Dimensions and disorders of adolescent adjustment: a quantitative genetic analysis of unselected samples and selected extremes

Behavioural genetics in the 21st century

Behavioural genetics will continue to flow into the mainstream of behavioural research as more behavioural scientists incorporate this approach in their research. Future research will go beyond simply asking whether and how much genetic factors influence behaviour to ask questions about development, about relations among traits, and about the interplay between nature and nurture. The identification of specific genes associated with behaviour will make it possible for behavioural scientists to ask more precise questions about how genotypes become phenotypes.

Genetically Informative Designs for the Study of Behavioural Development

Genetic and environmental influences on behaviour and development can be examined by studying more than one individual within a family, using quantitative genetic theory and behavioural genetic (BG) methodology. Specific environmental and genetic influences can be measured and effect sizes estimated, and many assumptions of the methodology can be explicitly tested. BG designs can identify specific aspects of the environment that have the greatest influence on behavioural variation, and they can pinpoint critical periods in which environmental influences are most malleable, both of which are useful when designing interventions. Traits that are shown to be the most heritable through traditional family resemblance methods can now be explored further and actual genes may be identified, using new molecular methods. By identifying specific genetic and environmental influences on behaviour, and modelling the structure of these influences over time, we can rapidly advance our understanding of human development.

A twin study of attachment style in young adults

The relative contribution of genetic and environmental influences to individual differences in attachment security is still incompletely understood. We assessed attachment style with the Experiences in Close Relationships questionnaire in a volunteer sample of 677 twins (43% male) ages 23-24 years drawn from the population-based Italian Twin Register, who belonged to 244 complete pairs (46% monozygotic) and 189 unmatched pairs. Genetic structural equation modeling was performed with the Mx program. Genetic effects accounted for 45% and 36% of individual differences in attachment-related anxiety and avoidance, respectively. Furthermore, the covariation between anxiety and avoidance was found to be mainly due to genetic factors, with heritability of the latent attachment security phenotype estimated at 62%. Unshared environmental factors explained the remaining proportion of variance. Although our findings are best regarded as preliminary given some study limitations, they suggest that both nature and nurture contribute to individual differences in adult attachment.

Observed externalizing behavior: a developmental comparison of genetic and environmental influences across three samples

Estimates of genetic and environmental influences on externalizing behavior are markedly inconsistent. In an attempt to refine and extend our knowledge of externalizing behavior, the current study examined the etiology of externalizing behavior using observational data in middle childhood and adolescence from three twin and sibling samples. Observational ratings offer a unique perspective on externalizing behavior rarely examined within behavioral genetic designs. Shared environmental influences were significant and moderate to large in magnitude across all three samples (i.e., 44, 77, and 38%), while genetic influences (31%) were significant only for the adolescent sample. All three samples showed greater shared environmental influences and less genetic influence than is typically found when examining self-, parent-, and teacher-reports of externalizing behavior. These findings are consistent with other reports that have found evidence for shared environmental influences on measures of child externalizing behavior-in direct contrast to a commonly held perception that shared environmental factors do not have significant influences on behavior beyond early childhood.

Heritability across the distribution: an application of quantile regression

We introduce a new method for analyzing twin data called quantile regression. Through the application presented here, quantile regression is able to assess the genetic and environmental etiology of any skill or ability, at multiple points in the distribution of that skill or ability. This method is compared to the Cherny et al. (Behav Genet 22:153-162, 1992) method in an application to four different reading-related outcomes in 304 pairs of first-grade same sex twins enrolled in the Western Reserve Reading Project. Findings across the two methods were similar; both indicated some variation across the distribution of the genetic and shared environmental influences on non-word reading. However, quantile regression provides more details about the location and size of the measured effect. Applications of the technique are discussed.

