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

Informant disagreement for separation anxiety disorder

OBJECTIVE

To characterize informant disagreement for separation anxiety disorder (SAD).

METHOD

The sample comprised 2,779 8- to 17-year-old twins from a community-based registry. Children and their parents completed a personal interview about the child's psychiatric history. Parents completed a personal interview about their own psychiatric history and a questionnaire about their marital relationship.

RESULTS

Informant agreement for SAD ranged between chance and extremely poor. Most cases of SAD were diagnosed by interview with only one informant. SAD diagnosed only by child interview was associated with an increased odds of father-rated oppositional defiant disorder, and vice versa. SAD diagnosed only by parent interview was predicted by the parental informant's history of antisocial personality disorder. SAD diagnosed only by paternal interview was also predicted by mother-rated marital conflict and dissatisfaction.

CONCLUSIONS

Parents and children rarely agree about the presence of any level of child separation anxiety. A symptom "or-rule" mostly indexes diagnoses based on interview with only one informant, but the relative validity of such diagnoses remains unclear.

Heritability of attention problems in children: longitudinal results from a study of twins, age 3 to 12

BACKGROUND

Twin studies of childhood behavior problems support the conclusion that individual differences in impulsivity, hyperactivity, and inattention are largely due to genetic influences. Non-genetic variation is due to environmental influences that are unique to the individual, and possibly to rater contrast effects. In the present longitudinal twin study, we report on the size of genetic and environmental effects on individual differences in attention problems at ages 3, 7, 10 and 12 years.

METHODS

Mothers were asked to complete the CBCL for their twin offspring when the children were 3 (n = 11,938), 7 (n = 10,657), 10 (n = 6,192), and 12 years old (n = 3,124). We focus on the Overactivity (OA) scale in the Child Behavior Checklist (CBCL/2-3), and on the Attention Problem (AP) scale of the CBCL/4-18. The data were analyzed using longitudinal structural equation modeling.

RESULTS

Broad heritability of OA and AP is estimated at nearly 75%, at each age. A contrast effect was observed at age 3 only. The results revealed less stability of OA at age 3 to AP at age 7 (r = .40), compared to the stability from AP at age 7 and beyond (r = .70). Genetic effects explained between 76% and 92% of the covariance between OA and AP.

CONCLUSIONS

OA and AP are highly heritable at all ages in both genders. The same set of genes appears to be expressed in boys and girls. The size of genetic and environmental contributions remains the same across the ages studied. Stability in OA and AP is accounted for by genetic influences. Children who do not display OA or AP at a given age are unlikely to develop these problems at a subsequent age.

Sibling relationships of anxiety disordered children--a research note

There is a paucity of knowledge on the role of sibling relationships in internalizing disorders. Research in nonclinical populations suggests an association between internalizing problems in children and negative sibling interactions. Further, an association is reported between internalizing problems and actual or perceived parental differential treatment. This study examines sibling relationship qualities and perceived parental differential treatment in a clinical sample. Participants included 24 anxiety disordered children (ages 8-13 years) and 25 nondisordered control children (ages 7-13 years). Anxiety disordered and nondisordered children do not differ with regard to perceived affection or hostility from a sibling. Anxiety disordered children, however, report significantly more parental differential treatment than do nondisordered children.

Family and high-risk studies of social anxiety disorder

OBJECTIVE

To present data on the role of familial factors in the etiology of social anxiety disorder.

METHOD

Findings presented from a family/high-risk study (the Yale Family Study) and a prospective community study of youth (the Munich Early Developmental Stages of Psychopathology (EDSP) Study).

RESULTS

The Yale Family Study demonstrated a substantial degree of familial aggregation of social anxiety disorder and specificity with respect to other anxiety subtypes among adult relatives. The Yale high-risk component and the EDSP Study confirm the association between parental and offspring social anxiety, but did not yield consistent evidence for an association between familial environmental factors and social anxiety.

CONCLUSION

Future studies are needed to examine mechanisms for the specificity of social anxiety disorder aggregation, to identify vulnerability factors for its development and to pinpoint environmental conditions that may enhance or suppress expression of underlying vulnerability.

Depression and bipolar support alliance consensus statement on the unmet needs in diagnosis and treatment of mood disorders in children and adolescents

OBJECTIVE

To focus attention on the critical unmet needs of children and adolescents with mood disorders and to make recommendations for future research and allocation of healthcare resources.

METHOD

The 36-member Consensus Development Panel consisted of experts in child/adolescent or adult psychiatry and psychology, pediatrics, and mental health advocacy. Reviews of the literature concerning youth mood disorders were performed on the subjects of risk factors, prevention, diagnosis, treatment, and services delivery, and opinions and experiences of mental health advocates were obtained.

RESULTS

The Consensus Development Panel listened to presentations and participated in discussions. Independent workgroups of clinicians, scientists, and mental health advocates considered the evidence and prepared preliminary statements. Workgroup leaders presented drafts for discussion by the Consensus Development Panel. The final document was reviewed by the entire group and edited to incorporate input from all participants.

CONCLUSIONS

Evidence suggests high rates of unmet needs for children and adolescents with depression or bipolar disorder. Training is largely limited to child mental health specialists; general psychiatrists, pediatricians, and other primary care physicians receive little or no training. As a result, treatment patterns may reflect adult treatment plans that are not validated for youths. Effective treatments have been identified and some preliminary prevention models have been developed, but they are not yet widely applied. Patients experience limited exposure to clinicians adequately trained to address their problems and little information to guide care decisions, particularly concerning bipolar disorder. National efforts are required to restructure healthcare delivery and provider training and to immediately develop more advanced research on pathophysiology, prevention, and services delivery effectiveness.

A twin study of anxiety-related behaviours in pre-school children

BACKGROUND

From middle childhood onwards, substantial evidence points to phenotypic differentiation between anxiety diagnostic categories such as generalised anxiety, separation anxiety, specific phobia, and obsessive-compulsive disorders. However, little is known about the genetics of these categories and especially about the phenotypic and genetic structure of related behaviours in pre-school children.

METHODS

We examined the phenotypic differentiation and genetics of mother-reported anxiety-related behaviours in 4,564 four-year-old twin pairs, from a population-based sample.

