Parent ratings of temperament in twins: explaining the 'too low' DZ correlations

Genetic and environmental factors influencing neonatal resting-state functional connectivity

Functional magnetic resonance imaging has been used to identify complex brain networks by examining the correlation of blood-oxygen-level-dependent signals between brain regions during the resting state. Many of the brain networks identified in adults are detectable at birth, but genetic and environmental influences governing connectivity within and between these networks in early infancy have yet to be explored. We investigated genetic influences on neonatal resting-state connectivity phenotypes by generating intraclass correlations and performing mixed effects modeling to estimate narrow-sense heritability on measures of within network and between-network connectivity in a large cohort of neonate twins. We also used backwards elimination regression and mixed linear modeling to identify specific demographic and medical history variables influencing within and between network connectivity in a large cohort of typically developing twins and singletons. Of the 36 connectivity phenotypes examined, only 6 showed narrow-sense heritability estimates greater than 0.10, with none being statistically significant. Demographic and obstetric history variables contributed to between- and within-network connectivity. Our results suggest that in early infancy, genetic factors minimally influence brain connectivity. However, specific demographic and medical history variables, such as gestational age at birth and maternal psychiatric history, may influence resting-state connectivity measures.

Genetic and environmental influences on temperament development across the preschool period

BACKGROUND

Preschoolers' temperament characteristics are associated with children's long-term development. Such links underscore the importance of understanding factors that shape temperament during preschool. This is the first study to examine genetic and environmental sources of developmental growth in three temperament dimensions: surgency, negative affectivity, and effortful control, during the preschool period.

METHODS

Biometric latent growth curve modeling was used to examine genetic, shared, and nonshared environmental contributions to the invariant level of and developmental growth in temperament, using a sample of 310 same-sex twin pairs (MZ = 123, DZ = 187) assessed at 3, 4, and 5 years of age. Temperament was assessed using primary caregiver's report on the Child Behavior Questionnaire-Short Form.

RESULTS

All three temperament dimensions demonstrated linear increases from ages 3 to 5 years. The invariant levels of all three temperament dimensions were explained by genetic and nonshared environmental factors. Growth in surgency was fully explained by nonshared environmental factors, while growth in negative affectivity was mainly explained by genetic factors. Growth in effortful control was explained by genetic and nonshared environmental factors, although neither were significant due to large bootstrap standard errors. For negative affectivity and effortful control, the genetic factors that contributed to developmental growth were independent from those associated with their invariant levels.

CONCLUSIONS

Collectively, these findings indicate that both genetic and nonshared environmental factors play important roles in the invariant levels of temperament. Findings also accord a critical role of children's nonshared environment in the development of surgency and to a lesser extent negative affectivity and effortful control. It is also notable that novel genetic effects contribute to developmental growth in negative affectivity and effortful control as children age, emphasizing the importance of integrating developmental models in genetic research.

The Special Role of Middle Childhood in Self-Control Development: Longitudinal and Genetic Evidence

Despite the importance of self-control for well-being and adjustment, its development from early childhood to early adolescence has been relatively understudied. We addressed the development of mother-reported self-control in what is likely the largest and longest longitudinal twin study of the topic to this day (N = 1,889 individual children with data from at least one of 5 waves: ages 3, 5, 6.5, 8-9 and 11 years). We examined rank-order change in self-control from early childhood to early adolescence, genetic and environmental contributions to variance in the trait and differential developmental trajectories. The relative contribution of genetic and environmental factors to change and stability was also examined. Results point at middle childhood as a period of potential transition and change. During this period the rank-order stability of self-control increases, heritability rates substantially rise, and a cross-over occurs in two of the self-control trajectories. Non-additive genetic effects contribute to both stability and change in self-control while the non-shared environment contributes mostly to change, with no effect for the shared environment. Our findings suggest that new genetic factors, that emerge around age 6.5 and whose effect on self-control is carried on along development, may partially account for changes in self-control around late middle childhood, and explain the growing stability in the trait approaching early adolescence. We discuss the implications of the special role of middle childhood for self-control development. This article is protected by copyright. All rights reserved.

Twin Study of the Relationship between Childhood Negative Emotionality and Hyperactivity/Inattention Problems

The present study aimed to determine the genetic and environmental etiology of the association between childhood negative emotionality (NE) and hyperactivity/inattention problems (HIP) using South Korean elementary school twins (mean age = 10.19 years, SD = 1.79 years). Telephone interviews were given to mothers of 919 twins (229 monozygotic males: 112 pairs and 5 individuals; 148 dizygotic males: 73 pairs and 2 individuals; 180 monozygotic females: 87 pairs and 6 individuals; 103 dizygotic females: 50 pairs and 3 individuals; 259 opposite-sex dizygotic twins: 127 pairs and 5 individuals) to assess their children's NE and HIP. Consistent with prior studies, the phenotypic correlation between NE and the HIP was moderate (r = .29; 95% CI = .24, .34). Model-fitting analysis revealed that additive genetic and nonshared environmental influences on NE were .45 (95% CI [.34, .54]) and .55 (95% CI [.46, .66]), respectively, and that additive and nonadditive genetic, and nonshared environmental influences on HIP were .08 (95% CI [.03, .26]), .41 (95% CI [.21, .51]) and .51 (95% CI = .42, .61), respectively. In addition, the additive genetic correlation between NE and HIP was 1.0 (95% CI [.52, 1.00]), indicating that additive genetic factors are entirely shared between the two phenotypes. Nonadditive genetic influences were unique to HIP and not responsible for the NE-HIP association. Nonshared environmental correlation was significant but modest (re = .18, 95% CI [.06, .30]).