Genetic and environmental influences upon the CBCL/6-18 DSM-oriented scales: similarities and differences across three different computational approaches and two age ranges

Inasmuch as the newly established DSM-oriented CBCL/6-18 scales are to be increasingly employed to assess clinical/high-risk populations, it becomes important to explore their aetiology both within the normal- and the extreme range of variation in general population samples and to compare the results obtained in different age groups. We investigated by the Quantitative Maximum Likelihood, the De Fries-Fulker, and the Ordinal Maximum Likelihood methods the genetic and environmental influences upon the five DSM-oriented CBCL/6-18 scales in 796 twins aged 8-17 years belonging to the general population-based Italian Twin Registry. When children were analysed together regardless of age, most best-fitting solutions yielded genetic and non-shared environmental factors as the sole influences for DSM-oriented CBCL/6-18 behaviours, both for the normal and the extreme variations. When analyses were conducted separately for two age groups, shared environmental influences emerged consistently for Affective and Anxiety Problems in children aged 8-11. Oppositional-Defiant, Attention Deficit/Hyperactivity, and Conduct Problems appeared-with few exceptions-influenced only by genetic and non-shared environmental factors in both age groups, according to all three computational approaches. The De Fries-Fulker method appeared to be more sensitive in detecting shared environmental effects. Analysing the same set of data with different analytic approaches leads to better-balanced views on the aetiology of psychopathological behaviours in the developmental years.

The genetics of mood disorders

Studying the genetics of mood disorders has never been more exciting. We have moved rapidly from establishing the genetic basis of depression to asking questions about how genes are expressed. This has been made possible by the capacity to collect and sequence DNA for large samples cheaply. But "multidisciplinary" approaches investigating interrelationships between risk factors have also been increasingly adopted, encouraging collaborations between those studying genes and those studying the brain, cognition, and/or the social environment. In this review, we first describe findings from quantitative and molecular studies investigating the genetic basis of depression. Second, we present overviews of three hot topics of genetic research: gene-environment interplay, which considers how genetic factors shape exposure and responses toward the social environment; endophenotypic research, which identifies neurophysiological and psychological mediators of genetic risk; and epigenetics, which explain how early environments can foster changes in gene expression, altering subsequent emotional 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.

Mental health of young people: a global public-health challenge

Mental disorders account for a large proportion of the disease burden in young people in all societies. Most mental disorders begin during youth (12-24 years of age), although they are often first detected later in life. Poor mental health is strongly related to other health and development concerns in young people, notably lower educational achievements, substance abuse, violence, and poor reproductive and sexual health. The effectiveness of some interventions for some mental disorders in this age-group have been established, although more research is urgently needed to improve the range of affordable and feasible interventions, since most mental-health needs in young people are unmet, even in high-income countries. Key challenges to addressing mental-health needs include the shortage of mental-health professionals, the fairly low capacity and motivation of non-specialist health workers to provide quality mental-health services to young people, and the stigma associated with mental disorder. We propose a population-based, youth focused model, explicitly integrating mental health with other youth health and welfare expertise. Addressing young people's mental-health needs is crucial if they are to fulfil their potential and contribute fully to the development of their communities.

Estimating genetic and environmental influences on depressive symptoms in adolescence: differing effects on higher and lower levels of symptoms

We estimate the relative effect sizes of genetic and environmental influences on both higher and lower levels of depressive symptoms with attention to persistence over a 1-year period in the genetically informative subsample of adolescents participating in the National Longitudinal Study of Adolescent Health (Add Health). Shared environmental effects were significant for persistent higher levels of depressive symptoms but not nonpersistent symptoms. Genetic effects were significant for both persistent and nonpersistent lower levels of depressive symptoms. Nongenetic factors that promote similarity between siblings for high levels of depressive symptoms are important and should be considered in both etiological and applied research. Genetic contributions to lack of susceptibility to depression should be considered in biological models of depression suppression.

