Using the Flanker Task to Examine Genetic and Environmental Contributions in Inhibitory Control Across the Preschool Period.

The limited research exploring genetic and environmental influences on inhibitory control (IC) in preschoolers has relied on parent ratings or simple delay tasks and has produced mixed results. The present study uses a cognitively-challenging Flanker task to examine genetic and environmental contributions to the development of early IC in a longitudinal sample of 310 same-sex twin pairs (123 MZ; 187 DZ; 51% female) assessed at ages 3, 4 and 5 years. IC was significantly heritable at each age (a2: age 3 = .36; age 4 = .36; age 5 = .35). Stability was entirely accounted for by genetic influences, and change was explained by genetic and nonshared environmental factors. No significant shared environmental influences were observed.

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.

Changing genetic architecture of body mass index from infancy to early adulthood: an individual based pooled analysis of 25 twin cohorts.

BACKGROUND: Body mass index (BMI) shows strong continuity over childhood and adolescence and high childhood BMI is the strongest predictor of adult obesity. Genetic factors strongly contribute to this continuity, but it is still poorly known how their contribution changes over childhood and adolescence. Thus, we used the genetic twin design to estimate the genetic correlations of BMI from infancy to adulthood and compared them to the genetic correlations of height. METHODS: We pooled individual level data from 25 longitudinal twin cohorts including 38,530 complete twin pairs and having 283,766 longitudinal height and weight measures. The data were analyzed using Cholesky decomposition offering genetic and environmental correlations of BMI and height between all age combinations from 1 to 19 years of age. RESULTS: The genetic correlations of BMI and height were stronger than the trait correlations. For BMI, we found that genetic correlations decreased as the age between the assessments increased, a trend that was especially visible from early to middle childhood. In contrast, for height, the genetic correlations were strong between all ages. Age-to-age correlations between environmental factors shared by co-twins were found for BMI in early childhood but disappeared altogether by middle childhood. For height, shared environmental correlations persisted from infancy to adulthood. CONCLUSIONS: Our results suggest that the genes affecting BMI change over childhood and adolescence leading to decreasing age-to-age genetic correlations. This change is especially visible from early to middle childhood indicating that new genetic factors start to affect BMI in middle childhood. Identifying mediating pathways of these genetic factors can open possibilities for interventions, especially for those children with high genetic predisposition to adult obesity.

Low social affiliation predicts increases in callous-unemotional behaviors in early childhood.

BACKGROUND: Callous-unemotional (CU) behaviors predict risk for aggression and rule-breaking. Low social affiliation (i.e. reduced motivation for and enjoyment of social closeness) is hypothesized to be a phenotypic marker for CU behaviors in early childhood. However, studies need to establish observational methods to objectively assess social affiliation as well as to establish parenting practices that can buffer pathways from low social affiliation to CU behaviors. METHODS: Using data from a longitudinal twin study of 628 children (age 2, 47% females; age 3, 44.9% females), we examined reciprocal associations between observed social affiliation, CU behaviors, and oppositional-defiant behaviors. We tested whether positive parenting moderated associations over time. RESULTS: We established that an observed measure of social affiliation derived from the Bayley's Behavior Rating Scale and Infant Behavior Record showed high inter-rater reliability and expected convergence with parent-reported temperament measures. Lower social affiliation at age 2 uniquely predicted CU behaviors, but not oppositional-defiant behaviors, at age 3. Finally, low social affiliation at age 2 predicted CU behaviors at age 3 specifically among children who experienced low, but not high, levels of parental positivity. CONCLUSIONS: An objective rating scale that is already widely used in pediatric settings reliably indexes low social affiliation and risk for CU behaviors. The dynamic interplay between parenting and low child social affiliation represents an important future target for novel individual- and dyadic-targeted treatments to reduce risk for CU behaviors.

Fearlessness and low social affiliation as unique developmental precursors of callous-unemotional behaviors in preschoolers.