RESULTS

Confirmatory factor analyses provided support for five correlated factors: General Distress, Separation Anxiety, Fears, Obsessive-Compulsive Behaviours, and Shyness/Inhibition. Genetic influences were found on all five factors, but the pattern of influences differed considerably across them, with particularly high heritability estimates for Obsessive-Compulsive Behaviours and Shyness/Inhibition, and substantial shared environmental influence on Separation Anxiety. Multivariate genetic analyses revealed moderate genetic correlations between the five factors. Genetic overlap was particularly pronounced between General Distress and the other anxiety-related behaviours, accounting for about half of their covariance. Genetic variance on Obsessive-Compulsive Behaviours was the least correlated with the other scales. The shared environmental influences correlated highly across the factors, accounting for the greatest proportion of covariation between Separation Anxiety, Fears and Obsessive-Compulsive Behaviours. The non-shared environment influences were largely variable specific.

CONCLUSIONS

These data provide evidence for phenotypic and genetic overlap as well as differentiation between aspects of anxiety-related behaviours in young children. We conclude that research with young children will benefit from more specific assessments of anxiety-related behaviours in addition to less differentiated assessments of 'internalising' symptoms.

Genetic epidemiology of self-reported lifetime DSM-IV major depressive disorder in a population-based twin sample of female adolescents

BACKGROUND

In adults, about 40% of the variance in risk of Major Depressive Disorder (MDD) is due to genetic factors, but little data exist on the heritability of youth MDD. The goal of this study was the genetic analysis of MDD in an epidemiologically and genetically representative sample of adolescent female twins.

METHODS

A sample of 3416 female adolescent twins systematically ascertained from birth records was assessed using a structured telephone interview that included a comprehensive DSM-IV-based section for the diagnostic assessment of MDD. Mean subject age at time of assessment was 15.5 and participation rate exceeded 85%. Genetic modeling was conducted taking into consideration the problem of censoring, i.e., that younger adolescents were not through their period of risk for adolescent onset of MDD.

RESULTS

Lifetime self-reported MDD prevalence ranged from 1% under age 12 to 17.4% at age 19 and older. The genetic variance in risk of MDD was 40.4% (95% confidence interval (CI): 23.9-55.1), with the remaining variance explained by non-shared environmental effects 59.6% (95%CI: 44.9-76.1). Shared environmental effects were not significant. A significant recall bias was observed with older respondents on average reporting later onsets for their first episode of MDD.

CONCLUSIONS

The genetic and environmental contributions to risk of MDD in this representative sample of female adolescent twins are remarkably analogous to findings from adult samples. These results are congruent with a conceptualization of adolescent MDD and adult MDD as having very similar etiologic determinants.

Resolving multiple epigenetic pathways to adolescent depression

BACKGROUND

Genotype x environment interaction (G x E) arises when genes influence sensitivity to the environment. G x E is easily recognized in experimental organisms that permit randomization of genotypes over fixed environmental treatments. Genotype-environment correlation (rGE) arises when genetic effects create or evoke exposure to environmental differences. Simultaneous analysis of G x E and such 'active' or 'evocative' rGE in humans is intractable with linear structural models widely used in behavioral genetics because environments are random effects often correlated with genotype. The causes of the environmental variation, therefore, need to be modeled at the same time as the primary outcome.

METHODS

A Markov Chain Monte Carlo approach is used to resolve three distinct pathways involving genes and life events affecting the development of post-pubertal depression in female twins and its relationship to pre-pubertal anxiety: 1) the main of genes and environment; 2) the interaction of genes and environment (G x E); and 3) genotype-environment correlation (rGE).

RESULTS

A model including G x E and rGE in addition to the main effects of genes and environment yields significant estimates of the parameters reflecting G x E and rGE. Omission of either G x E or rGE leads to overestimation of the effects of the measured environment and the unique random environment within families.

CONCLUSIONS

1) Genetic differences in anxiety create later genetic differences in depression; 2) genes that affect early anxiety increase sensitivity (G x E) to adverse life events; 3) genes that increase risk to early anxiety increase exposure to depressogenic environmental influences (rGE). Additional genetic effects, specific to depression, further increase sensitivity to adversity. Failure to take into account the effects of G x E and rGE will lead to misunderstanding how genes and environment affect complex behavior.

Genetic and environmental risk factors in adolescent substance use

BACKGROUND

The present study was undertaken with the goal of understanding the causes of association between substance use and both conduct disturbance (CD) and depression in adolescent boys and girls.

METHOD

Multivariate genetic structural equation models were fitted to multi-informant, multi-wave, longitudinal data collected in extensive home interviews with parents and children with respect to 307 MZ male, 392 MZ female, 185 DZ male, and 187 DZ female, same-sex twin pairs aged 12-17 years from the Virginia Twin Study of Adolescent Behavioral Development (VTSABD).

RESULTS

Although conduct disturbance and depression were moderately associated with substance use, the pattern of genetic and environmental risk differed for males and females and across the two disorders. Genetic factors were predominant in girls' substance use whereas boys' use was mediated primarily by shared environmental factors reflecting family dysfunction and deviant peers. The patterns of correlations across the two waves of the study were consistent with conduct disturbance leading to substance use in both males and females, but depression leading to smoking, drug use and, to a lesser extent, alcohol use in girls.

CONCLUSIONS

The comorbidity between substance use and depression, and between substance use and conduct disturbance in childhood/adolescence, probably reflects rather different mediating mechanisms--as well as a different time frame, with conduct disturbance preceding substance use but depression following it. In both, the co-occurrence partially reflected a shared liability but, in girls, genetic influences played an important role in the comorbidity involving depression, whereas in both sexes (but especially in boys) environmental factors played a substantial role. The extent to which these differences reflect genuine differences in the causal mechanisms underlying substance use and CD/depression in boys and girls revealed in the present analysis awaits replication from studies of other general population samples.

CHIP: Defining a dimension of the vulnerability to attention deficit hyperactivity disorder (ADHD) using sibling and individual data of children in a community-based sample

We are taking a quantitative trait approach to the molecular genetic study of attention deficit hyperactivity disorder (ADHD) using a truncated case-control association design. An epidemiological sample of children aged 5 to 15 years was evaluated for symptoms of ADHD using a parent rating scale. Individuals scoring high or low on this scale were selected for further investigation with additional questionnaires and DNA analysis. Data in studies like this are typically complicated. In the study reported on here, individuals have from 1 to 4 questionnaires completed on them and the sample is composed of a mixture of singletons and siblings. In this paper, we describe how we used a genetic hierarchical model to fit our data, together with a twin dataset, in order to estimate genetic factor loadings. Correlation matrices were estimated for our data using a maximum likelihood approach to account for missing data. We describe how we used these results to create a composite score, the heritability of which was estimated to be acceptably high using the twin dataset. This score measures a quantitative dimension onto which molecular genetic data will be mapped.

