Annotation: growing up as a twin: twin-singleton differences in psychological development

Sluggish Cognitive Tempo and Depressive Symptoms in Children and Adolescents Predict Adulthood Psychopathology

Sluggish cognitive tempo (SCT) is characterized by behavioral symptoms reflecting slowness and lethargy (e.g., sluggishness, appearing sleepy) and inconsistent alertness/mental confusion (e.g., daydreaming, fogginess). SCT is substantially correlated with the inattentive symptoms of attention-deficit/hyperactivity disorder (ADHD) and may be part of that domain, but in cross-sectional data, SCT is also strongly associated with both inattention and depression. To date, no study has examined the prospective associations of SCT symptoms in childhood/adolescence with symptoms of ADHD and internalizing problems in adulthood. Using a sample of 449 twin children and adolescent pairs, prospective multiple regression analyses examined whether self- and parent-reported SCT, depression, and parent-reported symptoms of ADHD predicted symptoms in adulthood 12 years later. SCT and depression at time one were strongly correlated (self-reported SCT and depression r = 0.84; parent-reported SCT and depression r = 0.78). When adult outcomes were separately regressed on each youth symptom dimension, self-reported SCT (β = 0.26, p < 0.0001) and depression (β = 0.13, p < 0.0001) each predicted adult symptoms of depression and self-reported SCT predicted inattention (β = 0.12, p = 0.0026). Parent-reported depression, but not parent-reported SCT, predicted self-reported adult depression symptoms (β = 0.17, p = 0.0003). In contrast, when each adult outcome was regressed simultaneously on youth self-reported SCT and depression, neither predicted adulthood inattention or depression. These findings indicate that SCT in childhood and adolescence is strongly associated concurrently and predictively with both inattention and depression. Theoretical and clinical applications of the construct of SCT must take its robust association with both inattention and depression into account.

Lexical processing of nouns and verbs at 36 months of age predicts concurrent and later vocabulary and school readiness

Children's lexical processing speed at 18 to 25 months of age has been linked to concurrent and later language abilities. In the current study, we extend this finding to children aged 36 months. Children (N = 126) participated in a lexical processing task in which they viewed two static images on noun trials (e.g., an ear of corn and a hat), or two dynamic video clips on verb trials (e.g., a woman stretching and the same woman clapping), and heard an auditory prompt labeling one of them (e.g., "Where is she stretching?"). They also participated in standard assessments of language and school readiness. The results indicated that lexical processing speed (i.e., how long they required to look to the labeled image or scene) was associated with measures of concurrent receptive vocabulary, as well as receptive vocabulary and school readiness two years later, although the associations are weaker than for younger children.

Is there something special about twin families? A comparison of parenting styles in twin and non-twin families

Twin comparisons offer a powerful quasi-experimental design to study the impact of the family of origin on children's life chances. Yet, there are concerns about the generalizability of results obtained from twin studies because twin families are structurally different and twins have a genetic resemblance. We examine these concerns by comparing mothers' reports on their parenting styles for twin and non-twin children between twin and non-twin families, as well as within twin families. We use two German studies for our comparisons: TwinLife and pairfam. Our results demonstrate that twins receive more differential treatment and more emotional warmth than non-twins; however, these differences are largely accounted for by age differences between children. Overall, our results indicate that results on parenting obtained from twin studies can be generalized to non-twin families.

The Twin Relationship Questionnaire

Abstract. Twins’ relationships evoke critical dilemmas for parents and teachers regarding raising and educating their twins. The current study provides comprehensive psychometric information about the Twin Relationship Questionnaire (TRQ; Fortuna, Goldner, & Knafo, 2010 ) as examined in nine samples, using mothers’ and fathers’ reports on 1,560 pairs of twins, tested in five measurement points from age 3 to age 9. On half of the subsamples, we conducted principal component analyses (PCA), whereas on the other subsamples, we conducted confirmatory factor analyses (CFA). Five factors were found in the PCA and confirmed by the CFA: conflict, closeness, dependence, dominance, and rivalry. All scales showed adequate internal consistencies and corrected item-total correlations. Correlation matrixes between scales demonstrate a triad of conflict, rivalry, and dominance scales, as opposed to the dyad of closeness and dependence. As evidence for the TRQ’s reliability and validity, we report the degree of agreement between mothers and fathers, the 10-month stability of mothers’ reports, and the associations of the TRQ subscales with experimentally assessed prosocial behaviors between the twins at age 6.5, and with the twins’ reports on their closeness at age 11. The replication of the findings across ages and raters reinforces the reliability and validity of the questionnaire.

Brooding, Inattention, and Impulsivity as Predictors of Adolescent Suicidal Ideation

Although suicide remains a leading cause of death for adolescents, risk factors beyond diagnoses and suicide attempt history remain unclear. We examined whether cognitive style and temperament impact risk for an early, yet still clinically relevant and distressing, form of suicidality: active suicidal ideation. We used binary logistic regression to test whether brooding, inattention, and impulsivity predicted significantly increased risk for suicidal ideation in a sample of 134 twins, 46 of whom endorsed active suicidal ideation (i.e., probands), as well as probands' cotwins and matched controls. When comparing probands with controls and controlling for depression diagnoses, brooding (B = 0.73, Odds Ratio [OR] = 2.07, p = 0.021), inattention (B = 1.09, OR = 2.98, p < 0.001), and impulsivity (B = 0.91, OR = 2.47, p = 0.001) differentiated probands from controls, individually. We compared probands with their cotwins using the same approach, which allowed us to account for variance in suicidal ideation risk related to twins' shared, familial characteristics (e.g., prenatal environment, neighborhood); inattention was the only significant predictor of suicidal ideation risk (B = 0.66, OR = 1.93, p = 0.020). We then fit a logistic regression model that included all three predictors. Only inattention predicted significantly increased likelihood of suicidal ideation in proband versus controls and proband versus cotwin comparisons (B = 0.88, OR = 2.40, p = 0.024 and B = 0.67, OR = 1.96, p = 0.045, respectively). These results highlight the potential utility of examining novel, more proximal risk factors for suicidal ideation in addition to more established distal factors, like suicide attempt history and psychiatric diagnoses.