BACKGROUND: Early callous-unemotional (CU) behaviors identify children at risk for severe and persistent aggression and antisocial behavior. Recent work suggests that fearlessness and low social affiliation are implicated in the etiology of CU behaviors, although more research is needed to clarify these etiological pathways, as well as the role of parenting. METHOD: Using a sample of preschoolers (N = 620), we examined pathways between observed fear in response to social and non-social stimuli and observed social affiliation during social interactions at age 3 and increases child CU behaviors and oppositional-defiant behaviors from ages 3 to 5. To elucidate the role of parenting in exacerbating or buffering the relationships between low fear and social affiliation and CU behaviors, we tested whether parental harshness or low warmth moderated these pathways. RESULTS: Fearlessness and low social affiliation uniquely predicted increases in CU behaviors, but not oppositional-defiant behaviors, from ages 3 to 5. Moreover, there was evidence for differential moderation of the fear pathway by harsh parenting, such that harsh parenting predicted increases in CU behaviors in fearless children but increases in oppositional-defiant behaviors in fearful children. CONCLUSIONS: Fearlessness and low social affiliation contribute to the development of CU behaviors. Harsh parenting can exacerbate the risky fearlessness pathway. Preventative interventions aimed at reducing risk for CU behaviors and persistent aggression and antisocial behavior should target socioaffiliative processes and provide parents with strategies and training to manage and scaffold rule-compliant behavior when children show low fearful arousal.

A Twin Study of Inhibitory Control at Age Two and ADHD Behavior Problems at Age Three.

Low levels of childhood inhibitory control (IC) are phenotypically and genetically associated with externalizing behavior problems and attention deficit hyperactivity disorder (ADHD). Unfortunately, there is little research on this topic in early childhood, when IC first emerges. This investigation extends the previous findings of contemporaneous genetic covariance between parent-rated and laboratory-assessed IC and ADHD at age 2 by examining longitudinal links between IC at age two and ADHD behavior problems at age three in a sample of 314 same-sex twin pairs (145 monozygotic or MZ, 169 dizygotic or DZ). There were significant phenotypic associations between both parent and laboratory IC assessments at age two and later ADHD behavioral problems (correlations ranged from - .15 to - .44). In our model-fitting strategy, we included measures of ADHD and IC at age 2 as predictors of ADHD at age 3. Longitudinal genetic analyses showed that phenotypic covariance between age two IC and ADHD behavior problems one year later were explained by overlapping genetic variance (genetic correlations ranged from - .28 to - .60). However, these effects were not unique to IC and reflect variance shared with ADHD at age 2. Parent-rated IC at age two showed higher phenotypic and genetic covariance with ADHD at age three than lab ratings of IC at age two. This is the first investigation examining genetic covariance between parent and lab-based IC at age two and ADHD behavior problems at age three. Findings show that after accounting for co-occurring ADHD, early temperamental IC is not a unique genetic risk factor for later ADHD.

The Boston University Twin Project (BUTP) II.

The Boston University Twin Project (BUTP) uses a multimethod, longitudinal approach to study the role of genetic and environmental factors on the development of child temperament and related behaviors in early childhood. There are two phases in this project. The first, described in the previous Twin Research and Human Genetics special issue on twin registries, focused on activity level and comprised over 300 twin pairs assessed in the home and laboratory at ages 2 and 3. In this article, we describe subject recruitment, sample characteristics, and study procedures and measures of the second phase of the BUTP. This recent study focuses more broadly on the development of multiple temperament dimensions and explores associations between temperament trajectories, parenting and child adjustment in a new cohort of approximately 300 twin pairs assessed at 3, 4 and 5 years of age.

Effects of socioeconomic status and executive function on school readiness across levels of household chaos.

Isolating child attributes and familial characteristics that support school readiness in children on the upper half of the socioeconomic spectrum can complement existing research on lower-socioeconomic status (SES) children and facilitate a more complete understanding of how children's performance varies across the full SES spectrum. This study examined if relations between SES, two components of executive function (EF; set-shifting and inhibitory control), and school readiness vary as a function of household chaos in 564 four-year-old children, primarily from middle-to upper-middle class families in the Northeast Region of the United States. Structural equation modeling of direct and indirect effects revealed three major findings: 1) higher levels of EF were related to better school readiness regardless of level of household chaos; 2) SES had an indirect effect on school readiness through set-shifting; and 3) household chaos was negatively associated with school readiness.

Parent- and Observer-Rated Positive Affect in Early Childhood: Genetic Overlap and Environmental Specificity.