Substance use disorders, externalizing psychopathology, and P300 event-related potential amplitude

We hypothesize the existence of an inherited predisposition for a spectrum of behaviors and traits characterized by behavioral disinhibition. This externalizing spectrum includes childhood disruptive disorders, antisocial behavior, substance use disorders, personality traits related to behavioral undercontrol, and the precocious expression of problem behavior. We further hypothesize that a genetically influenced central nervous system diathesis underlies this spectrum and is reflected in reduced P300 amplitude in a visual oddball event-related potential task. A review of evidence bearing on the model is derived from findings from the Minnesota Twin Family Study, a population-based, longitudinal investigation of twin youth. These findings indicate that the collection of attributes related to behavioral disinhibition is familial, heritable, and interrelated. Evidence supporting P3 amplitude reduction (P3-AR) as an index of genetic vulnerability for this externalizing spectrum includes its association with (a) familial risk for substance use and antisocial personality disorders, (b) diagnoses of childhood disruptive disorders and substance use disorders, (c) early onset of undersocialized behavior, and (d) quantitative phenotypes related to externalizing problems. In addition, the development of substance use disorders over a 3-year period is associated with P3-AR measured prior to their expression. These findings suggest that P3-AR indexes one aspect of the genetic diathesis for a spectrum of externalizing problem behavior.

Parenting and the development of conduct disorder and hyperactive symptoms in childhood: a prospective longitudinal study from 2 months to 8 years

BACKGROUND

This study investigated the early processes involved in the development of symptoms of conduct disorder and hyperactivity.

METHOD

The study employed a prospective design, over a period from 2 months to 8 years. Detailed observational data of early and later mother-child interactions were collected, infant prefrontal function (the A not B task) was assessed, and symptoms of child conduct disorder and hyperactivity were rated by maternal report at age 5 and 8 years.

RESULTS

The principal findings of the study were that emotional dysregulation on the A not B task at 9 months predicted symptoms of conduct disorder at 5 and 8 years, and delayed object reaching times on the same task predicted hyperactive symptoms at 5 years. These two developmental trajectories were associated with distinct patterns of early parenting that were strongly influenced by infant gender. Thus, in boys early emotional dysregulation was predicted by rejecting and coercive parenting, and delayed reaching on the A not B task by coercive parenting, whereas in girls only continuity from earlier infant behaviour could be demonstrated. There was strong continuity between these early infant behaviours and later child disturbance that was partially mediated by parenting for conduct disorder symptoms (maternal hostile parenting in boys, and maternal coercive parenting in girls), but not for hyperactive symptoms.

CONCLUSIONS

These data would suggest that only in boys was there evidence for the existence of a sensitive period for the development of hyperactive symptoms, and to a lesser extent, conduct disorder symptoms.

Psychopathology and delinquency in adolescents: a descriptive and developmental perspective

The current article presents a review on psychiatric pathology in delinquent adolescents. The putative developmental significance of this co-occurrence is discussed within Moffitt's [Psychol. Rev. 100 (1993) 674] developmental framework on antisocial behavior. Articles on psychopathology in delinquent adolescents were retrieved through search engines (MEDLINE, Psychlit) and by exploration of references in those articles. Substantial evidence is at hand that delinquent adolescents have higher rates of externalizing and internalizing disorders when compared to adolescents in the general population. Although substantial limitations hamper interpretation of the findings, the marked similarities across a diversity of samples from different countries suggest that the findings are fairly generalizable. Evidence was found for differences in prevalence and developmental significance of psychiatric pathology between adolescent limited (AL) offenders and life-course persistent (LCP) offenders. Although psychopathology was more severe in LCP offenders, AL offenders still had significant and potential harmful levels of psychopathology. These findings emphasize the necessity of developing mental health services to delinquent adolescents and the necessity of adequate diagnostic and therapeutic interventions in these adolescents.

Genetic influences on childhood competencies: a twin study

OBJECTIVE

To estimate genetic, environmental, and rater contrast influences on parental reports of Activities, Social, School, and Total Competence scales of the Child Behavior Checklist (CBCL).

METHOD

Parents of 492 twin pairs aged 8-12 years completed CBCLs. Genetic, shared and unique environmental, and rater bias effects were estimated for the Activities, Social, School, and Total Competence scales. Data on boys and girls were analyzed separately.

RESULTS

Moderate genetic influences were found only for the School scale (60%-76%), while shared environment accounted for most of the variance in Activities, Social, and Total Competence scales. Gender differences are reported. Similar to a prior twin study of CBCL problem syndromes, there was no evidence of rater bias.

CONCLUSIONS

Estimates of genetic influence on these child competence domains were high for School Competence, while social competence and activity competence evidenced higher levels of shared environmental influences. Organization and wording of CBCL items may avoid rater biases in reporting. These findings have implications for interventions to improve school, social, and activities competence.

Biased paternal transmission of SNAP-25 risk alleles in attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) is the most common childhood psychiatric disorder, affecting 5-10% of school-age children. Although the biological basis of this disorder is unknown, twin and family studies provide strong evidence that ADHD has a genetic basis involving multiple genes. A previous study found an association between ADHD and two polymorphisms in the 3' untranslated region (UTR) of SNAP-25, a gene encoding a synaptic vesicle docking protein known to play a role in the hyperactivity observed in the Coloboma mouse strain. In this paper, we test biased transmission of the 3' UTR SNAP-25 haplotype using a larger ADHD sample of 113 families with 207 affected children. Using the transmission disequilibrium test (TDT), we found a trend consistent with biased transmission of the TC haplotype of SNAP-25 in all transmissions and detected a significant distortion (P=0.027) when paternal transmissions were evaluated.

Heritability of attention problems in children: I. cross-sectional results from a study of twins, age 3-12 years

Multiple twin studies of attention problems (AP) from the Child Behavior Checklist or ADHD from the DSM criteria have reported on the genetic and environmental influences on these behaviors. The majority of these have studied AP and ADHD symptoms in twin samples combined across wide age spans, combined rater information and both genders. Thus, it is possible that the results are complicated by developmental, informant, and gender differences. The purpose of this study was to assess for the genetic and environmental contributions to overactive behavior (a syndrome highly related to AP in 7-, 10-, and 12-years olds) in 3-years olds (3,671 twin pairs), and attention problems in 7- (3,373 twin pairs), 10- (2,485 twin pairs), and 12-years olds (1,305 twin pairs) while controlling for developmental, gender and rater contrast contributions. Using a cross-sectional twin design, contributions from genetic additive, genetic dominance, unique environmental and rater contrast effects were estimated for CBCL maternal reports. We found that genetic influences on overactive behavior and attention problems are high across an age span that covers pre-school and elementary school age. Although girls display less problem behavior compared to boys, heritability estimates were found equal for both genders at each age. Environmental experiences that are unique to the individual accounted for the remaining influence. At the age of 3 years, a rater contrast effect was detected. We hypothesize that the contrast effect represents a maternal rater bias effect that is dependent on the age of the twins. The implications of these findings are discussed with reference to the clinical setting and in the context of future research.