The Predictive Value of Developmental Assessments at 1 and 2 for Intelligence Quotients at 6

Intelligence is an important individual difference factor related to mental health, academic achievement, and life success, yet there is a lack of research into its early cognitive predictors. This study investigated the predictive value of infant developmental assessment scores for school-age intelligence in a large, heterogeneous sample of single- and twin-born subjects (N = 521). We found that Early Learning Composite (ELC) scores from the Mullen Scales of Early Learning have similar predictive power to that of other infant tests. ELC scores at age 2 were predictive of Stanford-Binet abbreviated intelligence (ABIQ) scores at age 6 (r = 0.46) even after controlling for sex, gestation number, and parental education. ELC scores at age 1 were less predictive of 6-year ABIQ scores (r = 0.17). When the sample was split to test robustness of findings, we found that results from the full sample replicated in a subset of children born at ≥32 weeks gestation without birth complications (n = 405), though infant cognitive scores did not predict IQ in a subset born very prematurely or with birth complications (n = 116). Scores at age 2 in twins and singletons showed similar predictive ability for scores at age 6, though twins had particularly high correlations between ELC at age 1 and ABIQ at age 6.

Origins and Predictors of Friendships in 6- to 8-Year-Old Children Born at Neonatal Risk

OBJECTIVE

To test effects of gestational age (GA), early social experiences, and child characteristics on children's friendships and perceived peer acceptance.

STUDY DESIGN

As part of the prospective Bavarian Longitudinal Study (1147 children, 25-41 weeks GA), children's friendships (eg, number of friends, frequency of meeting friends) and perceived peer acceptance were assessed before school entry (6 years of age) and in second grade (8 years of age) using child and parent reports. The parent-infant relationship was evaluated during the 5 months after birth. Child characteristics (ie, height, motor impairment, cognitive ability, behavioral problems) were measured at 6 years of age. Multiple regressions estimated effects of GA, parent-infant relationship, and child characteristics.

RESULTS

Overall, children with higher GA had more friends, spent more time with friends, and were more accepted by peers at 6 years of age. Better parent-infant relationships, higher cognitive abilities, and fewer motor and behavioral problems predicted more friendships and higher peer acceptance after adjusting for sex, socioeconomic status, multiples, siblings, and special schooling. Across all GA groups, number of friends (child report: mean change, 1.77; 95% CI, 1.57-1.96) and peer acceptance (child report: mean change, 0.14; 95% CI, 0.09-0.19; parent report: mean change, 0.14; 95% CI, 0.11-0.17) increased with age, but the increase in number of friends was higher among preterm children (ie, interaction effect age*GA group: P = .034).

CONCLUSIONS

Our results provide evidence of a dose-response effect of low GA on children's friendships and perceived peer acceptance. Improvements in early parenting and motor, cognitive, and behavioral development may facilitate friendships and peer acceptance for all children across the gestation spectrum.

Moderators of wellbeing interventions: Why do some people respond more positively than others?

Interventions rarely have a universal effect on all individuals. Reasons ranging from participant characteristics, context and fidelity of intervention completion could cause some people to respond more positively than others. Understanding these individual differences in intervention response may provide clues to the mechanisms behind the intervention, as well as inform future designs to make interventions maximally beneficial for all. Here we focus on an intervention designed to improve adolescent wellbeing, and explore potential moderators using a representative and well-powered sample. 16-year old participants (N = 932) in the Twins Wellbeing Intervention Study logged online once a week to complete control and wellbeing-enhancing activities consecutively. Throughout the study participants also provided information about a range of potential moderators of intervention response including demographics, seasonality, personality, baseline characteristics, activity fit, and effort. As expected, some individuals gained more from the intervention than others; we used multi-level modelling to test for moderation effects that could explain these individual differences. Of the 15 moderators tested, none significantly explained individual differences in intervention response in the intervention and follow-up phases. Self-reported effort and baseline positive affect had a notable effect in moderating response in the control phase, during which there was no overall improvement in wellbeing and mental health. Our results did not replicate the moderation effects that have been suggested by previous literature and future work needs to reconcile these differences. They also show that factors that have previously been shown to influence baseline wellbeing do not also influence an individual's ability to benefit from a wellbeing intervention. Although future research should continue to explore potential moderators of intervention efficacy, our results suggest that the beneficial effect of positive activities in adolescents were universal across such factors as sex and socioeconomic status, bolstering claims of the scalability of positive activities to increase adolescent wellbeing.

Anxiety-related experience-dependent white matter structural differences in adolescence: A monozygotic twin difference approach

Anxiety is linked to deficits in structural and functional connectivity between limbic structures and pre-frontal cortices. We employed a monozygotic (MZ) twin difference design to examine the relationship between structural characteristics of the uncinate fasciculus (UF) measured by Diffusion Tensor Imaging (DTI) and anxiety symptoms in a sample of N = 100 monozygotic (genetically identical), adolescent twins. The MZ difference design allowed us focus on environmental factors that vary within twin pairs while controlling for genetic and environmental factors shared by twin pairs. Twins aged 13-18 years reported on symptoms of generalized anxiety and social phobia prior to participating in a neuroimaging visit. Regions of interest from the JHU ICBM atlas, including uncinate fasciculus and sagittal stratum as a control tract, were registered to the study template. We incorporated multiple diffusion tensor measures to characterize the white matter differences. Within twin pairs, the more anxious twin exhibited decreased fractional anisotropy (t = -2.22, p = 0.032) and axial diffusivity (t = -2.38, p = 0.022) in the left UF compared to the less anxious twin, controlling for age and gender. This study demonstrated the feasibility and advantages of adopting the MZ twin design for DTI measures in neuroimaging research.