Attentional Control Explains Covariation Between Symptoms of Attention-Deficit/Hyperactivity Disorder and Anxiety During Adolescence

Symptoms of attention-deficit/hyperactivity disorder (ADHD) and anxiety are common during adolescence and frequently co-occur. However, the genetic and environmental influences that underlie this co-occurrence are understudied. Using a large twin sample (N = 1,017), we examined cross-sectional genetic and environmental influences on ADHD and anxiety symptoms during childhood. We also explored whether these influences were shared with attentional control, a putative mechanism for symptom comorbidity. We found evidence for common genetic and nonshared environmental influences on the covariation among attentional control, ADHD, and anxiety symptoms, supporting the putative role of attentional control as a mechanism by which comorbid problems may develop. Genetic factors also accounted for symptom co-occurrence after controlling for covariation with attentional control, suggesting the presence of additional unmeasured mechanisms.

The Association of Intrapair Birth-Weight Differences With Internalizing and Externalizing Behavior Problems

Problem behaviors are of increasing public health concern. Twin studies have revealed substantial genetic and environmental influences on children's behavior, and examining birth-weight difference could allow the identification of the specific contribution of multiple non-shared prenatal environmental factors. The Twins and Multiple Births Association Heritability Study, a UK, volunteer-based study, recruited mothers of twins aged 18 months to 5 years; 960 twins (480 pairs) were included in the analysis. Twins' mothers answered questions relative to their pregnancy and their twins' characteristics, and completed the Child Behavior Checklist (CBCL) 1½-5. The association between the absolute birth-weight difference and each CBCL scale's score difference was analyzed by means of multiple linear regressions. Expected mean CBCL score differences were calculated. In monozygotic (MZ) twins, statistically and clinically significant associations were found between intrapair birth-weight difference and difference in total problems, internalizing problems, and emotional reactiveness. No significant results were observed neither in dizygotic (DZ) twins when analyzed as a separate group nor in MZ and DZ twins combined. The results of the present study suggest that with increasing the absolute birth-weight difference, the intrapair difference in total problems, internalizing behaviors and emotionality increases, with smaller twins being at major risk for later behavior problems. Moreover, these results suggest a causal association between birth weight and behavior development.

Parental Relationship with Twins from Pregnancy to 3 Months: The Relation Among Parenting Stress, Infant Temperament, and Well-Being

Objective: The transition to parenthood, from pregnancy to postpartum period, is a critical process, particularly for couples expecting twins. There is very little literature regarding the links between anxiety, depression, dyadic adjustment, parental stress, and infant temperament spanning from pregnancy to postpartum. This study has two aims: first, to examine whether mothers' and fathers' anxiety, depression, and dyadic adjustment, assessed at the sixth month of pregnancy and 3 months postpartum, are associated with infants' negative affectivity (NA) and parenting stress; second, to examine whether there is any difference between fathers' and mothers' levels of parenting stress and perception of the twins' temperament, as well as to evaluate, separately for mothers and fathers, whether the levels of parenting stress and perception of child temperament differ for each twin. Method: The study participants were 58 parents (29 couples) and their healthy 58 twin babies (51.7% boys, 48.3% girls). Mothers' ages ranged from 30 to 44 years, (MAge = 36.3 years, SD = 3.2 years), and fathers' ages ranged from 32 to 52 years, (MAge = 38.2 years, SD = 4.4 years). The parents, during the pregnancy period and 3 months after delivery, filled out the Edinburgh Postnatal Depression Scale, the State-Trait Anxiety Inventory, and the Dyadic Adjustment Scale. Three months after delivery they also filled out the Parenting Stress Index-Short Form and the Infant Behavior Questionnaire Revised. Results: The analyses showed a significant correlation between parental anxiety/depression symptoms and infants' NA and parenting stress (in both mothers and fathers). Moreover, compared to fathers, mothers reported higher scores on specific dimensions of the infants' NA, [t(28) = -2.62 and p < 0.05; t(28) = 2.09 and p < 0.05], and parenting stress, [t(28) = 2.19 and p < 0.05; t(28) = 2.23 and p < 0.05], but only for Twin 2. Finally, the results showed that mothers' perceptions of child temperament vary between two twins, [e.g., distress to limitations: t(28) = 2.08 and p < 0.05]. Discussion: This study highlights the peculiarity of twin parenthood during the fourth trimester. In particular, the differences between twins' mothers' and fathers' perceptions are relevant from a clinical perspective and for perinatal professionals. It would be interesting to study the long-term impact of mothers' and fathers' differing perceptions of their twins.

Are children's activity levels determined by their genes or environment? A systematic review of twin studies

CONTEXT

The importance of physical activity to paediatric health warrants investigation into its determinants. Objective measurement allows a robust examination of genetic and environmental influences on physical activity.

OBJECTIVE

To systematically review the evidence on the extent of genetic and environmental influence on children's objectively-measured activity levels from twin studies.

DATA SOURCES AND SEARCH TERMS

Medline, EMBASE, PsycINFO, Health and Psychosocial Instruments and all Ovid Databases. Search terms: "accelerometer" OR "actometer" OR "motion sensor" OR "heart rate monitor" OR "physical activity energy expenditure" AND "twin". Limited to Human, English language and children (0-18 years).