The sources of individual differences in both observed and parent-rated positive affect (PA) were examined in a sample of 304 3-year-old twin pairs (140 MZ, 164 DZ). Based on model-fitting analyses, individual differences in observed PA were attributed to moderate genetic and high nonshared environmental factors, but not shared environmental factors. In contrast, shared environmental effects accounted for over half of the variance in parent-rated PA and genetic and nonshared environmental effects were more modest. The genetic correlation across the two measures was high, indicating substantial overlap between genetic factors influencing the two. It was these overlapping genetic effects that fully explained the phenotypic correlation between both measures. There was no significant covariance between the environmental influences on parent rated and observed PA. Thus, the two measures of PA in early childhood have common genetic underpinnings, whereas environmental influences are measure-specific. Measurement implications are discussed.

Genetic and environmental links between motor activity level and attention problems in early childhood.

Cross-lagged biometric models were used to examine genetic and environmental links between actigraph-assessed motor activity level (AL) and parent-rated attention problems (AP) in 314 same-sex twin pairs (MZ = 145, DZ = 169) at ages 2 and 3 years. At both ages, genetic correlations between AL and AP were moderate (ra2 = .35; ra3 = .39) indicating both overlap and specificity in genetic effects across the two domains. Within- and across-age phenotypic associations between AL and AP were entirely due to overlapping genetic influences. There was a unidirectional effect of AL at age 2 predicting later AP. For AP, genetic and environmental influences from age 2 were transmitted to age 3 via stability effects and from AL. For AL, across-age effects were transmitted only via stability. These results suggest that overactivity in late infancy may impact the later development of problems related to inattention, and that genetic factors explain the association between the two domains.

Tipping points? Curvilinear associations between activity level and mental development in toddlers.

BACKGROUND: The Theory of Optimal Stimulation (Zentall & Zentall, Psychological Bulletin, 94, 1983, 446) posits that the relation between activity level (AL) and cognitive performance follows an inverted U shape where midrange AL predicts better cognitive performance than AL at the extremes. METHODS: We explored this by fitting linear and quadratic models predicting mental development from AL assessed via multiple methods (parent ratings, observations, and actigraphs) and across multiple situations (laboratory play, laboratory test, home) in over 600 twins (2- and 3-year olds). RESULTS: Only observed AL in the laboratory was curvilinearly related to mental development scores. Results replicated across situations, age, and twin samples, providing strong support for the optimal stimulation model for this measure of AL in early childhood. CONCLUSIONS: Different measures of AL provide different information. Observations of AL which include both qualitative and quantitative aspects of AL within structured situations are able to capture beneficial aspects of normative AL as well as detriments of both low and high AL.

Difficult temperament and negative parenting in early childhood: a genetically informed cross-lagged analysis.

A genetically informed longitudinal cross-lagged model was applied to twin data to explore etiological links between difficult temperament and negative parenting in early childhood. The sample comprised 313 monozygotic (MZ) and dizygotic (DZ) twin pairs. Difficult temperament and negative parenting were assessed at ages 2 and 3 using parent ratings. Both constructs were interrelated within and across age (rs .34-.47) and showed substantial stability (rs .65-.68). Difficult temperament and negative parenting were influenced by genetic and environmental factors at ages 2 and 3. The genetic and nonshared environmental correlations (rs .21-.76) at both ages suggest overlap at the level of etiology between the phenotypes. Significant bidirectional associations between difficult temperament and negative parenting were found. The cross-lagged association from difficult temperament at age 2 to negative parenting at age 3 and from negative parenting at age 2 and difficult temperament at age 3 were due to genetic, shared environmental, and nonshared environmental factors. Substantial novel genetic and nonshared environmental influences emerged at age 3 and suggest change in the etiology of these constructs over time.

Genetic and Environmental Links Between Natural Language Use and Cognitive Ability in Toddlers.

Although the phenotypic correlation between language and nonverbal cognitive ability is well-documented, studies examining the etiology of the covariance between these abilities are scant, particularly in very young children. The goal of this study was to address this gap in the literature by examining the genetic and environmental links between language use, assessed through conversational language samples, and nonverbal cognition in a sample of 3-year-old twins (N = 281 pairs). Significant genetic and nonshared environmental influences were found for nonverbal cognitive ability and language measures, including mean length of utterance and number of different words, as well as significant genetic covariance between cognitive ability and both language measures.