Maternal smoking during pregnancy and risk to boys' conduct disturbance: an examination of the causal hypothesis

BACKGROUND

We undertook this study to determine whether the widely replicated link between maternal smoking and conduct disturbance (Cd) is better explained by a model of direct causation or of mother-offspring transmission of a latent Cd variable.

METHODS

Family data collected on 538 adolescent twin boys from the Virginia Twin Study of Adolescent Behavioral Development (VTSABD) was used to compare two alternative models: 1) a model composed of a latent transmissible factor that influences mother's juvenile conduct symptoms, smoking during pregnancy, and subsequent Cd and smoking in her adolescent boys; and 2) a model specifying a direct causal path from mother's smoking to child Cd.

RESULTS

The maternal-offspring transmission model fit the data as well as a model specifying a direct causal path from maternal smoking to child Cd.

CONCLUSIONS

Our results suggest that the association between maternal smoking during pregnancy and boys' Cd symptoms may be attributed to the transmission of a latent Cd factor and not to a direct effect of the smoking. Our results challenge previous findings of a direct effect of prenatal smoke exposure on risk to Cd once other etiologic factors are considered.

Monozygotic twins discordant for attention-deficit/hyperactivity disorder: ascertainment and clinical characteristics

OBJECTIVE

Nongenetic factors and phenomenology of attention-deficit/hyperactivity disorder (ADHD) were examined in monozygotic (MZ) twin pairs discordant for ADHD.

METHOD

Recruitment included telephone screening (n = 297 pairs), behavioral ratings obtained from parents and teachers (n = 59 pairs), and, finally, in-person assessment (n = 25 pairs; structured classroom observation, diagnostic interview, psychoeducational evaluation, birth record review, establishment of monozygosity, and anatomic brain imaging). Affected twins were further contrasted with previously studied affected singletons.

RESULTS

Of the 25 MZ twin pairs qualifying for in-person evaluation, only 10 proved discordant for ADHD. Affected twins were mostly comparable with affected singletons on clinical measures, although fathers' self-ratings of childhood ADHD status were significantly lower in twins than in singletons.

CONCLUSIONS

Discordance for ADHD in MZ twins appears to be ascribable to greater environmental discordance and decreased familiality. Despite these differences, affected twins were phenotypically comparable with affected singletons. Thus MZ twins discordant for ADHD, while rare, can inform research on the etiology and pathophysiology of this disorder.

Assessment of the within-family environment in twins: absolute versus differential ratings, and relationship with conduct problems

BACKGROUND

The aim was to compare the psychometric properties of absolute versus differential ratings of the within-family environments and to examine their associations with conduct problems in 8-16-year-old twins.

METHODS

The sample comprised 1117 pairs of like-sex male and female twins and their parents, recruited from the school population of Virginia. The within-family environment was assessed from the Twin Inventory of Relationships and Experiences (TIRE), which provided measures of differential parental criticism, parental preference for one twin, and twins' peers' conduct problems, as rated by mothers, fathers and each of the twins. Twins' conduct problems were assessed by a modified version of the Olweus questionnaire.

RESULTS

Although differential ratings and absolute ratings had similar psychometric properties, the use of absolute ratings to differentiate the twins resulted in a construct with weak inter-rater agreement and low stability over time. Differential ratings of the twins' environment showed significant associations with differences in conduct problems between the twins, whereas the difference in absolute ratings of each twin did not. The differential parental criticism effect was uninfluenced by the overall level of criticism in the family.

CONCLUSIONS

A direct contrast between siblings or twins, as part of the rating procedure (as provided by differential ratings), may be a more efficient way to assess aspects of the within-family environment that are reliable and which are associated with psychopathology. Both differential parental criticism and peers' conduct disturbance were associated with conduct problems in the twins.

Assessing the effects of age, sex and shared environment on the genetic aetiology of depression in childhood and adolescence

BACKGROUND

Depressive symptoms and disorder are experienced by a significant proportion of young people and have long-lasting deleterious effects. The aims of the current investigation were to examine the aetiology of depressive symptoms using a twin design. In particular to examine the effects of sex, age, maternal depression and anxiety symptoms and to examine the aetiology of high depression scores.

METHODS

Questionnaires were sent to the families of a population-based sample of twins aged between 8 and 17 years. Parents and children over the age of 11 were asked to complete the Mood and Feelings Questionnaire and Hospital Anxiety and Depression Scale (mothers only). Responses were obtained from 1463 families and data were analysed using genetic model fitting and DeFries and Fulker regression analysis.

RESULTS

Depressive symptoms, particularly when self-rated, were significantly genetically influenced. There was evidence of significant heterogeneity according to age, with shared environmental factors more important and genetic factors less important for children aged 8 to 10 than for adolescents aged 11 to 17 years. Some but not all of the shared environmental influences on parent-rated depressive symptoms were accounted for by maternal symptoms of depression and anxiety. There was a significant effect of gender for self-rated depressive symptoms. For boys, genetic factors were of greater importance and common environmental influences of less importance than for girls. Shared environmental effects had a substantial influence on high self-rated depression scores. Adolescents who scored highly on self-rated depression questionnaires experienced significantly more shared life events and their mothers had significantly higher internalising symptoms than adolescents who scoredwithin the normal range.

CONCLUSIONS

The results of this study add to the evidence that the aetiology of depressive symptoms differs by age, with genetic factors becoming more important from childhood to adolescence. Some but not all of the shared environmental effect observed for mother-rated depression scores is due to maternal depression and anxiety symptoms. For self-rated depressive symptoms, the importance of genetic and environmental factors may also differ by sex, with genetic influences more important for boys. The aetiology of high depression symptom scores, when self-rated, appears to differ from scores within the normal range in that shared environmental factors appear to be more important. Further research is needed to identify these shared environmental factors using longitudinal models that test genetic and environmental mediation.

Process, mechanism, and explanation related to externalizing behavior in developmental psychopathology

Advances in conceptualization and statistical modeling, on the one hand, and enhanced appreciation of transactional pathways, gene-environment correlations and interactions, and moderator and mediator variables, on the other, have heightened awareness of the need to consider factors and processes that explain the development and maintenance of psychopathology. With a focus on attentional problems, impulsivity, and disruptive behavior patterns, I address the kinds of conceptual approaches most likely to lead to advances regarding explanatory models in the field. Findings from my own research program on processes and mechanisms reveal both promise and limitations. Progress will emanate from use of genetically informative designs, blends of variable and person-centered research, explicit testing of developmental processes, systematic approaches to moderation and mediation, exploitation of "natural experiments," and the conduct of prevention and intervention trials designed to accentuate explanation as well as outcome. In all, breakthroughs will occur only with advances in translational research-linking basic and applied science-and with the further development of transactional, systemic approaches to explanation.