The Phenomenology of Non-Aggressive Antisocial Behavior During Childhood

Although the phenomenology of overt or aggressive antisocial behavior during childhood is well-documented, far less is known about covert or non-aggressive, rule-breaking (RB) antisocial behavior. Gaps in knowledge include issues as basic as RB's typical symptom presentation during childhood and which symptoms differ across sex. The current study sought to fill these gaps in the literature by establishing the prevalence and psychometric properties of specific RB behaviors in a sample of 1022 twin boys and 1010 twin girls between the ages of 6 and 10 years. Legal RB behaviors (e.g., breaking rules, swears, lying or cheating) were present to varying degrees in most children, regardless of whether or not they passed the clinical threshold for RB. They were also more common in boys than in girls regardless of their clinical status. In sharp contrast, illegal RB behaviors (e.g., stealing, vandalism, setting fires) were rarely observed in typically-developing children, but were seen at moderate levels in boys and girls with clinically-significant levels of RB. Moreover, sex differences in illegal RB behaviors were observed only for those youth with clinically meaningful levels of RB. Such findings collectively imply that while legal RB behaviors can be found (albeit at different frequencies) in children with and without clinically meaningful levels of RB, illegal RB behaviors may function as relatively 'unambiguous' indicator of clinically-significant levels of RB.

Developmental and behavioral outcomes of uncomplicated monochorionic diamniotic twins born in the third trimester

BACKGROUND

Relatively little is known about the neurodevelopmental and behavioral outcomes of monochorionic diamniotic (MCDA) twin pregnancies where there are no antenatal complications peculiar to monochorionicity or prematurity.

METHODS

Twenty-two MCDA twins (44 children) with an average age of 4.3 years, and with no antenatal complications detected by 28 weeks of gestation, were recruited from a feto-maternal unit database. Parents completed a battery of neurodevelopmental and behavioral assessment questionnaires.

RESULTS

Eighteen children (41%) were identified as having developmental or behavioral concerns, predominantly of mild severity, which in turn were associated with a lower birth weight of medium effect size (Cohen's d=0.59).

CONCLUSION

MCDA twins delivered in the third trimester with no antenatal monochorionic complications in the first two trimesters appear to be at risk for subtle neurodevelopmental difficulties, associated with a lower birth weight. Ongoing developmental surveillance of these children during preschool-age is indicated for early identification and intervention.

A novel sibling-based design to quantify genetic and shared environmental effects: application to drug abuse, alcohol use disorder and criminal behavior

BACKGROUND

Twin studies have been criticized for upwardly biased estimates that might contribute to the missing heritability problem.

METHOD

We identified, from the general Swedish population born 1960-1990, informative sibships containing a proband, one reared-together full- or half-sibling and a full-, step- or half-sibling with varying degrees of childhood cohabitation with the proband. Estimates of genetic, shared and individual specific environment for drug abuse (DA), alcohol use disorder (AUD) and criminal behavior (CB), assessed from medical, legal or pharmacy registries, were obtained using Mplus.

RESULTS

Aggregate estimates of additive genetic effects for DA, AUD and CB obtained separately in males and females varied from 0.46 to 0.73 and agreed with those obtained from monozygotic and dizygotic twins from the same population. Of 54 heritability estimates from individual classes of informative sibling trios (3 syndromes × 9 classes of trios × 2 sexes), heritability estimates from the siblings were lower, tied and higher than those from obtained from twins in 26, one and 27 comparisons, respectively. By contrast, of 54 shared environmental estimates, 33 were lower than those found in twins, one tied and 20 were higher.

CONCLUSIONS

With adequate information, human populations can provide many methods for estimating genetic and shared environmental effects. For the three externalizing syndromes examined, concerns that heritability estimates from twin studies are upwardly biased or were not generalizable to more typical kinds of siblings were not supported. Overestimation of heritability from twin studies is not a likely explanation for the missing heritability problem.

Age-Related Differences and Heritability of the Perisylvian Language Networks

Acquisition of language skills depends on the progressive maturation of specialized brain networks that are usually lateralized in adult population. However, how genetic and environmental factors relate to the age-related differences in lateralization of these language pathways is still not known. We recruited 101 healthy right-handed subjects aged 9-40 years to investigate age-related differences in the anatomy of perisylvian language pathways and 86 adult twins (52 monozygotic and 34 dizygotic) to understand how heritability factors influence language anatomy. Diffusion tractography was used to dissect and extract indirect volume measures from the three segments of the arcuate fasciculus connecting Wernicke's to Broca's region (i.e., long segment), Broca's to Geschwind's region (i.e., anterior segment), and Wernicke's to Geschwind's region (i.e., posterior segment). We found that the long and anterior arcuate segments are lateralized before adolescence and their lateralization remains stable throughout adolescence and early adulthood. Conversely, the posterior segment shows right lateralization in childhood but becomes progressively bilateral during adolescence, driven by a reduction in volume in the right hemisphere. Analysis of the twin sample showed that genetic and shared environmental factors influence the anatomy of those segments that lateralize earlier, whereas specific environmental effects drive the variability in the volume of the posterior segment that continues to change in adolescence and adulthood. Our results suggest that the age-related differences in the lateralization of the language perisylvian pathways are related to the relative contribution of genetic and environmental effects specific to each segment.