The effect of birth-weight with genetic susceptibility on depressive symptoms in childhood and adolescence

Low birth-weight has been associated with depression and related outcomes in adults, and with problem behaviours in children. This study aimed to examine the association between low birth-weight for gestation and depressive symptoms in children and adolescents and to examine whether the relationship is moderated by genetic risk for depression. An epidemiological, genetically sensitive design was used including 2,046 twins aged 8-17 years (1,023 families). Data were obtained by parental report and analysed using regression analysis. A small but significant association between birth-weight for gestation and early depressive symptoms was observed. The unit increase in depressive symptoms per unit decrease in birth-weight for gestation was greater for individuals at genetic or familial risk for depression. For low birth-weight children, genetic risk for depression moderated the influence of birth-weight for gestation in predicting early depressive symptoms. Birth-weight for gestation is moderated by genetic and familial risk for depression in influencing early depression symptoms. These observations have clinical implications in that the impact of being small for gestational age on depressive symptoms is greater in children at familial/genetic risk although the association between birth weight and depression does not imply causality.

Genetic and Environmental Influences on Antisocial Behaviors: Evidence from Behavioral–Genetic Research

This article reviews behavioral-genetic research into human antisocial behavior. The focus is on studies of antisocial behavior that have been leading the way in investigating environmental and genetic influences on human behavior. The first generation of studies, which provided quantitative estimates attesting that genes and environments each influence about half of the population's variation in antisocial behaviors is interpreted. Then how behavioral-genetic methods are being applied to test developmental theory and to detect environmental causes of antisocial behavior is illustrated. Evidence for interactions between genes and the environment in the etiology of antisocial behavior is also examined. The article ends by envisioning future work on gene-environment interplay in the etiology of antisocial behavior.

Linking changes in whole family functioning and children's externalizing behavior across the elementary school years

This study used observational assessments of 57 2-parent families working and playing together when their eldest child was in kindergarten and again in Grade 4 to identify distinct patterns of family functioning derived from structural family systems theory for (a) cohesive, (b) separate, and (c) triangulated families. Little consistency in family type from early to middle childhood was indicated. No significant mean differences were found in teacher reports of children's externalizing behavior in their Grade 1 classrooms for children in cohesive or triangulated families. Fourth graders in triangulated families were seen as more aggressive at school than were their peers in cohesive or separate families. Changes in observed family functioning across a 4-year period (kindergarten to Grade 4) were also systematically linked to changes in teachers' ratings of children's externalizing behavior from Grades 1 to 4.

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.

Associations between behaviour problems and verbal and nonverbal cognitive abilities and disabilities in early childhood

BACKGROUND

We investigated associations between behaviour problems and verbal and nonverbal cognitive abilities at 2, 3 and 4 years of age both for the entire distribution and for the lowest 5% and 10% of the verbal and nonverbal cognitive disabilities.

METHODS

A community sample of 4,000 pairs of twins born in England and Wales in 1994 and 1995 was assessed by their parents at 2, 3 and 4 years using the Revised Rutter Parent Scale for Preschool Children (RRPSPC, behaviour problems), the MacArthur Communicative Development Inventory (MCDI, verbal development), and the Parent Report of Children's Abilities (PARCA, nonverbal cognitive development).

RESULTS

For the entire sample, behaviour problem scores were modestly associated with lower MCDI and PARCA scores - correlations were less than .30. Similarly modest effect sizes were found for relationships between behaviour problem scores and the lowest 5% and 10% of the MCDI and of the PARCA distributions. Associations were stronger for nonverbal than for verbal development, increased from 2 to 3 to 4 years, and, at the extremes of the distributions, were stronger for boys than for girls. Multivariate genetic analyses indicated that both genetic and shared environmental factors mediate the links between behaviour problems and cognitive development both for the total distribution and for the extremes. Genetic links may be stronger for the extremes than for the total sample.

CONCLUSIONS

We conclude that, in this community sample of young children, associations between behaviour problems and verbal and nonverbal cognitive development are generally modest for the entire distribution and are no greater at the extremes than expected on the basis of the associations for the entire distribution.

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.