The dopamine D4 receptor and the hyperactivity phenotype: a developmental-epidemiological study

Attention-deficit hyperactivity disorder (ADHD) affects 2-6% of school-age children and is a precursor of behavioural problems in adolescence and adulthood. Underlying the categorical definition of ADHD are the quantitative traits of activity, impulsivity, and inattention which vary continuously in the population. Both ADHD and quantitative measures of hyperactivity are heritable, and influenced by multiple genes of small effect. Several studies have reported an association between clinically defined ADHD and the seven-repeat allele of a 48-bp tandem repeat polymorphism in the third exon of the dopamine D4 receptor gene (DRD4). We tested this association in a large, unselected birth cohort (n = 1037) using multiple measures of the hyperactivity phenotype taken at multiple assessment ages across 20 years. This longitudinal approach allowed us to ascertain whether or not DRD4 has a general effect on the diagnosed (n = 49) or continuously distributed hyperactivity phenotype, and related personality traits. We found no evidence to support this association.

Development and natural history of mood disorders

To expand and accelerate research on mood disorders, the National Institute of Mental Health (NIMH) developed a project to formulate a strategic research plan for mood disorder research. One of the areas selected for review concerns the development and natural history of these disorders. The NIMH convened a multidisciplinary Workgroup of scientists to review the field and the NIMH portfolio and to generate specific recommendations. To encourage a balanced and creative set of proposals, experts were included within and outside this area of research, as well as public stakeholders. The Workgroup identified the need for expanded knowledge of mood disorders in children and adolescents, noting important gaps in understanding the onset, course, and recurrence of early-onset unipolar and bipolar disorder. Recommendations included the need for a multidisciplinary research initiative on the pathogenesis of unipolar depression encompassing genetic and environmental risk and protective factors. Specifically, we encourage the NIMH to convene a panel of experts and advocates to review the findings concerning children at high risk for unipolar depression. Joint analyses of existing data sets should examine specific risk factors to refine models of pathogenesis in preparation for the next era of multidisciplinary research. Other priority areas include the need to assess the long-term impact of successful treatment of juvenile depression and known precursors of depression, in particular, childhood anxiety disorders. Expanded knowledge of pediatric-onset bipolar disorder was identified as a particularly pressing issue because of the severity of the disorder, the controversies surrounding its diagnosis and treatment, and the possibility that widespread use of psychotropic medications in vulnerable children may precipitate the condition. The Workgroup recommends that the NIMH establish a collaborative multisite multidisciplinary Network of Research Programs on Pediatric-Onset Bipolar Disorder to achieve a better understanding of its causes, course, treatment, and prevention. The NIMH should develop a capacity-building plan to ensure the availability of trained investigators in the child and adolescent field. Mood disorders are among the most prevalent, recurrent, and disabling of all illnesses. They are often disorders of early onset. Although the NIMH has made important strides in mood disorders research, more data, beginning with at-risk infants, children, and adolescents, are needed concerning the etiology and developmental course of these disorders. A diverse program of multidisciplinary research is recommended to reduce the burden on children and families affected with these conditions.

The association between psychopathology in fathers versus mothers and children's internalizing and externalizing behavior problems: a meta-analysis

In light of the selective focus on maternal (vs. paternal) psychopathology as a risk factor for child development, this meta-analysis examines the relative strength of the association between psychopathology in mothers versus fathers and the presence of internalizing and externalizing disorders in children. Associations were stronger between maternal than paternal psychopathology and the presence of internalizing (but not externalizing) problems in children, with all average effect sizes being small in magnitude. Relations were moderated by variables that highlight theoretically relevant differences between psychopathology in mothers versus fathers (e.g., age of children studied, type of parental psychopathology) and by variables related to methodological differences across studies (e.g., method of assessing psychopathology in parents and children, type of sample recruited, familial composition).

Maternal sensitivity, infant attachment, and temperament in early childhood predict adjustment in middle childhood: the case of adopted children and their biologically unrelated parents

In a longitudinal study, internationally adopted children (N = 146) placed before 6 months of age were followed from infancy to age 7. Results showed that girls were better adjusted than boys, except in cognitive development, and that easy temperament was associated with higher levels of social, cognitive, and personality development and fewer behavior problems. Higher quality of child-mother relationships, in terms of attachment security and maternal sensitivity, uniquely predicted better social and cognitive development. The combination of attachment disorganization and difficult temperament predicted less optimal ego-control and lower levels of cognitive development. It is concluded that even in adopted children, who are not biologically related to their adoptive parents, early mother-infant interactions and attachment relationships predict later socioemotional and cognitive development, beyond infant temperament and gender.

Home-cage activity in heterogeneous stock (HS) mice as a model of baseline activity

Behavioral genetic work in humans indicates that clinical hyperactivity is best viewed as the extreme end of activity levels in the population. However, current animal models of hyperactivity are not studied as quantitative traits as they are either knockout models or inbred strains. Furthermore, these animal models generally demonstrate elevated locomotion in novel environments, but not in their home-cages. This is the opposite of the symptoms seen in the human condition where childhood hyperactivity is generally more pronounced in constant, unstimulating situations. In this study we filmed an outbred population of 44 heterogeneous stock (HS) mice under red light during their active phase, to assess the reliability of individual differences in home-cage behavior and extract an index of home-cage activity (HCA) level. We then compared this measure to locomotor behavior in a novel environment--the open-field. Reliable individual differences in home-cage behaviors such as running, swinging on bars, and burrowing were found, and principal component factor analysis yielded a general activity factor, which accounted for 32% of the variance and correlated 0.90 with a subjective impression of activity level. The correlation between HCA and locomotor activity in the open-field was 0.23, which was non-significant. However, the association with HCA level appeared to increase over the five minutes of the open-field, presumably as the mice habituated. Furthermore, although mice displaying particularly high and low HCA were indistinguishable early in the open-field task, they became significantly differentiated over time. We conclude that home-cage behaviors and the open-field, after habituation, display good face and construct validity, and may provide a good model of baseline activity for quantitative trait loci (QTL) discovery and functional genomics in the HS mice.