RESULTS

Seven sets of analyses were included in the review. Six analyses examined children's daily-life activity and found that the shared environment had a strong influence on activity levels (weighted mean 60%), with a smaller contribution from genetic factors (weighted mean 21%). Two analyses examined short-term, self-directed activity in a standard environment and found a smaller shared environment effect (weighted mean 25%) and a larger genetic estimate (weighted mean 45%).

CONCLUSIONS

Although genetic influences may be expressed when children have brief opportunities for autonomous activity, activity levels in daily-life are predominantly explained by environmental factors. Future research should aim to identify key environmental drivers of childhood activity.

Genetic and Environmental Etiology of the Relationship Between Childhood Hyperactivity/Inattention and Conduct Problems in a South Korean Twin Sample

Recently, there has been increased research into the etiology of the comorbidity between hyperactivity/inattention problems (HIP) and conduct problems (CP). However, the nature of the etiology of the comorbidity has remained unclear. Mothers of 507 pairs of twins, comprised of 221 monozygotic (MZ) and 286 dizygotic (DZ) twin pairs aged from 6 to 13 years (mean = 9.6 years; SD = 2.0 years), completed the HIP and the CP scale of the Strengths and Difficulties Questionnaire (SDQ) via a telephone interview. The phenotypic correlation between HIP and CP was 0.43 (p < .01). MZ and DZ twin correlations were, respectively, 0.48 (95%CI: 0.37–0.58) and 0.06 (95%CI: -0.06–0.19) for HIP and 0.38 (95%CI: 0.26–0.49) and 0.35 (95%CI: 0.25–0.45) for CP. The bivariate model-fitting results revealed additive genetic correlation of 1.0 (95% CI: 0.72–1.00), a complete overlap of additive genetic variance component between HIP and CP, supporting the importance of correlated additive genetic risk factors for the comorbid condition of HIP and CP. HIP was additionally influenced by non-additive genetic factors that did not contribute to the relationship between HIP and CP. There was a significant but moderate child-specific environmental correlation (re = 0.37) between HIP and CP. CP was additionally influenced by shared family environmental influences. While the results of the present study are generally consistent with the findings from Western twin studies of the relationship between HIP and CP, they add a new finding to the extant literature by showing that it is additive rather than non-additive genetic factors that are responsible for the co-occurrence of HIP and CP.

Increasing phenotypic and genetic variations in hyperactivity/inattention problems from age 3 to 13 years: a cross-sectional twin study

A twin design was used to examine the developmental nature of genetic, environmental, and phenotypic variations in hyperactivity and inattention problems (HIP). Mothers of 662 complete pairs of twins (273 monozygotic [MZ] pairs and 389 dizygotic [DZ] pairs) aged from 3 to 13 years (mean [SD] age = 8.3 [2.9] years) responded to the items of the HIP scale of the Strengths and Difficulties questionnaire via a telephone interview. Maximum likelihood MZ and DZ twin correlations in the total sample were 0.47 (95% CI: 0.37-0.55) and -0.01 (95% CI: -0.11-0.09). A standard univariate model incorporating age as a modifier was applied to the raw data. Results of model-fitting analyses showed that the phenotypic variation of HIP monotonically increased from age 3 to age 12 and that this increase was completely due to an increase in genetic variance, suggesting that it is genes that expand individual difference in ADHD symptoms with age during childhood. Child-specific environmental variance was constant during this age period. In terms of relative influences, total genetic factors increased from 33% (95% CI: 27-44%) at age 3 to 51% (95% CI: 28-71%) at age 13 and this increase was accompanied by a decrease in relative influences of child-specific environmental factors from 67% (95% CI: 56-73%) at age 3 to 49% (95% CI: 29-72%) at age 13. These estimates of genetic influences were somewhat lower than those found in most twin studies of ADHD symptoms. However, the increasing trend of genetic influences with age during childhood was consistent with the results of a recent meta-analysis of ADHD symptoms.

Prenatal smoking and genetic risk: examining the childhood origins of externalizing behavioral problems

An ever-growing body of research has begun to focus closely on the role of prenatal smoke exposure in the development of conduct problems in children. To this point, there appears to be a correlation between prenatal nicotine exposure and behavioral problems. We build on this prior research by examining the coalescence of prenatal smoke exposure and genetic risk factors in the prediction of behavior problems. Specifically, the current study analyzed data from a nationally representative sample of twin pairs collected during early childhood. Our findings suggested that an interaction existed between prenatal smoke exposure and genetic risk factors which corresponded to increased risk of behavior problems. These findings provide evidence of a gene-environment interaction, in that prenatal smoke exposure conditioned the influence of genetic risk factors in the prediction of aggressive behavior. Interestingly, the association between genetic risk and prenatal smoking was sex-specific, and only reached statistical significance in females. Given the nature of our findings, it may shed light on why heterogeneity exists concerning the relationship between prenatal smoke exposure and externalizing behavioral problems in children.

Examining the etiological associations among higher-order temperament dimensions

A multivariate independent pathway model was used to examine the shared and unique genetic and environmental influences of Positive Affect (PA), Negative Affect (NA), and effortful control (EC) in a sample of 686 twin pairs (M age = 10.07, SD = 1.74). There were common genetic influences and nonshared environmental influences shared across all three temperament dimensions and shared environmental influences in common to NA and EC. There were also significant independent genetic influences unique to PA and NA and significant independent shared environmental influences unique to PA. This study demonstrates that there are genetic and environmental influences that affect the covariance among temperament dimensions as well as unique genetic and environmental influences that influence the dimensions independently.