Genetics and Psychology: Beyond Heritability

The questions whether and how much genetic factors affect psychological dimensions and disorders represent important first steps in understanding the origins of individual differences. Because it is now widely accepted that genetic influences contribute importantly to individual differences throughout psychology, genetic research is moving beyond merely estimating heritability to asking questions about how genetic mechanisms work. We focus on two examples of ways in which genetic research is going beyond heritability. The first is to use genetically sensitive designs to identify specific environmental influences, taking into account two of the most important findings from behavioral genetics: nonshared environment and genotype-environment correlation. The second is to use the new tools of molecular genetics to identify specific genes responsible for the substantial heritability of a variety of behavioral traits.

Internalizing behavior problems and scholastic achievement in children: cognitive and behavioral pathways as mediators of outcome

Examined a conceptual model in which dual developmental pathways (behavioral and cognitive) are hypothesized to account for the relation among internalizing behavior problems, intelligence, and later scholastic achievement using a cross-sectional sample of 325 children. Classroom behavior and select aspects of cognitive functioning (vigilance, short-term memory) were hypothesized to mediate the relations among internalizing problems, IQ, and long-term scholastic achievement. Hierarchical tests applied to a nested series of models demonstrated that (a) individual differences in measured intelligence among children are associated with variations in classroom performance and cognitive functioning, (b) classroom performance and cognitive functioning make unique contributions to prediction of later achievement over and above the influence of intelligence, (c) anxious/depressive features are correlated but separable constructs, and (d) anxiety/depression and withdrawal contribute to prediction of classroom performance and cognitive functioning over and above the effects of intelligence. Classroom performance and cognitive functioning thus appear to mediate the effects of internalizing behaviors as well as intelligence. Particular attention to the presence and potential impact of social withdrawal on children's functioning, both alone and concomitant with anxiety/depression, appears warranted during the course of clinical evaluations owing to the strong continuity among these variables.

Genetic and environmental contributions to loneliness in children

This report presents the results of the first behavioral genetic studies of children's loneliness. Data were collected using both an adoption design and a twin-sibling design. As part of the Colorado Adoption Project, 133 sibling pairs (69 biologically related pairs and 64 unrelated pairs in adoptive families) completed a general loneliness scale when they were 9, 10, 11, and 12 years old. As part of the San Diego Sibling Study, 142 sibling pairs (22 monozygotic twin, 40 dizygotic twin, and 80 full-sibling pairs) between the ages of 8 and 14 years old completed a scale assessing loneliness at school. Both studies showed significant heritability and nonshared environmental influences for children's loneliness.

DNA

The authors predict that in a few years, many areas of psychology will be awash in specific genes responsible for the widespread influence of genetics on behavior. As the focus shifts from finding genes (genomics) to understanding how genes affect behavior (behavioral genomics), it is important for the future of psychology as a science that pathways between genes and behavior be examined not only at the molecular biological level of cells or the neuroscience level of the brain but also at the psychological level of analysis. After a brief overview of quantitative genetic research, the authors describe how genes that influence complex traits like behavioral dimensions and disorders in human and nonhuman animals are being found. Finally, the authors discuss behavioral genomics and predict that DNA will revolutionize psychological research and treatment early in the 21st century.

Depressive symptoms in children and adolescents: etiological links between normality and abnormality: a research note

This research note considers whether normal and abnormal depressive symptoms are caused by the same or differing etiologies. The comparison of the heritability of individual differences with extreme group heritability, along with the use of multiple cut-offs to define abnormal groups, can be used to answer this question and to bridge the gap between dimensional and categorical approaches to developmental psychopathology. Depressive symptoms from 395 same-sex child twin pairs were analysed in this way. Genetic factors contributed to a similar extent both to individual differences (h2 = .48) and to extreme group membership using multiple cut-offs (hg2 = 2.0-2.3). The common environment did not contribute significantly in any of the analyses but showed a trend for a greater role in extreme group membership. These results are in good agreement with previous data.