Emotional and behavioral problems among female twins: an evaluation of the equal environments assumption

OBJECTIVE

To evaluate the equal environments assumption (EEA) of the twin method for mother-reported symptoms of child and adolescent emotional and behavioral problems.

METHOD

Four emotional and behavioral problem scales (symptoms of separation anxiety disorder, attention-deficit/hyperactivity disorder, oppositional defiant disorder, and conduct disorder) and four environmental similarity measures (sharing friends, sharing classes, dressing alike, and perceived zygosity) were assessed by telephone interviews with the biological mothers of 1,948 female adolescent and young adult twin pairs. The effect of environmental similarity on the magnitude of the monozygotic and dizygotic twin correlations and on the parameter estimates from genetic model-fitting was examined for each symptom scale.

RESULTS

The measures of environmental similarity were not strongly or consistently related to the similarity of twins for mother-reported emotional and behavioral problems. On average, controlling for environmental similarity did not substantially affect estimates of genetic and environmental influences.

CONCLUSIONS

These results lend support for the validity of the EEA and suggest that estimates of genetic and environmental influences obtained from twin studies of mother-reported child and adolescent emotional and behavioral problems are not unduly biased by the greater environmental similarity of monozygotic than dizygotic twins.

Clinical implications of a link between fetal alcohol spectrum disorder and attention-deficit hyperactivity disorder

OBJECTIVE

To provide an overview of the animal and human research literature on the link between fetal alcohol spectrum disorder (FASD) and attention-deficit hyperactivity disorder (ADHD).

METHOD

We conducted a comprehensive literature review that addressed the history of, and current research on, fetal alcohol syndrome (FAS) and FASD, as well as that on ADHD in children.

RESULTS

In animal and human research, there is emerging clinical, neuropsychological, and neurochemical evidence of a link between FASD and ADHD.

CONCLUSIONS

The evidence of the link between these 2 conditions has implications for clinical management. The clinical quality of ADHD in children with FASD often differs from that of children without FASD. For children with FASD, ADHD is more likely to be the earlier-onset, inattention subtype, with comorbid developmental, psychiatric, and medical conditions. Children with FASD are commonly not mentally retarded but present complex learning disabilities, especially a mixed receptive-expressive language disorder with deficits in social cognition and communication (reminiscent of sensory aphasia and apraxia), working memory problems, and frequently, a mathematics disorder. Comorbid psychiatric conditions include anxiety, mood, conduct, or explosive disorders. As well, cardiac, renal, or skeletal problems are more likely to be present. Because these children have a disturbance in brain neurochemistry, or even brain structure (that is, in the corpus callosum), their response to standard psychostimulant medication can be quite unpredictable.

The heritability of depressive symptoms: multiple informants and multiple measures

BACKGROUND

Earlier research suggests large rater differences in heritability estimates of children's depressive symptoms in the context of significant age and sex-limitation effects.

METHODS

With data from an ongoing, population-based twin-family study, we estimated genetic and environmental influences on children's depression with models allowing for sex-specific effects. Our assessments of twins included self-reports and ratings made by their classmate peers, their parents and their teachers, allowing estimates of genetic and environmental effects with data from different informants. Model-fitting used maximum likelihood estimation of log-transformed data from a sample of 1,366 11- and 12-year-old twin pairs.

RESULTS

Estimates of additive genetic effects were significant for both boys and girls across all four informants, and, standardized to reflect the percentage of phenotypic variance accounted for, those estimates ranged from .28 to .71. Significant effects from common environmental sources were found in ratings of teachers and parents, where, typically, one individual rated both co-twins, but neither in peer nominations nor self-ratings. The correlation of teacher and parent ratings was modest, and bivariate model-fitting found no correlation in either genetic or shared environmental effects between teacher and parental ratings. At this young age, sex-limitation effects were found only in teacher ratings, where genetic effects were greater in girls than in boys.

CONCLUSIONS

Results underscore the utility of using multiple informants to measure children's behavior and provide a foundation for follow-up of these twins in later adolescence.

Quantitative trait locus for reading disability on chromosome 6p is pleiotropic for attention-deficit/hyperactivity disorder

Comorbidity is pervasive among both adult and child psychiatric disorders; however, the etiological mechanisms underlying the majority of comorbidities are unknown. This study used genetic linkage analysis to assess the etiology of comorbidity between reading disability (RD) and attention-deficit hyperactivity disorder (ADHD), two common childhood disorders that frequently co-occur. Sibling pairs (N = 85) were ascertained initially because at least one individual in each pair exhibited a history of reading difficulties. Univariate linkage analyses in sibling pairs selected for ADHD from within this RD-ascertained sample suggested that a quantitative trait locus (QTL) on chromosome 6p is a susceptibility locus for ADHD. Because this QTL is in the same region as a well-replicated QTL for reading disability, subsequent bivariate analyses were conducted to test if this QTL contributed to comorbidity between the two disorders. Analyses of data from sib pairs selected for reading deficits revealed suggestive bivariate linkage for ADHD and three measures of reading difficulty, indicating that comorbidity between RD and ADHD may be due at least in part to pleiotropic effects of a QTL on chromosome 6p.

Dopamine transporter polymorphism associated with externalizing behavior problems in children

Early childhood externalizing behavior is a stable and heritable pattern of aggressive and delinquent behavior that often leads to the development of serious psychiatric disorders such as conduct disorder and attention deficit hyperactivity disorder. We examined the relationship between parent reported externalizing behavior (assessed at ages 4, 7, and 9 years) and the VNTR polymorphism of the 3' untranslated region of SLC6A3 (DAT1) in a community sample of 790 children ascertained as part of our longitudinal twin and adoption studies. We applied the sibling-based methodology developed by Fulker et al. [1999: Am J Hum Genet 64:259-267] for estimating allelic association with quantitative traits, while controlling for population stratification. An extension of these methods allowed for the inclusion of monozygotic twins, dizygotic twins, siblings, and singletons. We have demonstrated that the 9-repeat variant of the DAT1 is a significant risk allele for externalizing behavior at ages 4 (P=0.001) and 7 years (P=0.02). Although the effect size was negligible at age 9 (P=0.92), a formal test of the developmental decrease in effect across the three ages was non-significant (P=0.70).

The relationship between DSM-IV oppositional defiant disorder and conduct disorder: findings from the Great Smoky Mountains Study

BACKGROUND

We examine models of the relationship between oppositional defiant disorder (ODD) and conduct disorder (CD) in a community sample. Particular attention is paid to the generalisability of findings based on clinic-referred boys.

METHODS

The analyses were based on four waves of data from the Great Smoky Mountains Study covering children in the community aged 9-16 years. Child and parent reports of DSM-IV symptoms, diagnoses, and a range of family and environmental adversities were collected using the Child and Adolescent Psychiatric Assessment.