Atypical activation during the Embedded Figures Task as a functional magnetic resonance imaging endophenotype of autism

Atypical activation during the Embedded Figures Task has been demonstrated in autism, but has not been investigated in siblings or related to measures of clinical severity. We identified atypical activation during the Embedded Figures Task in participants with autism and unaffected siblings compared with control subjects in a number of temporal and frontal brain regions. Autism and sibling groups, however, did not differ in terms of activation during this task. This suggests that the pattern of atypical activation identified may represent a functional endophenotype of autism, related to familial risk for the condition shared between individuals with autism and their siblings. We also found that reduced activation in autism relative to control subjects in regions including associative visual and face processing areas was strongly correlated with the clinical severity of impairments in reciprocal social interaction. Behavioural performance was intact in autism and sibling groups. Results are discussed in terms of atypical information processing styles or of increased activation in temporal and frontal regions in autism and the broader phenotype. By separating the aspects of atypical activation as markers of familial risk for the condition from those that are autism-specific, our findings offer new insight into the factors that might cause the expression of autism in families, affecting some children but not others.

Heritability of hallucinations in adolescent twins

Hallucinations are common in normal individuals and patients with schizophrenia and other psychotic disorders. Traditionally psycho-social approaches have emphasized the importance of environmental factors that contribute to variation of hallucinations. Using the Launay-Slade Hallucination Scale-Revised (LSHS-R), we investigated genetic and environmental influences on hallucinations in 598 pairs of healthy South Korean adolescent twins. Parameter estimates in the best-fitting model indicated that additive genetic and individual specific environmental factors for the LSHS-R were 33% (95% CI: 23-42%) and 67% (95% CI: 60-77%), respectively. There was no evidence for sex-specific genes for hallucinations. The magnitudes of genetic and environmental influences on hallucinations were similar in males and females. These results have implications in future molecular genetic studies that search for genes for hallucinations.

Bigger families fare better: a novel method to estimate rater contrast effects in parental ratings on ADHD symptoms

Many twin studies on parental ratings of attention deficit hyperactivity disorder (ADHD) symptoms report low or negative DZ correlations. The observed differences in MZ and DZ variances indicate sibling contrast effects, which appear to reflect a bias in parent ratings. Knowledge of the factors that contribute to this rater contrast effect is, however, limited. Using parent-rated ADHD symptoms from the Twins' Early Development Study and a novel application of a twin model, we explored a range of socio-demographic variables (ethnicity, socio-economic status, and family size), as potential contributors to contrast effects and their interactive effect with gender composition of twin pairs. Gender did moderate contrast effects but only in DZ opposite-sex twin pairs. Family size also showed a moderating effect on rater contrast effects, which was further modified by gender. We further observed an effect of rating scale, with the DSM-IV ADHD subscale of the Revised Conners' Parent Rating Scale more resistant to contrast effects than shorter scales of ADHD symptoms. The improved identification of situations where the accuracy of the most common informant of childhood ADHD symptoms-parents-is compromised as a result of rater bias, might have implications for future research on ADHD.

The magnitude of genetic and environmental influences on parental and observational measures of behavioral inhibition and shyness in toddlerhood

Behavioral inhibition is a temperamental trait that refers to slow approach to novel items, shyness towards new people, and fearfulness in new situations, and individuals may develop inhibited response styles by as early as 2 years of age. There are important methodological considerations in the assessment of early temperament, with parental report and observational measures providing both corroborative and unique data. The present study examined behavioral inhibition measured by parental report and observational measures in a genetically informative sample to delineate the agreement between the methods and the uniqueness of each method, and to estimate the magnitude of genetic and environmental influences on the common and unique variance. The biometric, psychometric, and rater bias models were conducted to study the covariance between measurement modalities. Overall, the results suggested a common phenotype was assessed by both parents and observers. The latent phenotype underlying parental and observational measures of behavioral inhibition was moderately to substantially heritable.

Social-emotional development through a behavior genetics lens: infancy through preschool

The field of developmental behavior genetics has added significantly to the collective understanding of what factors influence human behavior and human development. Research in this area has helped to explain not only how genes and environment contribute to individual differences but also how the interplay between genes and environment influences behavior and human development. The current chapter provides a background of the theory and methodology behind behavior genetic research and the field of developmental behavior genetics. It also examines three specific developmental periods as they relate to behavior genetic research: infancy, toddlerhood, and early preschool. The behavior genetic literature is reviewed for key socioemotional developmental behaviors that fit under each of these time periods. Temperament, attachment, frustration, empathy, and aggression are behaviors that develop in early life that were examined here. Thus, the general purpose of this chapter is to provide an overview of how genes and environment, as well as the interplay between them, relate to early socioemotional behaviors.

Maternal eating disorders and infant temperament: findings from the Norwegian mother and child cohort study

OBJECTIVE

We hypothesized that women with eating disorders would be more likely to rate their infants' temperament higher on negative emotionality than women without eating disorders.

METHOD

Of 3,013 mothers with eating disorders, 44 reported anorexia nervosa (AN), 436 bulimia nervosa (BN), 2,475 binge eating disorder (BED), and 58 EDNOS purging type (EDNOS-P). The referent group comprised 45,964 mothers with no eating disorder. A partial proportional odds model was used to estimate the relation among maternal eating disorder presentations and infant temperament ratings while adjusting for covariates.

