is well documented, little is known about the genetic and/or environmental mechanisms responsible for the association. The present study explored links between parent and teacher ratings of hyperactive behavior problems and teacher-assessed achievement in a sample of 1,876 twin pairs (mean age 7.04 years). The results did not differ across rater, nor were there significant differences between males or females or for twins in the same or different classrooms. Hyperactivity was significantly correlated with achievement. Multivariate model-fitting analyses revealed significant genetic and nonshared environmental covariance between the two phenotypes. In addition, bivariate heritabilities were substantial, indicating that the phenotypic correlations between hyperactivity and achievement were largely mediated by genetic influences.

Etiological heterogeneity in the development of antisocial behavior: the Virginia Twin Study of Adolescent Behavioral Development and the Young Adult Follow-Up

BACKGROUND

Longitudinal, genetically informed, prospective data collected on a large population of male twins (n=1037) were used to examine developmental differences in the etiology of antisocial behavior.

METHOD

Analyses were carried out on both mother- and child-reported symptoms of conduct disorder (CD) in 10- to 17-year-old twins from the Virginia Twin Study of Adolescent Behavioral Development (VTSABD) and self-reported antisocial behavior by the twins as young adults from the Young Adult Follow-Up (YAFU) study.

RESULTS

The following trends were identified: (1) a single genetic factor influencing antisocial behavior beginning at age 10 through young adulthood ('life-course persistent'); (2) a shared-environmental effect beginning in adolescence ('adolescent-onset'); (3) a transient genetic effect at puberty; and (4) a genetic influence specific to adult antisocial behavior.

CONCLUSIONS

Overall, these etiological findings are consistent with predictions from Moffitt's developmental theory of antisocial behavior. The genetic effect at puberty at ages 12-15 is also consistent with a genetically mediated influence on the timing of puberty affecting the expression of genetic differences in antisocial outcomes.

Genetic and cultural transmission of antisocial behavior: an extended twin parent model

Considerable evidence from twin and adoption studies indicates that both genetic and shared environmental factors play a substantial role in the liability to antisocial behavior. Although twin and adoption designs can resolve genetic and environmental influences, they do not provide information about assortative mating, parent-offspring transmission, or the contribution of these factors to trait variation. We examined the role of genetic and environmental factors for conduct disorder (CD) using a twin-parent design. This design allows the simultaneous estimation of additive genetic, shared and individual-specific environmental effects, as well as sex differences in the expression of genes and environment in the presence of assortative mating and combined genetic and cultural transmission. A retrospective measure of CD was obtained from twins and their parents or guardians in the Virginia Twin Study of Adolescent Behavior Development and its Young Adult Follow up sample. Both genetic and environmental factors play a significant role in the liability to CD. Major influences on individual differences appeared to be additive genetic (38%-40%) and unique environmental (39%-42%) effects, with smaller contributions from the shared environment (18%-23%), assortative mating (-2%), cultural transmission (approximately 2%) and resulting genotype-environment covariance. This study showed significant heritability, which is slightly increased by assortative mating, and significant effects of primarily nonparental shared environment on CD.

Validating psychiatric endophenotypes: inhibitory control and attention deficit hyperactivity disorder

ADHD is a heritable condition of childhood for which several risk alleles have been identified. However, observed effect sizes have been small and few replicated polymorphisms have been identified. There are many reasons for the lack of one-to-one correspondence between genotype and phenotype in ADHD. Endophenotypes are non-clinical markers of genetic risk which may facilitate gene discovery in complex disorders like ADHD. The most common endophenotypes under consideration in ADHD are neuropsychological measures of executive function, although a range of psychological, physiological and neuroanatomical endophenotypes have been proposed. If carefully chosen, endophenotypes have the potential to increase the power of genetic research to identify susceptibility genes. If not carefully selected, endophenotypes may generate false negative and false positive results. This paper reviews the theoretical rationale for endophenotypes and proposes a priori criteria by which ADHD endophenotypes should be selected and validated. The literature on motor response inhibition is reviewed to illustrate the validation process which is recommended in the selection of other candidate endophenotypes.

Environmental contributions to adolescent delinquency: a fresh look at the shared environment

Few genetically-informative studies have attempted to explicitly identify the shared environmental (i.e., those environmental influences that contribute to sibling similarity) factors now known to contribute to adolescent delinquency. The current study therefore examined whether the parent-child relationship served as one source of these shared environmental influences. Participants were 610 adoptive and biological families from the Sibling Interaction and Behavior Study (SIBS). Parents and adolescents reported on their parent-child conflict and parental involvement with child, and adolescents reported on their own delinquent behaviors. We employed structural equation modeling and supplementary multilevel modeling, finding consistent evidence that the association between delinquency and the parent-child relationship is at least partially shared environmental in origin. Such findings provide an important extension of previous twin studies, as they suggest that passive genotype-environment correlations do not explain earlier findings of shared environmental influences on this association.