Callous-unemotional behaviors in early childhood: Genetic and environmental contributions to stability and change.

Callous-unemotional (CU) behaviors demonstrate meaningful individual differences in early childhood, even in nonclinical samples with low mean levels of CU, but the factors underlying this variation have not been examined. This study investigated genetic and environmental contributions to individual differences and to sources of continuity and change in CU in toddler twins (145 monozygotic, 169 dizygotic) assessed at ages 2 and 3 years. CU, as assessed by the Child Behavior Checklist 1.5-5 (Achenbach & Rescorla, 2000), was moderately stable across age (r = .45, p < .0001). Longitudinal biometric analyses revealed genetic and nonshared environmental influences on CU at both ages, with no significant contribution from shared environmental factors. Stability from age 2 to 3 was due to genetic factors, whereas change was due to both genetic and nonshared environmental influences. This genetic and nonshared environmental change was substantial, suggesting malleability of CU in early childhood. Over 50% of the genetic influences and 100% of the nonshared environmental influences on CU at age 3 were independent of those that operated at age 2. Implications of novel sources of variance across age are discussed.

Education in Twins and Their Parents Across Birth Cohorts Over 100 years: An Individual-Level Pooled Analysis of 42-Twin Cohorts.

Whether monozygotic (MZ) and dizygotic (DZ) twins differ from each other in a variety of phenotypes is important for genetic twin modeling and for inferences made from twin studies in general. We analyzed whether there were differences in individual, maternal and paternal education between MZ and DZ twins in a large pooled dataset. Information was gathered on individual education for 218,362 adult twins from 27 twin cohorts (53% females; 39% MZ twins), and on maternal and paternal education for 147,315 and 143,056 twins respectively, from 28 twin cohorts (52% females; 38% MZ twins). Together, we had information on individual or parental education from 42 twin cohorts representing 19 countries. The original education classifications were transformed to education years and analyzed using linear regression models. Overall, MZ males had 0.26 (95% CI [0.21, 0.31]) years and MZ females 0.17 (95% CI [0.12, 0.21]) years longer education than DZ twins. The zygosity difference became smaller in more recent birth cohorts for both males and females. Parental education was somewhat longer for fathers of DZ twins in cohorts born in 1990-1999 (0.16 years, 95% CI [0.08, 0.25]) and 2000 or later (0.11 years, 95% CI [0.00, 0.22]), compared with fathers of MZ twins. The results show that the years of both individual and parental education are largely similar in MZ and DZ twins. We suggest that the socio-economic differences between MZ and DZ twins are so small that inferences based upon genetic modeling of twin data are not affected.

Variant at serotonin transporter gene predicts increased imitation in toddlers: relevance to the human capacity for cumulative culture.

Cumulative culture ostensibly arises from a set of sociocognitive processes which includes high-fidelity production imitation, prosociality and group identification. The latter processes are facilitated by unconscious imitation or social mimicry. The proximate mechanisms of individual variation in imitation may thus shed light on the evolutionary history of the human capacity for cumulative culture. In humans, a genetic component to variation in the propensity for imitation is likely. A functional length polymorphism in the serotonin transporter gene, the short allele at 5HTTLPR, is associated with heightened responsiveness to the social environment as well as anatomical and activational differences in the brain's imitation circuity. Here, we evaluate whether this polymorphism contributes to variation in production imitation and social mimicry. Toddlers with the short allele at 5HTTLPR exhibit increased social mimicry and increased fidelity of demonstrated novel object manipulations. Thus, the short allele is associated with two forms of imitation that may underlie the human capacity for cumulative culture. The short allele spread relatively recently, possibly due to selection, and its frequency varies dramatically on a global scale. Diverse observations can be unified via conceptualization of 5HTTLPR as influencing the propensity to experience others' emotions, actions and sensations, potentially through the mirror mechanism.

Twin's Birth-Order Differences in Height and Body Mass Index From Birth to Old Age: A Pooled Study of 26 Twin Cohorts Participating in the CODATwins Project.