RESULTS

Women with AN, BN, EDNOS-P, and BED were 2.3, 1.4, 2.8, and 1.4 times more likely to report extreme fussiness than the referent group of women with no eating disorder, respectively.

DISCUSSION

Mothers with eating disorders may rate their infants as more difficult because of information-processing biases or because their infants are emotionally difficult. Maternal perception of infant temperament may be a risk factor for children's emotional development.

Sources of continuity and change in activity level in early childhood

Actigraphs and parent and observer ratings were used to explore genetic influences on continuity and change in activity level (AL) in early childhood. Over 300 pairs of twins wore actigraphs for a 48-hr period in the home and laboratory at ages 2 and 3. AL was genetically influenced at both ages with little evidence of differential heritability across age. For all measures, genetic influences contributed to phenotypic continuity. With the exception of the actigraph measure of AL in the home, new genetic effects emerged at age 3 indicating that genetic factors influence both continuity and change in AL in early childhood. Nonshared environmental influences were also a source of change in AL across the transition from infancy to early childhood.

Working memory and parent-rated components of attention in middle childhood: a behavioral genetic study

The purpose of the current study was to investigate potential genetic and environmental correlations between working memory and three behavioral aspects of the attention network (i.e., executive, alerting, and orienting) using a twin design. Data were from 90 monozygotic (39% male) and 112 same-sex dizygotic (41% male) twins. Individual differences in working memory performance (digit span) and parent-rated measures of executive, alerting, and orienting attention included modest to moderate genetic variance, modest shared environmental variance, and modest to moderate nonshared environmental variance. As hypothesized, working memory performance was correlated with executive and alerting attention, but not orienting attention. The correlation between working memory, executive attention, and alerting attention was completely accounted for by overlapping genetic covariance, suggesting a common genetic mechanism or mechanisms underlying the links between working memory and certain parent-rated indicators of attentive behavior.

The role of child temperament in parental child feeding practices and attitudes using a sibling design

Although previous research indicates that parental child feeding practices are one component of a bidirectional relationship between children and parents, little is known about how child temperament operates in this relationship. The purpose of this study was to investigate relationships between child temperament and parental feeding practices and attitudes using a sibling design. By collecting data regarding pairs of siblings, we were able to investigate sibling differences and differential parental treatment. We examined mothers' and fathers' perceptions of their two children's temperaments as well as reports of the feeding practices and attitudes they use with each child. Fifty-five mothers and fathers completed questionnaires including the Carey Temperament Scales and the Child Feeding Questionnaire (CFQ). Results from correlation analyses showed that 6 of the 9 father reports of temperament between two siblings were positively related, whereas 1 of the 9 mother reports were positively related. Mothers' and fathers' perceptions of temperament were positively correlated for a single child. Some patterns were found between parental reports of sibling temperament and child feeding practices and attitudes, suggesting that temperament plays a role in how parents feed their children.

A longitudinal analysis of anger and inhibitory control in twins from 12 to 36 months of age

Inhibitory control (IC) is a dimension of child temperament that involves the self-regulation of behavioral responses under some form of instruction or expectation. Although IC is posited to appear in toddlerhood, the voluntary control of emotions such as anger begins earlier. Little research has analyzed relations between emotional development in infancy and later emerging IC. We examined phenotypic associations and genetic and environmental influences on parent- and laboratory-assessed anger and IC in a twin sample from 12 to 36 months of age. Typically, twins with low levels of IC had high levels of anger. Behavioral genetic findings confirmed significant genetic influences on anger and IC as assessed by parents, and on lab-based anger assessments. Shared environmental factors contributed to twin similarity on lab-assessed anger and IC at 36 months. Phenotypic covariance between anger and IC was largely due to overlapping genetic factors for parent ratings, and environmental factors in the laboratory.

Genetic and environmental factors shape infant sleep patterns: a study of 18-month-old twins

OBJECTIVE

Between 25% and 30% of children and adolescents experience sleep disorders. These disorders are complex phenotypes that are regulated by many genes, the environment, and gene-environment interactions. The objective of this study was to evaluate the contribution of genetic and environmental factors to sleep behaviors in early childhood and to contribute to the knowledge on appropriate therapeutic approaches, using a twin design.

PATIENTS AND METHODS

Data on sleeping behavior were collected from 314 18-month-old twin pairs (127 monozygotic and 187 dizygotic)using a parent-rated questionnaire. We used structural equation modeling to estimate genetic and environmental variance components for different sleep behaviors (cosleeping, sleep duration, and night awakenings).

RESULTS

Shared environment explained almost all (98.3%) of the total variance in cosleeping. Sleep duration was substantially influenced by shared environmental factors (64.1% nocturnal sleep and 61.2% diurnal sleep), with a moderate contribution of additive genetic effects (30.8% and 36.3% for nocturnal and diurnal sleep, respectively). For nocturnal waking episodes, we found a shared environmental contribution of 63.2% and a heritability estimate of 35.3%.

CONCLUSIONS

Most sleep disturbances during early childhood are explained by common shared environmental factors, and behavioral interventions adopted by parents and focused on modifying sleep behavior could contribute to solving sleep disturbances in this age group. However, the influence of genetic factors should not be underestimated, and research in this area could clarify the physiologic architecture of sleeping and contribute to selecting appropriate personalized therapeutic approaches.