RESULTS

Cross-sectional analyses indicated that CD and ODD largely shared similar correlates, although some aspects of parenting appeared more related to CD than ODD. This pattern was broadly similar in boys and girls. Longitudinal analyses confirmed that ODD was a strong risk factor for CD in boys and there was a suggestion that ODD was a stronger risk factor for CD than for other common disorders. Atypical family structure was an important factor in the transition between ODD and CD in boys. In girls ODD provided no increased risk for later CD but was associated with increased risk for continued ODD, depression, and anxiety.

CONCLUSIONS

These results are more consistent with a developmental relationship between ODD and CD in boys than girls.

Observer effects and heritability of childhood attention-deficit hyperactivity disorder symptoms

BACKGROUND

Twin studies have found that childhood attention-deficit hyperactivity disorder (ADHD) has a strong genetic component. Estimates of heritability, the extent of non-additive genetic effects and of 'sibling contrast' effects vary between different studies.

AIMS

To use multiple informants to assess the extent to which observer effects influence such estimates in an epidemiological sample of twins.

METHOD

Questionnaire packs were sent to the families and teachers of twins aged 5-16 years in the Bro Taf region of South Wales. The twins were ascertained from community paediatric registers.

RESULTS

Both parent- and teacher- rated data showed a high degree of heritability for ADHD measured as a symptom dimension, but the correlation between the two types of rater was modest. Bivariate analyses suggested that parent and teacher ratings reflect the effects of different genes. Self-report data from twins aged 11-16 years showed no evidence of genetic effects.

CONCLUSIONS

Although ADHD is shown to be highly heritable by both parent- and teacher-rated data, the underlying genotypes may be substantially different. This has implications for study designs aiming to find genes that contribute to the disorder.

Bulimic symptoms in the Virginia Twin Study of Adolescent Behavioral Development: correlates, comorbidity, and genetics

BACKGROUND

This paper addresses bulimia symptoms in a large community sample of twins aged 8 to 17 years. We aim to identify environmental correlates of bulimia symptoms and relationships with other psychiatric disorder symptoms. The twin design allows examination of the structure of genetic and environmental effects.

METHODS

DSM-IIIR bulimia symptoms and consequential impairment were measured by interview in the first wave of the Virginia Twin Study of Adolescent Behavioral Development. Comorbidity with other psychiatric symptoms and environmental correlates were examined and the relative contributions of genes and environment were assessed using structural equation modeling.

RESULTS

An item-response theory model indicated that the range of bulimic symptoms represented a single underlying trait. Bulimia symptoms were more common in postmenarche girls and positively associated with body-mass index. Subdiagnostic symptomatology was associated with impairment in psychosocial functioning. Bulimia symptoms were strongly associated with other psychiatric disorders symptoms including anxiety and depression. Genetic model fitting identified strong additive genetic effects on the symptom score. Accounting for a potential violation of the equal environment assumption for identical and fraternal twins slightly reduced estimated genetic variance.

CONCLUSIONS

The pattern of comorbidity suggests overlap between bulimia symptoms and those of internalizing disorders. Substantial genetic variance (44%) was evident in the most conservative model.

Relation between anxiety and depressive disorders in childhood and adolescence

Current research indicates that there is a strong relationship between pediatric anxiety disorders and depression. Assessment measures show high rates of correlation between depression and anxiety and much of the overlap may be related to a common domain of negative affectivity. Anxious youth and depressed youth share a cognitive style marked by a negative bias in information processing. Anxiety disorders and depression are frequently comorbid in children and adolescents. About 25-50% of depressed youth have comorbid anxiety disorders and about 10-15% of anxious youth have depression. Twin and family studies have demonstrated that pediatric anxiety disorders and depression likely share some common genetic factors or influences. Selective serotonin reuptake inhibitors and cognitive-behavioral therapy have been shown in randomized controlled trials to be efficacious for both pediatric depression and anxiety disorders. Integrating the treatment literature with studies of phenomenology, biology and genetics indicates that pediatric anxiety disorders and depression may share a genetically determined neurobiological component that could involve neural circuits that include or are modulated by serotonergic neurons. This component could contribute to the negative affective temperament that appears to be common in both pediatric depression and anxiety disorders.

The genetic aetiology of childhood depression: a review

BACKGROUND

We review the evidence for the familiality of major depressive disorder (MDD) and the genetic aetiology of depressive symptoms in children and adolescents.

METHODS

Databases and reference lists were searched for family, twin and adoption studies of childhood MDD and childhood depressive symptoms. Data from independent family studies that fulfilled specified inclusion criteria were pooled and odds ratios were calculated for top-down and bottom-up family studies.

RESULTS

Estimates of familial risk differ by control group and by study design (odds ratio range 1.70, 3.98). Twin studies show that depressive symptoms in young people are heritable although rater and measurement issues are important. Adoption studies show little evidence for a genetic influence on depressive symptoms.

CONCLUSIONS

MDD in young people is familial although control group and study design affect the magnitude of the familial risk. Estimates of heritability from twin and adoption studies vary widely and few firm conclusions can be made regarding the genetic aetiology of depressive symptoms in childhood. Areas that require future work include the examination of rater effects, measurement issues, the effects of age and comorbidity and reasons for the discrepancy between twin and adoption findings.

Can the Family Assessment Device (FAD) be used with school aged children?

This article examines the usefulness of the Family Assessment Device(FAD) in assessing family functioning with school-aged children (under 12 years of age) compared to children 12 and older and mothers. FAD reports from 194 children with asthma (132 under 12 years; 62 12 years and older) were evaluated in comparison to mothers' FAD reports as well as mothers' reports on two other family functioning measures: the Family Inventory of Life Events (FILE) and the Impact on Family (IOF) scale, both relevant to functioning in families with children with asthma. Although FAD scale reliabilities were lowest in younger children (alpha = 48-.79), good concurrent validity was found with mothers' reports on the three different measures of family functioning. Older childrens' FAD scales showed good reliability (> .70) but poorer agreement with the various maternal measures. With modifications to make items more structured and simplified, the FAD could be tried with younger children, although documentation of improved reliability is needed before more widespread use with school-aged children is advocated. The measure already shows good agreement with maternal report in this younger age group, which likely would be strengthened with improved reliability. FAD reports in older children correlated less well with maternal measures, but may represent nonredundant information on family health from an independent perspective and are worth considering in addition to mothers' perspectives.