SIGNIFICANCE STATEMENT

Our study shows that, by early childhood, frontotemporal (long segment) and frontoparietal (anterior segment) connections of the arcuate fasciculus are left and right lateralized, respectively, and remain lateralized throughout adolescence and early adulthood. In contrast, temporoparietal (posterior segment) connections are right lateralized in childhood, but become progressively bilateral during adolescence. Preliminary twin analysis suggested that lateralization of the arcuate fasciculus is a heterogeneous process that depends on the interplay between genetic and environment factors specific to each segment. Tracts that exhibit higher age effects later in life (i.e., posterior segment) appear to be influenced more by specific environmental factors.

Differences in Religiousness in Opposite-Sex and Same-Sex Twins in a Secular Society

Sex differences in religion are well known, with females generally being more religious than males, and shared environmental factors have been suggested to have a large influence on religiousness. Twins from opposite-sex (OS) and same-sex (SS) pairs may differ because of a dissimilar psycho-social rearing environment and/or because of different exposures to hormones in utero. We hypothesized that OS females may display more masculine patterns of religiousness and, vice versa, that OS males may display more feminine patterns. We used a web-based survey conducted in Denmark, which is a secular society. The survey included 2,997 twins aged 20-40 years, identified through the population-based Danish Twin Registry. We applied la Cour and Hvidt's adaptation of Fishman's three conceptual dimensions of meaning: Cognition, Practice, and Importance, and we used Pargament's measure of religious coping (RCOPE) for the assessment of positive and negative religious coping patterns. Differences between OS and SS twins were investigated using logistic regression for each sex. The analyses were adjusted for dependence within twin pairs. No significant differences in religiousness and religious coping were found for OS and SS twins except that more OS than SS females were members of the Danish National Evangelical Lutheran Church and fewer OS than SS females were Catholic, Muslim, or belonged to other religious denominations. Moreover, OS males at age 12 had higher rates of church attendance than did SS males. This study did not provide evidence for masculinization of female twins with male co-twins with regard to religiousness. Nor did it show any significant differences between OS and SS males except from higher rates of church attendance in childhood among males with female co-twins.

The etiologic role of genetic and environmental factors in criminal behavior as determined from full- and half-sibling pairs: an evaluation of the validity of the twin method

BACKGROUND

Twin studies have shown that criminal behavior (CB) is influenced by both genetic and shared environmental factors. Could these results be replicated using full-siblings and half-siblings?

METHOD

In 911 009 full-siblings reared together (FSRT), 41 872 half-siblings reared together (HSRT) and 52 590 half-siblings reared apart (HSRA), CB was assessed from the Swedish Crime Register. Modeling, including testing for age differences and rearing status, was performed using the OpenMx package.

RESULTS

Five sibling models were fitted examining FSRT and HSRT 0-2 years different in age, and both FSRT and HSRT, and FSRT, HSRT and HSRA 0-10 years different in age with and without a specified shared environment indexing age differences. Heritability estimates for CB ranged from 33 to 55% in females and 39 to 56% in males, similar to those found in our prior twin study on the same population. Estimates for the shared environment varied from 1 to 14% in females and 10 to 23% in males, lower than those estimated in the twin study. The specified shared environment indexed by sibling age differences was significant in all models tested.

CONCLUSIONS

Heritability estimates for CB from full- and half-siblings closely approximated those found from twins in the same population, validating the twin method. Shared environmental estimates were lower, suggesting the presence of shared environmental factors for CB specific to twins. When rearing status can be assessed, full- and half-siblings offer an additional method for assessing the role of genetic and environmental factors in complex disorders. However, age differences in siblings may need to be included in the models.

Trajectories of Sensory Over-Responsivity from Early to Middle Childhood: Birth and Temperament Risk Factors

Sensory over-responsivity, a subtype of sensory modulation disorder, is characterized by extreme negative reactions to normative sensory experiences. These over-reactions can interfere with daily activities and cause stress to children and their families. The etiology and developmental course of sensory over-responsivity is still largely unknown. We measured tactile and auditory over-responsivity in a population-based, typically developing sample of twins (N=978) at age two years via a caregiver report temperament questionnaire and again at age seven years via a sensory over-responsivity symptom inventory. Participating twins were treated as singletons although all analyses controlled for clustering within families. Children were divided into four trajectory groups based on risk status at both ages: low symptom (N=768), remitted (N=75), late-onset (N=112), and chronic (N=24). A subset of children who screened positive for SOR in toddlerhood (N = 102) took part in a pilot study focused on sensory over-responsivity at four years of age. Children in the chronic group had more severe symptoms of sensory sensitivity at age four years, including more motion sensitivity, than the other trajectory groups. Children in the chronic group had a younger gestational age and were more likely to be low birth-weight than the low symptom group. Differences between remitted and late-onset groups and the low-symptoms group were inconsistent across measures. Sensory over-responsivity was modestly correlated across ages (r = .22 for tactile over-responsivity and r = .11 for auditory over-responsivity), but symptoms were more stable among children born prematurely or who had more fearful and less soothable temperaments. A clear implication is that assessment over development may be necessary for a valid sensory processing disorder diagnosis, and a speculative implication is that sensory over-responsivity symptoms may be etiologically heterogeneous, with different causes of transient and stable symptoms.

Prospective association of childhood receptive vocabulary and conduct problems with self-reported adolescent delinquency: tests of mediation and moderation in sibling-comparison analyses

Associations among receptive vocabulary measured at 4-9 years, mother-reported childhood conduct problems at 4-9 years, and self-reported adolescent delinquency at 14-17 years were assessed using data from a prospective study of the offspring of a large U.S. nationally representative sample of women. A novel quasi-experimental strategy was used to rule out family-level confounding by estimating path-analytic associations within families in a sibling comparison design. This allowed simultaneous tests of the direct and indirect effects of receptive vocabulary and childhood conduct problems, and of their joint moderation, on adolescent delinquency without family-level environmental confounding. The significant association of receptive vocabulary with later adolescent delinquency was indirect, mediated by childhood conduct problems. Furthermore, a significant interaction between receptive vocabulary and childhood conduct problems reflected a steeper slope for the predictive association between childhood conduct problems and adolescent delinquency when receptive vocabulary scores were higher. These findings of significant indirect association were qualitatively identical in both population-level and within-family analyses, suggesting that they are not the result of family-level confounds.