Rethinking shared environment as a source of variance underlying attention-deficit/hyperactivity disorder symptoms: comment on Burt (2009)

Burt (2009) recently published a meta-analysis of twin studies on behaviors associated with childhood psychopathologies, concluding that the finding that traits associated with attention-deficit/hyperactivity disorder (ADHD) were the only behaviors that did not show a significant influence of shared environment (C) was surprising. We agree, highlighting four methodological issues that may account for this finding: (a) the use of nonlinear transformations to normalize skewed data; (b) low power to detect C and the subsequent presentation of reduced models; (c) the negative confounding of dominant genetic (D) and C influences in twin models with data exclusively from monozygotic and dizygotic twin pairs reared together; and (d) the correction used for contrast effects (a form of rater bias), which may lead to an overestimate of additive genetic (A) or D parameters at the expense of C. We offer suggestions for future research to address these issues, and we emphasize the need for additional research to examine possible shared environmental factors related to ADHD.

Twin studies and their implications for molecular genetic studies: endophenotypes integrate quantitative and molecular genetics in ADHD research

OBJECTIVE

To describe the utility of twin studies for attention-deficit/hyperactivity disorder (ADHD) research and demonstrate their potential for the identification of alternative phenotypes suitable for genomewide association, developmental risk assessment, treatment response, and intervention targets.

METHOD

Brief descriptions of the classic twin study and genetic association study methods are provided, with illustrative findings from ADHD research. Biometrical genetics refers to the statistical modeling of data gathered from one or more group of known biological relation; it was apparently coined by Francis Galton in the 1860s and led to the "Biometrical School" at the University of London. Twin studies use genetic correlations between pairs of relatives, derived using this theoretical framework, to parse the individual differences in a trait into latent (unmeasured) genetic and environmental influences. This method enables the estimation of heritability, i.e., the percentage of variance due to genetic influences. It is usually implemented with a method called structural equation modeling, which is a statistical technique for fitting models to data, typically using maximum likelihood estimation. Genetic association studies aim to identify those genetic variants that account for the heritability estimated in twin studies. Measurements other than those used for the clinical diagnosis of the disorder are popular phenotype choices in current ADHD research. It is argued that twin studies have great potential to refine phenotypes relevant to ADHD.

RESULTS

Prior studies have consistently found that the majority of the variance in ADHD symptoms is due to genetic factors. To date, genomewide association studies of ADHD have not identified replicable associations that account for the heritable variation. Possibly, the application of genomewide association studies to these alternative phenotypic measurements will assist in identifying the pathways from genetic variants to ADHD.

CONCLUSION

Power to detect associations should be improved by the study of highly heritable endophenotypes for ADHD and by reducing the number of phenotypes to be considered. Therefore, twin studies are an important research tool in the development of endophenotypes, defined as alternative, more highly heritable traits that act at earlier stages of the pathway from genes to behavior. Although genetic variation in liability to ADHD is likely polygenic, the proposed approach should help to identify improved alternative measurements for genetic association studies.

Gene-environment interaction of child temperament

OBJECTIVE

The aims of this study were to review studies on the molecular genetics of child temperament and prospectively analyze infant temperament as a function of the interaction between infant and mother: 5-HTT, DRD4, and MAO-A functional polymorphisms and the mother's emotional state.

METHOD

A prospective study of 317 newborns and their mothers was performed. Infant temperament and the mother's anxiety and confidence in caregiving were evaluated at 8 and 32 weeks after childbirth using the Mother and Baby Scale. The mother's emotional state was evaluated using the Edinburgh Postpartum Depression Scale and the State-Trait Anxiety Inventory. These variables were correlated with 5-HTTLPR and Stin2 variants in the 5-HTT gene and the DRD4 variable number tandem repeats Exon 3 and MAO-A variable number tandem repeats genotypes of both the infants and their mothers.

RESULTS

The irritability scores of infants with the 5-HTTLPR s allele showed a linear relationship with their mothers' anxiety of caregiving at 8 (p = .011) and 32 weeks (p = .001), whereas the irritability of infants carrying the HTTLPR ll genotype was independent of their mothers' anxiety.

CONCLUSIONS

The review of the literature in this field and the results of this study support that the 5-HTTLPR polymorphism moderates the influence of the mother's anxiety on infant irritability.

Environmental influences on children's physical activity: quantitative estimates using a twin design

BACKGROUND

Twin studies offer a 'natural experiment' that can estimate the magnitude of environmental and genetic effects on a target phenotype. We hypothesised that fidgetiness and enjoyment of activity would be heritable but that objectively-measured daily activity would show a strong shared environmental effect.

METHODOLOGY/PRINCIPAL FINDINGS

In a sample of 9-12 year-old same-sex twin pairs (234 individuals; 57 MZ, 60 DZ pairs) we assessed three dimensions of physical activity: i) objectively-measured physical activity using accelerometry, ii) 'fidgetiness' using a standard psychometric scale, and iii) enjoyment of physical activity from both parent ratings and children's self-reports. Shared environment effects explained the majority (73%) of the variance in objectively-measured total physical activity (95% confidence intervals (CI): 0.63-0.81) with a smaller unshared environmental effect (27%; CI: 0.19-0.37) and no significant genetic effect. In contrast, fidgetiness was primarily under genetic control, with additive genetic effects explaining 75% (CI: 62-84%) of the variance, as was parent's report of children's enjoyment of low 74% (CI: 61-82%), medium 80% (CI: 71-86%), and high impact activity (85%; CI: 78-90%), and children's expressed activity preferences (60%, CI: 42-72%).