We analyzed birth order differences in means and variances of height and body mass index (BMI) in monozygotic (MZ) and dizygotic (DZ) twins from infancy to old age. The data were derived from the international CODATwins database. The total number of height and BMI measures from 0.5 to 79.5 years of age was 397,466. As expected, first-born twins had greater birth weight than second-born twins. With respect to height, first-born twins were slightly taller than second-born twins in childhood. After adjusting the results for birth weight, the birth order differences decreased and were no longer statistically significant. First-born twins had greater BMI than the second-born twins over childhood and adolescence. After adjusting the results for birth weight, birth order was still associated with BMI until 12 years of age. No interaction effect between birth order and zygosity was found. Only limited evidence was found that birth order influenced variances of height or BMI. The results were similar among boys and girls and also in MZ and DZ twins. Overall, the differences in height and BMI between first- and second-born twins were modest even in early childhood, while adjustment for birth weight reduced the birth order differences but did not remove them for BMI.

The influence of infant characteristics and attention to social cues on early vocabulary.

Individual differences in early language development are related to a number of infant characteristics and skills, including temperament traits, cognitive ability, and social cognition. The current study aimed to assess the complex mechanisms underlying these relations by evaluating the ability to use social cues in a word-learning context along with temperament, cognitive ability, and expressive vocabulary in 71 18-month-old infants. A moderated mediation analysis indicated that infants' nonverbal cognitive ability affects their early vocabulary through its effect on their ability to use social cues, but only in infants with low nonverbal cognitive ability. Implications for both typical and atypical development are discussed.

Zygosity Differences in Height and Body Mass Index of Twins From Infancy to Old Age: A Study of the CODATwins Project.

A trend toward greater body size in dizygotic (DZ) than in monozygotic (MZ) twins has been suggested by some but not all studies, and this difference may also vary by age. We analyzed zygosity differences in mean values and variances of height and body mass index (BMI) among male and female twins from infancy to old age. Data were derived from an international database of 54 twin cohorts participating in the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins), and included 842,951 height and BMI measurements from twins aged 1 to 102 years. The results showed that DZ twins were consistently taller than MZ twins, with differences of up to 2.0 cm in childhood and adolescence and up to 0.9 cm in adulthood. Similarly, a greater mean BMI of up to 0.3 kg/m2 in childhood and adolescence and up to 0.2 kg/m2 in adulthood was observed in DZ twins, although the pattern was less consistent. DZ twins presented up to 1.7% greater height and 1.9% greater BMI than MZ twins; these percentage differences were largest in middle and late childhood and decreased with age in both sexes. The variance of height was similar in MZ and DZ twins at most ages. In contrast, the variance of BMI was significantly higher in DZ than in MZ twins, particularly in childhood. In conclusion, DZ twins were generally taller and had greater BMI than MZ twins, but the differences decreased with age in both sexes.

The CODATwins Project: The Cohort Description of Collaborative Project of Development of Anthropometrical Measures in Twins to Study Macro-Environmental Variation in Genetic and Environmental Effects on Anthropometric Traits.

For over 100 years, the genetics of human anthropometric traits has attracted scientific interest. In particular, height and body mass index (BMI, calculated as kg/m2) have been under intensive genetic research. However, it is still largely unknown whether and how heritability estimates vary between human populations. Opportunities to address this question have increased recently because of the establishment of many new twin cohorts and the increasing accumulation of data in established twin cohorts. We started a new research project to analyze systematically (1) the variation of heritability estimates of height, BMI and their trajectories over the life course between birth cohorts, ethnicities and countries, and (2) to study the effects of birth-related factors, education and smoking on these anthropometric traits and whether these effects vary between twin cohorts. We identified 67 twin projects, including both monozygotic (MZ) and dizygotic (DZ) twins, using various sources. We asked for individual level data on height and weight including repeated measurements, birth related traits, background variables, education and smoking. By the end of 2014, 48 projects participated. Together, we have 893,458 height and weight measures (52% females) from 434,723 twin individuals, including 201,192 complete twin pairs (40% monozygotic, 40% same-sex dizygotic and 20% opposite-sex dizygotic) representing 22 countries. This project demonstrates that large-scale international twin studies are feasible and can promote the use of existing data for novel research purposes.