Genetic and Environmental Contributions to Covariation Between DHEA and Testosterone in Adolescent Twins

Although several studies have shown that pubertal tempo and timing are shaped by genetic and environmental factors, few studies consider to what extent endocrine triggers of puberty are shaped by genetic and environmental factors. Doing so moves the field from examining correlated developmentally-sensitive biomarkers toward understanding what drives those associations. Two puberty related hormones, dehydroepiandrosterone and testosterone, were assayed from salivary samples in 118 MZ (62 % female), 111 same sex DZ (46 % female) and 103 opposite-sex DZ twin pairs, aged 12-16 years (M = 13.1, SD = 1.3). Pubertal status was assessed with a composite of mother- and self-reports. We used biometric models to estimate the genetic and environmental influences on the variance and covariance in testosterone and DHEA, with and without controlling for their association with puberty, and to test for sex differences. In males, the variance in testosterone and pubertal status was due to shared and non-shared environmental factors; variation in DHEA was due to genetic and non-shared environmental factors. In females, variance in testosterone was due to genetic and non-shared environmental factors; genetic, shared, and non-shared environmental factors contributed equally to variation in DHEA. In males, the testosterone-DHEA covariance was primarily due to shared environmental factors that overlapped with puberty as well as shared and non-shared environmental covariation specific to testosterone and DHEA. In females, the testosterone-DHEA covariance was due to genetic factors overlapping with pubertal status, and shared and non-shared environmental covariation specific to testosterone and DHEA.

Twins conceived using assisted reproduction: parent mental health, family relationships and child adjustment at middle childhood

STUDY QUESTION

Compared with singletons, what is the parent mental health, parent-child and couple relationship satisfaction, and child adjustment of 6- to 12-year-old assisted reproduction technology (ART) twins and their families?

SUMMARY ANSWER

There are no differences between 6- and 12-year-old ART twin and singleton families in parent mental health or family relationships; however, twins had significantly fewer behavior and attention problems than singletons in middle childhood.

WHAT IS KNOWN ALREADY

When ART twins are younger than 5 years old, parents have more mental health difficulties and poorer parent-child relationship quality, and no differences have been found in ART twin and singletons' psychosocial adjustment. However, studies have only examined the implications of ART twin status in families with infant and toddler aged children.

STUDY DESIGN, SIZE, DURATION

A cross-sectional study of 300 6-12-year-old ART children (n = 124 twins and n = 176 singletons) from 206 families at a reproductive endocrinology clinic in the USA.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Patients from one clinic with a child born between 1998 and 2004 were invited to participate in an online survey (82% recruitment rate). Participants provided information on each 6- to 12-year-old ART child in the family, and responded to questions on parent mental health, family relationships and child adjustment.

MAIN RESULTS AND THE ROLE OF CHANCE

There were no differences in parent mental health or family relationships in families with 6- to 12-year-old ART twins versus singletons. However, twins (M = 2.40, SE = 0.35) had significantly fewer behavior problems than singletons (M = 3.47, SE = 0.36; F(1, 201) = 4.54, b = 1.08, P < 0.05). Twins (M = 1.86, SD = 0.23) also had fewer attention problems than singletons (M = 2.64, SD = 0.23; F(1, 156) = 5.75, b = 0.78, P < 0.05). Results also suggest that full-term twins had significantly fewer attention problems (M = 1.37, SE = 0.33; F(1, 222) = 2.65, P = 0.05) than premature twins (M = 2.32, SE = 0.32, b = 0.95, P < 0.05), full-term singletons (M = 2.25, SE = 0.21, b = 0.88, P < 0.05) and premature singletons (M = 2.84, SE = 0.49, b = 1.47, P = 0.01). There were no significant differences between the other groups.

LIMITATIONS, REASONS FOR CAUTION

Although the response rate is high (82%) and family demographics are representative of US ART patients, patients are from one US clinic. Responses also are from one family member and may be subject to social desirability biases. Additionally, our data did not include identification of monozygotic and dizygotic twins.

WIDER IMPLICATIONS OF THE FINDINGS

Studies on infant and toddler ART twins suggest these families have parents with more mental health difficulties and lower parent-child relationship quality than singleton families. This study indicates the negative effects of twin status may have ameliorated by middle childhood, and twins may even have more optimum psychosocial adjustment than singletons in this developmental period.

STUDY FUNDING/COMPETING INTERESTS

This research is based on a collaborative research effort supported by University of Minnesota Agriculture Experiment Station Project Number MN-52-107, a University of Minnesota Grant-in-Aid of Research, Artistry and Scholarship grant, a University of Minnesota College of Education and Human Development Research Development Investment Grant and the M. Janice Hogan Fellowship. The authors of this article have no commercial or corporate interests to declare.