CONCLUSIONS

Consistent with our hypothesis, the shared environment was the dominant influence on children's day-to-day activity levels. This finding gives a strong impetus to research into the specific environmental characteristics influencing children's activity, and supports the value of interventions focused on home or school environments.

Temperament in middle childhood: A behavioral genetic analysis of fathers' and mothers' reports

The genetic and environmental sources of variance in mothers' and fathers' ratings of child temperament in middle childhood were estimated and compared. Parents of 88 MZ twin pairs and 109 same-sex DZ twin pairs completed a temperament questionnaire. For Effortful Control, significant genetic and environmental effects were indicated across mothers' and fathers' ratings, but parent differences were found for the Negative Affectivity factor. When present, sibling contrast effects were not consistent for mothers and fathers. Parental ratings of the Effortful Control factor were best explained by the Biometric model whereas the Negative Affectivity factor was best explained by the Rater Bias model. Overall, mothers' and fathers' ratings yielded similar evidence of genetic and environmental etiology of temperament in middle childhood.

Actigraph data are reliable, with functional reliability increasing with aggregation

Motion sensor devices such as actigraphs are increasingly used in studies that seek to obtain an objective assessment of activity level. They have many advantages, and are useful additions to research in fields such as sleep assessment, drug efficacy, behavior genetics, and obesity. However, questions still remain over the reliability of data collected using actigraphic assessment. We aimed to apply generalizability theory to actigraph data collected on a large, general-population sample in middle childhood, during 8 cognitive tasks across two body loci, and to examine reliability coefficients on actigraph data aggregated across different numbers of tasks and different numbers of attachment loci. Our analyses show that aggregation greatly increases actigraph data reliability, with reliability coefficients on data collected at one body locus during 1 task (.29) being much lower than that aggregated across data collected on two body loci and during 8 tasks (.66). Further increases in reliability coefficients by aggregating across four loci and 12 tasks were estimated to be modest in prospective analyses, indicating an optimum trade-off between data collection and reliability estimates. We also examined possible instrumental effects on actigraph data and found these to be nonsignificant, further supporting the reliability and validity of actigraph data as a method of activity level assessment.

Agree or Agree to Disagree? Assessing the Convergence between Parents and Observers on Infant Temperament

The assessment of infant temperament has been typically accomplished with parent questionnaires. When compared with temperament behaviours observed in the laboratory, parents and observers generally do not agree, leading some researchers to question the validity of parent report. This paper reports on a representative sample of infants whose families resided in non-metropolitan counties and whose temperament was measured in three ways: (1) standard parent report (Infant Behavior Questionnaire); (2) observer ratings across two lengthy home visits; and (3) observer coding of second-by-second reactions to specific emotion-eliciting tasks. In order to account for both trait and method variance, structural equation modelling was applied to a sample of 955 infants (M age = 7.3 months) using variables from the three methods that reflected the dimensions of positivity and negativity. Although models based solely on method factors and trait factors fit the data well, results indicated that a model that included method and trait factors provided the best fit. Results also indicated that parents and observers (either across the home visit or to specific tasks) converge, to a degree, on ratings of the positivity dimension but diverge on the negativity dimension.

Genetic influences on activity level in early childhood: do situations matter?

Although genetic influences on individual differences in activity level (AL) are well documented, few studies have considered the etiology of AL from a contextual perspective. In the present study, cross-situational and context-specific genetic effects on individual differences in AL at age 2 were examined. The AL of 312 twin pairs (144 monozygotic and 168 dzygotic) was mechanically assessed with actigraphs in the home and in laboratory test and play situations. AL displayed significant genetic variance in all 3 situations. Moreover, actigraph scores significantly correlated across situations. Multivariate genetic model-fitting analyses found that the observed cross-situational continuity in AL was due entirely to genetic factors. Situational differences in AL arise from genetic, shared environmental, and nonshared environmental influences.

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

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

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

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

Genetic influences on mechanically-assessed activity level in children

BACKGROUND

Activity level is an important component of children's temperament, as well as being part of the core symptom domain of hyperactivity-impulsivity within attention deficit hyperactivity disorder (ADHD). Yet it is poorly understood, due partly to limitations on parent and teacher ratings, which are typically used as measurements of these symptoms.

METHODS

We aimed to study the etiology of objectively-measured activity level across two situations, using actigraphs. A population-based sample of 463 7-9-year-old twin pairs were assessed individually both when apart undergoing laboratory-based cognitive testing and when together during a break in testing.

RESULTS

Heritability of activity level was estimated as 24% during the test session and at 30% during the break in testing. Shared environmental influences accounted for 27% of the variance in activity level during the test session and 42% of activity level measured during the break. A genetic correlation of 1.0 indicated that the same genes influenced activity level across the two situations, justifying the use of a composite measure of the two situations. This produced a heritability estimate of 36%.

CONCLUSIONS

Objectively-measured activity level shows a moderate degree of genetic influence, with a common set of genes influencing activity level across situations. This supports the use of actigraphs as an additional source of information in studies that aim to improve phenotype definition for molecular genetic studies of activity level and ADHD.

Genetic heterogeneity between the three components of the autism spectrum: a twin study

OBJECTIVE

This study investigated the etiology of autistic-like traits in the general population and the etiological overlap between the three aspects of the triad of impairments (social impairments, communication impairments, restricted repetitive behaviors and interests) that together define autism spectrum disorders.