Biological, psychological and social processes in the conduct disorders

BACKGROUND

This paper reviews recent evidence on the causes and maintenance of aggressive and disruptive behaviours in childhood and adolescence. It considers the relative merits of several different ways of conceptualising such problems, in relation to the contribution of biological, psychological and social factors.

METHOD

It focuses on conduct problems appearing in young childhood, which greatly increase the likelihood of persistent antisocial behaviours in adolescence and adult life in association with wider interpersonal and social role impairments. It considers the contribution of individual factors, including impaired verbal skills, deficits in executive functions, and an imbalance between behavioural activation and inhibition systems. These are viewed in interaction with commonly associated environmental disadvantages such as hostile or intrusive parenting. The roles of attributional biases, unrealistic self-evaluations, and insecure attachment are considered in relation to affect regulation, and effective social action. The contributions of the wider social environments of peers, neighbourhood and socio-economic conditions are evaluated.

CONCLUSIONS

The paper concludes that, although considerable progress has been made over the past ten years, there is a need to further refine our conceptualisation of the behaviours to be explained, to develop a coherent theory of the causal and maintaining processes, and to carry out prospective studies with adequate numbers of high risk children.

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.

Development of gender differences in depression: an elaborated cognitive vulnerability-transactional stress theory

Descriptive epidemiological studies are reviewed, showing that the female preponderance in depression begins to emerge around age 13. A developmentally sensitive, elaborated cognitive vulnerability-transactional stress model of depression is proposed to explain the "big fact" of the emergence of the gender difference in depression. The elaborated causal chain posits that negative events contribute to initial elevations of general negative affect. Generic cognitive vulnerability factors then moderate the likelihood that the initial negative affect will progress to full-blown depression. Increases in depression can lead transactionally to more self-generated dependent negative life events and thus begin the causal chain again. Evidence is reviewed providing preliminary support for the model as an explanation for the development of the gender difference in depression during adolescence.

Child psychiatric symptoms and psychosocial impairment: relationship and prognostic significance

BACKGROUND

Relatively little is known about the relationships between psychiatric symptoms, diagnosis and psychosocial impairment.

AIMS

To examine these contemporaneous relationships and prognostic significance in a large general population sample.

METHOD

Symptoms of major depression, conduct and oppositional defiant disorders were assessed by interview in two waves of the Virginia Twin Study of Adolescent behavioural Development (2800 children aged 8-16 years).

RESULTS

Many children below the DSM-III-R diagnostic threshold, especially for depression, had symptom-related impairment, whereas many children reaching the symptom threshold for conduct and oppositional defiant disorders were little impaired. Impairment score was linearly related to symptom count, with no evidence of any additional impairment at the diagnostic threshold. For depression, only symptoms predicted later symptoms and diagnosis. For conduct and oppositional defiant disorders, impairment was additionally predictive of later symptoms and diagnosis.

CONCLUSIONS

Impairment, in addition to symptoms, is important for both nosology and prognosis.

Genetic and environmental influences on the temporal association between earlier anxiety and later depression in girls

BACKGROUND

The purpose of the present study was to investigate the role of genetic and environmental factors in the association between depressive symptoms and symptoms of overanxious disorder, simple phobias, and separation anxiety in 8-13-year-old and 14-17-year-old girls.

METHODS

Multivariate genetic models were fitted to child-reported longitudinal symptom data gathered from clinical interview on 415 MZ [corrected] and 194 DZ [corrected] female twin pairs from the Virginia Twin Study of Adolescent Behavioral Development (VTSABD) [corrected].

RESULTS

Model-fitting results suggest there are distinct etiological [corrected] patterns underlying the association between depression and the different anxiety syndromes during the course of development: 1) specific genetic influences on depression after age 14 reflect liability to symptoms of earlier overanxious disorder (OAD) and simple phobias; 2) aspects of the shared environment that influence symptoms of depression before age 14 contribute to symptoms of separation anxiety and simple phobias later in adolescence [corrected]; 3) the shared environmental influence on [corrected] depression in 14+ girls also affects liability to symptoms of concurrent OAD and persistent separation anxiety.

CONCLUSIONS

These results suggest that depression before and after age 14 may be etiologically distinct syndromes. Earlier symptoms of OAD and (to a lesser extent phobic symptoms) [corrected] reflect the same genetic risk, and separation anxiety symptoms both before and after age 14 reflect the same environmental risk that influence liability to depressive symptoms expressed in middle to late adolescence.

QTL association analysis of the DRD4 exon 3 VNTR polymorphism in a population sample of children screened with a parent rating scale for ADHD symptoms

Current developments in molecular genetics have led to a rapid increase in research aimed at the identification of genetic variation that influences complex human phenotypes. One phenotype that has aroused a great deal of interest is the behavioral trait hyperactivity and the related clinical disorder attention-deficit hyperactivity disorder (ADHD). The driving force behind the molecular genetic research in this area is the overwhelming evidence from quantitative genetic studies that show high heritablility (h(2) = 0.7-0.9) for the behaviors characterizing the diagnosis of ADHD, whether the disorder is viewed as a categorical entity or a continuous trait. To date, molecular studies have aimed at identifying susceptibility genes for ADHD, defined using operational diagnostic criteria, and have focused on variation within genes that regulate dopamine neurotransmission. Several studies report ADHD to be associated with the 7-repeat allele of a 48 bp repeat polymorphism (DRD4-7) in exon 3 of the dopamine D4 receptor gene (DRD4). In this study, we take a dimensional perspective of ADHD and examine the relationship of this DRD4 polymorphism in a sample of children selected from the general population on the basis of high and low scores on the five ADHD items of the Strengths and Difficulties Questionnaire (SDQ) as rated by their parents. We found a significant relationship between DRD4-7 and high-scoring individuals [chi-square = 8.63; P = 0.003; OR = 2.09 (95% CI 1.24 < OR < 3.54), F-statistic = 7.245; P = 0.008].

Testing hypotheses on specific environmental causal effects on behavior

There have been strong critiques of the notion that environmental influences can have an important effect on psychological functioning. The substance of these criticisms is considered in order to infer the methodological challenges that have to be met. Concepts of cause and of the testing of causal effects are discussed with a particular focus on the need to consider sample selection and the value (and limitations) of longitudinal data. The designs that may be used to test hypotheses on specific environmental risk mechanisms for psychopathology are discussed in relation to a range of adoption strategies, twin designs, various types of "natural experiments," migration designs, the study of secular change, and intervention designs. In each case, consideration is given to the need for samples that "pull-apart" variables that ordinarily go together, specific hypotheses on possible causal processes, and the specification and testing of key assumptions. It is concluded that environmental risk hypotheses can be (and have been) put to the test but that it is usually necessary to use a combination of research strategies.