Do Twins Differ From Single-Born Children on Rates of Behavioral Difficulty in Early Childhood? A Study of Sibling Relationship Risk Factors

The hypothesis that twinning raises risk for behavioral difficulties in childhood is persistent, yet there is limited and inconsistent empirical evidence. Simple mean comparison without control for confounders provides data on prevalence rates but cannot provide knowledge about risk or etiology. To assess the effect of twin relationship on behavior, comparison of patterns of association with single-born siblings may be informative. Analyses of data from an Australian sample of twins and single-born children (N = 305, mean age 4 years 9 months, and a follow-up 12 months later) were undertaken. The outcome measure was the Strengths and Difficulties Questionnaire. Predictor and control measures were obtained from parent report on the sibling/co-twin relationship behavior, family demographics, and obstetric history. We assessed difference between twins and single-born children in two respects: (a) mean behavioral difficulties, and (b) patterns of association between sibling relationship and behavioral difficulties, controlling for confounders. Results showed no differences in mean levels of behavioral difficulties between twins and single-born siblings identifying the importance of statistical control for family and obstetric adversity. Differences in patterns of association were found; for twin children, conflict in their co-twin relationship predicted externalizing behaviors, while for single-born children conflict predicted internalizing behaviors. The findings of mean differences between twin and single-born children in social background, but not in behavioral difficulties, underscore the necessity of statistical control to identify risk associated with twinning compared with risk associated with family and obstetric background factors.

Resilience and vulnerability in adolescents: genetic influences on differential response to risk for delinquency

Prior research has identified a vast number of correlates for delinquent behavior during adolescence, yet a considerable number of errors in prediction remain. These errors suggest that behavioral development among a portion of youths is not well understood, with some exhibiting resilience and others a heightened vulnerability to risks. Examining cases that do not confirm prediction outcomes provides an opportunity to achieve a greater understanding of the relationships between risk factors and delinquency, which can be used to improve theoretical explanations of behavior. This study explores the contribution of genetic and environmental factors to differences in individual responses to cumulative risk for delinquency among a sample of adolescent twins (N = 784 pairs, 49 % female) in the National Longitudinal Study of Adolescent Health. The results indicate that additive genetic and unique environmental factors significantly contribute to variation in responses to cumulative risk across 14 risk factors spanning individual, familial, and environmental domains. When analyzed separately, the majority of the difference between vulnerable youths and the overall population was attributed to genetic influences, while differences between resilient youths and the population were primarily attributed to environmental influences. The findings illustrate the importance of examining both genetic and environmental influences in order to enhance explanations of adolescent offending.

A demonstration of the generalizability of twin-based research on antisocial behavior

Researchers typically analyze samples of twin pairs in order to decompose trait variance into genetic and environmental components. This methodological technique, referred to as twin-based research, rests on several assumptions that must be satisfied in order to produce unbiased results. While research has analyzed the tenability of certain assumptions such as equal environments, less attention has been given to whether results gleaned from samples of twins generalize to the broader population of non-twins. The current study analyzed data drawn from the National Longitudinal Study of Adolescent Health and findings suggested twins do not systematically differ from the general population of non-twins on many measures of behavior and development. Furthermore, the effects of specific covariates on measures of antisocial behavior did not appear to differ across twin status. In sum, evidence concerning the etiology of antisocial behavior (e.g., heritability estimates) gleaned from twin-based research is likely to generalize to the non-twin population.

The Effects of Sibling Relationships on Social Adjustment Among Japanese Twins Compared With Singletons

This study examined the link between sibling relationships and children's social adjustment by comparing twin siblings and siblings with different ages (singleton siblings), and clarified the role of reciprocity in sibling relationships on children's social development. Mothers of 58 monozygotic twin pairs, 48 dizygotic twin pairs, and 86 singleton sibling pairs reported their children's sibling relationships and social adjustment. This study showed that the effects of sibling relationships on the prosocial behaviors and conduct problems of each child are stronger for twins than for singleton siblings. Moreover, positivity toward one's sibling increased peer problems only among monozygotic twins. The opposite tendency was present among dizygotic twins and singleton siblings. This study suggests the importance for children's social development of having many interactions with siblings and establishing reciprocity in sibling relationships. Moreover, our results suggest that the quality of sibling relationships among monozygotic twins may be different from those among dizygotic twins and singleton siblings.

Human aggression across the lifespan: genetic propensities and environmental moderators

This chapter reviews the recent evidence of genetic and environmental influences on human aggression. Findings from a large selection of the twin and adoption studies that have investigated the genetic and environmental architecture of aggressive behavior are summarized. These studies together show that about half (50%) of the variance in aggressive behavior is explained by genetic influences in both males and females, with the remaining 50% of the variance being explained by environmental factors not shared by family members. Form of aggression (reactive, proactive, direct/physical, indirect/relational), method of assessment (laboratory observation, self-report, ratings by parents and teachers), and age of the subjects-all seem to be significant moderators of the magnitude of genetic and environmental influences on aggressive behavior. Neither study design (twin vs. sibling adoption design) nor sex (male vs. female) seems to impact the magnitude of the genetic and environmental influences on aggression. There is also some evidence of gene-environment interaction (G × E) from both twin/adoption studies and molecular genetic studies. Various measures of family adversity and social disadvantage have been found to moderate genetic influences on aggressive behavior. Findings from these G × E studies suggest that not all individuals will be affected to the same degree by experiences and exposures, and that genetic predispositions may have different effects depending on the environment.

Being Opposite: Is There Advantage for Social Competence and Friendships in Being an Opposite-Sex Twin?

Arecent large-scale study of Finnish adolescent twins (Pulkkinen et al., 2003) reported that individuals from opposite-sex twin pairs were more socially adaptive than individuals from same-sex pairs or singletons. This finding raised questions about the social learning effects of being an opposite-sex twin. The current article predicted on the basis of this finding, and evidence from singleton populations, that having an opposite-sex twin would yield social advantage. It sought to examine the social competencies of opposite-sex twins and compare them with same-sex twins and singletons. The study focused on the preschool years (age 3 to 6), a period in which the majority of children encounter their first large group, nonfamilial social experiences. The study obtained reports from parents and teachers of children aged 3 to 6 years: 72 children (36 pairs) who were dizygotic opposite-sex twins (DZOS), 50 children (25 pairs) who were dizygotic same-sex twins (DZSS), and 85 singletons of the same age and sex as the twins, who had at least one sibling. Reports were made using standardized measures of social competencies, behavior problems, language development and friendships. The main effects found were of differences in social competency between twins and singletons. Twins had lower social competency scores. No differences between same-sex and opposite-sex twins were found. The findings did not support the hypothesis of social advantage for opposite-sex twins in early childhood.