METHOD

Parents of 3,400 8-year-old twin pairs from the Twins Early Development Study completed the Childhood Asperger Syndrome Test, a screening instrument for autism spectrum symptoms in mainstream samples. Genetic model-fitting of categorical and continuous data is reported.

RESULTS

High heritability was found for extreme autistic-like traits (0.64-0.92 for various cutoffs) and autistic-like traits as measured on a continuum (0.78-0.81), with no significant shared environmental influences. All three subscales were highly heritable but showed low covariation. In the genetic modeling, distinct genetic influences were identified for the three components.

CONCLUSIONS

These results suggest the triad of impairments that define autism spectrum disorders is heterogeneous genetically. Molecular genetic research examining the three components separately may identify different causal pathways for the three components. The analyses give no indication that different genetic processes affect extreme autistic impairments and autistic impairments as measured on a continuum, but this can only be directly tested once genes are identified.

Contributions of Genes and Environments to Stability and Change in Externalizing and Internalizing Problems During Elementary and Middle School

We examined longitudinally collected behavioral reports by teachers on a unique twin sample at the ages of 7, 8, 9, 10, 11, and 12 years. As twin and adoption studies implicate the role of genetic influence on behavioral problems found to be stable in epidemiological samples, the current study employs a developmental behavior genetic model to examine the extent to which genetic and environmental contributions to problem behaviors are stable and/or change during development. In this sample of 410 monozygotic (MZ) and 354 dizygotic (DZ) twins, MZ twins were rated as more similar than DZ twins on average. In general, boys were more frequently rated as displaying externalizing behaviors than were girls across each of the six observations, while girls' internalizing problems were found not to be significantly different from boys'. For both sexes, stability in externalizing problem behaviors was due to a single common genetic factor whose effects acted pleiotropically at each age in the presence of unique environmental influences that were transmitted from age-to-age. Change was largely due to uncorrelated age-specific non-shared environmental and additive genetic effects. Contributions to stability for internalizing problems were due to age-to-age transmission of earlier expressed genetic effects. Change for girls and boys internalizing problems were largely due to environmental experiences unique to siblings along with uncorrelated age-specific genetic effects. These results further inform the notion that individual environments are important factors in the etiology of problem behaviors, but suggest that heritable contributions to phenotypic stability are largely the same across middle childhood and early adolescence. Clinical implications of these findings are discussed.

Behavioral genetics and child temperament

Most temperament theories presume a biological basis to those behavioral tendencies thought to be temperamental in origin. Behavioral genetic methods can be used to test this assumption. Twin and adoption studies suggest that individual differences in infant and child temperament are genetically influenced. However, behavioral genetics has much more to offer to the study of temperament than simple heritability estimates. The present paper describes some recent findings from behavioral genetics research in temperament that go well beyond the basic nature-nurture question. These findings include the importance of nonshared environmental influences on temperament, genetic continuity and environmental change during development, links between temperament and behavior problems, and harnessing the power of molecular genetics to identify specific genes responsible for genetic influence on early temperament.

Night and day: are siblings as different in temperament as parents say they are?

Twin studies suggest that parent ratings of temperament exaggerate differences between twins. The present study examined whether such contrast effects also operate for nontwin siblings. The activity level (AL) and shyness of 95 nontwin sibling pairs (ages 3 to 8 years) were assessed via parent ratings and objective measures (actigraph and observer ratings). Siblings showed no resemblance in either parent-rated AL or shyness; however, sibling resemblance for actigraph AL and observer-rated shyness was substantial. Thus, parents do contrast their nontwin siblings when rating these 2 temperament dimensions. Moreover, the importance of sibling differences in temperament to the sibling relationship and differential maternal treatment varied across the different measures of AL and shyness, suggesting that parent perceptions may play a role in these associations.

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.

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.

Genetics of personality: are we making progress?

For centuries, scientists are intrigued by the differences in personality between individuals. As early as in the ancient Greek civilization, people tried to formulate theories to systematize this diversity. With the increased interest in behavior genetics, personality was also considered a challenging phenotype. From the early start, studies suggested a heritable component in personality. After the successes of molecular genetic studies in unraveling the genetic basis of (mostly) monogenic diseases, the focus shifted towards complex traits, including psychiatric disorders. It was observed in several studies that personality measures differed between patients with psychiatric disorders and healthy controls. Therefore, normal personality was considered a viable endophenotype in the search for genes involved in psychiatric disorders such as affective disorders, ADHD and substance dependence. Genes that were to be found in studies on personality could be candidate genes for particular psychiatric disorders. In the course of time, however the study of genes for personality turned out to be at least as hard as the search for genes involved in other complex disorders. In this review, past studies, present problems and future directions concerning the study of personality genetics are discussed.

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.

The epigenesis of planum temporale asymmetry in twins

Variation in hemispheric asymmetry of the planum temporale (PT) has been related to verbal ability. The degree to which genetic and environmental factors mediate PT asymmetry is not known. This study examined the heritability for planar asymmetry in 12 dizygotic (DZ) and 27 monozygotic (MZ) male twin pairs who were between 6 and 16 years of age. There was weak but positive evidence for heritability of planar asymmetry. Co-twin similarity for planar asymmetry and Sylvian fissure morphology increased when excluding twins discordant for writing hand and when excluding twins exhibiting birth weight differences >20% from the analyses. Birth weight differences were also related to twin differences in total cerebral volume, but not central sulcus asymmetry. These results suggest that exogenous perinatal factors affect the epigenesis of planar asymmetry development.