A Twin-Singleton Comparison of Developmental Trajectories of Externalizing and Internalizing Problems in 6- to 12-Year-Old Children

Research on twin-singleton differences in externalizing and internalizing problems in childhood is largely cross-sectional and yields contrasting results. The goal of this study was to compare developmental trajectories of externalizing and internalizing problems in 6- to 12-year-old twins and singletons. Child Behavior Checklist (CBCL) maternal reports of externalizing and internalizing problems were obtained for a sample of 9651 twins from the Netherlands Twin Register and for a representative general population sample of 1351 singletons. Latent growth modeling was applied to estimate growth curves for twins and singletons. Twin-singleton differences in the intercepts and slopes of the growth curves were examined. The developmental trajectories of externalizing problems showed a linear decrease over time, and were not significantly different for twins and singletons. Internalizing problems seem to develop similarly for twins and singletons up to age 9. After this age twins' internalizing symptoms start to decrease in comparison to those of singletons, resulting in less internalizing problems than singletons by the age of 12 years. Our findings confirm the generalizability of twin studies to singleton populations with regard to externalizing problems in middle and late childhood. The generalizability of studies on internalizing problems in early adolescence in twin samples should be addressed with care. Twinship may be a protective factor in the development of internalizing problems during early adolescence.

Twins Have Slightly Higher Self-Concepts Than Singletons in the Elementary School Period: A Study of South Korean Twins and Singletons

The twin method assumes that the trait under study is similar in twins and singletons so that the results from twin samples can be generalized to the singleton population. The purpose of the present study was to compare self-concept in twins and singletons matched in parental level of education. The 6 cluster scales of the Piers-Harris Children's Self-Concept Scale (P-H) were administered to 630 singleton children and 635 twins aged 7 to 12 years. Model-fitting analyses yielded two conclusions. First, self-concept was generally similar between first- and second-born twins and between monozygotic and dizygotic twins across the 6 cluster scales of the P-H. Second, twins consistently exceeded singletons for all 6 cluster scales of the P-H. However, the effect sizes of differences between twins and singletons were small except for one scale, Physical Appearance and Attributes.

Feelings and the need for information and counselling of expectant parents of twins

In this study, expectant parents of twins were questioned about their feelings and need for information and counselling on the parenting of twins. Data were obtained from self-rating questionnaires and an interview with the expectant mother. From the results it is argued that expectant parents of twins need specific information and counselling about the medical, emotional and parenting issues of having twins. Guidelines for organising this counselling are proposed.

The Personalities of Twins: Just Ordinary Folks

Twin studies have demonstrated that personality traits show moderate genetic influence. The conclusions drawn from twin studies rely on the assumptions that twins are representative of the population at large and that monozygotic and dizygotic twins are comparable in every way that might have bearing on the traits being studied. To evaluate these assumptions, we used Multidimensional Personality Questionnaire (MPQ) data from three samples drawn from the Minnesota Twin Registry (totaling 12,971 respondents) to examine the effect sizes associated with mean differences on the 11 MPQ scales and 3 higher-order MPQ factors for singletons versus twins and MZ twins versus DZ twins. The singletons in the samples were family members of the participating twins. We also used ratios of scale variances to examine the significance of variance differences. The only mean or variance difference replicated across all three samples was greater Social Closeness (about .1 standard deviation) for twins than for singletons. This difference was obtained for both males and females. It would appear that, with respect to personality, twins are not systematically different from other people. Our results also highlight the importance of replication in psychological research because each of our large samples showed differences not replicated in other samples.

Genetic and environmental influences on impulsivity: a meta-analysis of twin, family and adoption studies

A meta-analysis of twin, family and adoption studies was conducted to estimate the magnitude of genetic and environmental influences on impulsivity. The best fitting model for 41 key studies (58 independent samples from 14 month old infants to adults; N=27,147) included equal proportions of variance due to genetic (0.50) and non-shared environmental (0.50) influences, with genetic effects being both additive (0.38) and non-additive (0.12). Shared environmental effects were unimportant in explaining individual differences in impulsivity. Age, sex, and study design (twin vs. adoption) were all significant moderators of the magnitude of genetic and environmental influences on impulsivity. The relative contribution of genetic effects (broad sense heritability) and unique environmental effects were also found to be important throughout development from childhood to adulthood. Total genetic effects were found to be important for all ages, but appeared to be strongest in children. Analyses also demonstrated that genetic effects appeared to be stronger in males than in females. Method of assessment (laboratory tasks vs. questionnaires), however, was not a significant moderator of the genetic and environmental influences on impulsivity. These results provide a structured synthesis of existing behavior genetic studies on impulsivity by providing a clearer understanding of the relative genetic and environmental contributions in impulsive traits through various stages of development.

Trajectories of CBCL attention problems in childhood

The first aim of this study was to identify developmental trajectories of Attention Problems in twins followed from age 6 to 12 years. Second, we investigated whether singletons follow similar trajectories. Maternal longitudinal ratings on the Attention Problems (AP) subscale of the Child Behavior Checklist were obtained for a sample of 12,486 twins from the Netherlands Twin Register and for a general population sample of 1,346 singletons. Trajectories were analyzed by growth mixture modeling in twins, and compared with singletons. Teacher ratings on the AP subscale of the Teachers' Report Form were available for 7,179 twins and 1,211 singletons, and were used for cross-sectional mean comparisons at each age. All analyses were conducted for boys and girls separately. We identified three linear trajectories in both boys and girls, i.e., stable low (62-71%), low-increasing (15-18%), and high-decreasing (14-21%). Singletons followed three identical trajectories, with similar class proportions. Teacher ratings yielded no differences in mean levels of Attention Problems between twins and singletons. The development of Attention Problems from age 6 to 12 years can be characterized by stable low, low-increasing, and high-decreasing developmental trajectories. Twins and singletons are comparable with respect to the development of Attention Problems in childhood.

No major effect of twinning on autistic traits

BACKGROUND

It has been questioned whether the process of twinning might be a risk factor for autism spectrum conditions (ASC) and autistic traits.

AIM

We sought to determine whether autistic traits and probable disorder, as measured by the Childhood Autism Spectrum Test (CAST), were more pronounced in twins compared to singletons.

SAMPLES

Data were analyzed from two large population-based samples of UK children, twins (n = 5,142 twin pairs, aged 8 years) and singletons (n = 2,805, aged 5-9 years).

RESULTS

Distributions of CAST scores in both groups were negatively skewed and scores for twins were more variable than singletons. Mean CAST total scores and standard errors (SE) were not significantly different for twins (5.1; SE 0.04) compared to singletons (4.9; SE 0.08). Moreover, contrary to expectations, the likelihood of scoring above the threshold for possible ASC was significantly lower in the twins than the singletons (OR = 0.69; P = 0.002). Subsidiary analyses of CAST scores according to sex, twin type, and subscale scores representing the subdomains of autism found a few significant differences (P<0.01), but the effect sizes for these differences were small and none exceeded η(2) = 0.005. The explanation for these small differences remains obscure, but the very small effect sizes mean they are of little importance.

CONCLUSIONS

Our results do not provide evidence to support twinning as a risk factor in the development of autistic traits.

Age-of-onset or behavioral sub-types? A prospective comparison of two approaches to characterizing the heterogeneity within antisocial behavior

There are two common approaches to sub-typing the well-documented heterogeneity within antisocial behavior: age-of-onset (i.e., childhood-onset versus adolescence-onset; see Moffitt 1993) and behavioral (i.e., physical aggression versus non-aggressive rule-breaking). These approaches appear to be associated, such that aggression is more characteristic of childhood-onset antisocial behavior whereas rule-breaking is linked to both child- and adolescence-onset antisocial behavior. However, it remains unclear which approach, if either, better explains the heterogeneity within antisocial behavior. We examined this question in a prospective sample of male twins, assessed at the ages of 11, 14, 17, and 24 years. Although the age-of-onset subtypes predicted adult antisocial behavior in the expected direction when analyzed alone, this association dissipated once we controlled for aggression and rule-breaking. Such findings suggest that the behavioral sub-types of antisocial behavior may be a stronger predictor of later antisocial outcomes than is its age-of-onset.

Testing structural models of DSM-IV symptoms of common forms of child and adolescent psychopathology

Confirmatory factor analyses were conducted of Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) symptoms of common mental disorders derived from structured interviews of a representative sample of 4,049 twin children and adolescents and their adult caretakers. A dimensional model based on the assignment of symptoms to syndromes in DSM-IV fit better than alternative models, but some dimensions were highly correlated. Modest sex and age differences in factor loadings and correlations were found that suggest that the dimensions of psychopathology are stable across sex and age, but slightly more differentiated at older ages and in males. The dimensions of symptoms were found to be hierarchically organized within higher-order "externalizing" and "internalizing" dimensions, which accounted for much of their variance. Major depression and generalized anxiety disorder were substantially correlated with both the "externalizing" dimension and the "internalizing" dimension, however, suggesting the need to reconceptualize the nature of these higher-order dimensions.

Revisiting the association between reading achievement and antisocial behavior: new evidence of an environmental explanation from a twin study

Previous studies have reported, but not explained, the reason for a robust association between reading achievement and antisocial behavior. This association was investigated using the Environmental Risk (E-Risk) Longitudinal Twin Study, a nationally representative 1994-1995 birth cohort of 5- and 7-year-olds. Results showed that the association resulted primarily from environmental factors common to both reading and antisocial behavior and was stronger in boys. Environmental factors also explained the relation between reading disability and conduct disorder. Leading candidate environmental risk factors weakly mediated the association. For boys the best explanation was a reciprocal causation model: poor reading led to antisocial behavior, and vice versa. In contrast, the relation between reading achievement and attention deficit hyperactivity disorder was best explained by common genetic influences.

Correlates of regular cigarette smoking in a population-based sample of Australian twins

AIMS

To investigate the role of measured risk factors and the influence of genetic and environmental factors on regular cigarette smoking. Design Members of monozygotic and dizygotic, including unlike-sex twin pairs (n = 6257) from a young adult cohort from the Australian Twin Registry.

METHODS

Cox proportional hazards models were used to determine whether putative risk factors were significantly associated with regular cigarette smoking. Risk factors were classified into four tiers: tier 1 (parental history, including parental education, alcoholism and cigarette smoking), tier 2 (early home and family influences), tier 3 (early life events, e.g. trauma) and tier 4 (psychiatric symptoms/disorders with onset prior to 14 years), after controlling for gender, zygosity and their interactions. Genetic models were fitted to examine the heritability of smoking behavior before and after controlling for significant covariates from the four tiers.

FINDINGS

Parental history of cigarette smoking and alcoholism, parental closeness and home environment, as well as incidence of childhood sexual abuse or other trauma, a history of early onset panic attacks and conduct problems were associated with regular cigarette smoking. Important age interactions were found, particularly for family background risk factors. Regular cigarette smoking was moderately heritable, even after accounting for significant covariates.

CONCLUSIONS

Several measured risk factors are associated with regular smoking. While some of the genetic influences on regular smoking may be shared with these risk factors, a significant proportion of the genetic vulnerability to regular smoking is phenotype-specific.