Sibling differentiation in adolescence: implications for behavioral genetic theory

The presence of sibling "differentiating processes"--defined as processes in which increased sibling similarity in environmental or genetic factors leads to differences in sibling outcomes-poses a challenge for standard behavioral genetic theory and research. The presence of differentiation processes may affect estimates of genetic and environmental parameters in ways that have not been fully recognized. Utilizing data from the Nonshared Environment and Adolescent Development project, this study examined whether differentiating processes existed for seven composite indices of positive and negative adolescent adjustment. The 720 sibling pairs in the study were broken down into groups by age difference (0-4 years) between siblings. The hypothesis that siblings close in age would demonstrate lower correlations on adjustment measures was generally supported at two time points, three years apart. However, siblings one year apart at Time 1 were more similar to each other than were siblings two years apart, suggesting that shared environmental influences counteract sibling differentiation processes for these siblings. The overall trend supporting sibling differentiation was found to be unrelated to measures of sibling positivity and negativity.     

Genetic and environmental influences on prereading skills and early reading and spelling development in the United States, Australia, and Scandinavia

Genetic and environmental influences on prereading skills in preschool and on early reading and spelling development at the end of kindergarten were compared among samples of identical and fraternal twins from the U.S. (Colorado), Australia, and Scandinavia. Mean comparisons revealed significantly lower preschool print knowledge in Scandinavia, consistent with the relatively lower amount of shared book reading and letter-based activities with parents, and lack of emphasis on print knowledge in Scandinavian preschools. The patterns of correlations between all preschool environment measures and prereading skills within the samples were remarkably similar, as were the patterns of genetic, shared environment, and non-shared environment estimates: in all samples, genetic influence was substantial and shared environment influence was relatively weak for phonological awareness, rapid naming, and verbal memory; genetic influence was weak, and shared environment influence was relatively strong for vocabulary and print knowledge. In contrast, for reading and spelling assessed at the end of kindergarten in the Australian and U.S. samples, there was some preliminary evidence for country differences in the magnitude of genetic and environmental influences. We argue that the apparently higher genetic and lower shared environment influence in the Australian sample was related to a greater emphasis on formal reading instruction, resulting in more advanced reading and spelling skills at the end of kindergarten, and thus there was greater opportunity to observe genetic influences on response to systematic reading instruction among the Australian twins.     

Longitudinal twin study of early reading development in three countries: Preliminary results

We have initiated parallel longitudinal studies in Australia (Byrne, PI), the United States (Olson, PI), and Norway (Samuelsson, PI) of identical and fraternal twins who are being tested in preschool for prereading skills, and in kindergarten, first grade, and second grade for the development of early reading, spelling, and related cognitive skills. Comparisons of the similarities of identical and fraternal twins will reveal the relative influence of genetic, shared family environment, and nonshared environment on individual differences at and across different stages of development. Family and twin-specific environmental information is also being directly assessed through parent questionnaires and observations by testers. Most of the data collected so far have been from preschool twins (146 in Australia, 284 in the United States, and 70 in Norway). Preliminary analyses for the preschool cognitive measures showed reliable genetic influences on phonological awareness and several measures of memory and learning. In contrast, vocabulary, grammar, and morphology showed significant shared environment and negligible genetic effects. A print knowledge composite showed both genetic and shared environment influence.     

Genetic and environmental influences on sex-typed behavior during the preschool years

The genetic and environmental etiologies of sex-typed behavior were examined during the preschool years in a sample of 3,990 three- to four-year-old twin and non-twin sibling pairs. Results showed moderate genetic and significant shared environmental influence for boys and substantial genetic and moderate shared environmental influence for girls. For both boys and girls, twin-specific shared environmental effects contributed to twins' similarity in gender role behavior and accounted for approximately 22% of the shared environmental variance. These findings extend previous research conducted with older samples by showing not only important genetic contributions to gender role behavior but also an important role for shared environment. The inclusion of non-twin siblings showed that some of the shared environmental influence is specific to twins.     

An adoption study of the etiology of teacher and parent reports of externalizing behavior problems in middle childhood

Twin studies of externalizing behavior problems in middle childhood and early adolescence suggest that there is moderate-to-substantial genetic variance and modest-to-moderate shared environmental variance in these behaviors. The present study is novel in three ways: it is a sibling adoption study, it employs both teacher and parent ratings of children's behaviors averaged over five assessments, and it explores aggression and delinquency separately. The sample included 78 adoptive sibling pairs and 94 biologically related sibling pairs in the Colorado Adoption Project. Parents and teachers completed ratings of the children's externalizing behavior problems at ages 7, 9, 10, 11, and 12 years. Boys and adopted children were rated as being somewhat higher in externalizing behavior problems. Sex differences in delinquency were more pronounced in adoptive than in nonadoptive families. Teachers' ratings showed evidence for moderate heritability and no shared environment for aggression and showed modest shared environment for delinquency. Parents' ratings showed evidence for moderate amounts of heritability and shared environment for both aggression and delinquency.     

Peer Network Overlap in Twin,Sibling, and Friend Dyads

Research suggests that sibling–peer connections are important for understanding adolescent problem behaviors. Using a novel behavioral genetic design, the current study investigated peer network overlap in 300 child–child pairs (aged 7–13 years) in 5 dyad types: monozygotic (MZ), dizygotic twins, full siblings (FSs), friend pairs, and virtual twins (i.e., same‐aged, genetically unrelated siblings). Genetic relatedness, sex composition, and age differences contributed to peer overlap in sibling dyads. MZ twins showed the highest overlap (82%), opposite‐sex FS pairs showed the lowest overlap (27%), and friend pairs (48%) were close to the mean (53%). Social contact variables and self‐reported relationship intimacy predicted additional variance in peer overlap. The roles of genotype–environment correlational and shared environmental processes in the sibling–peer connections are discussed.     

Perceived competence and self-worth during adolescence: a longitudinal behavioral genetic study

This investigation is the first longitudinal behavioral genetic study of self-concept during adolescence. It is a follow-up of a previous study examining genetic and environmental contributions to children's perceived self-competence and self-worth using a twin/sibling design. The study investigated adolescents' reports 3 years later and stability across two time points. Participants included 248 pairs of same-sex twins, full siblings, and stepsiblings between 10 and 18 years old. The results showed that six of the seven subscales were heritable at the second time point. None of the scales showed significant shared environmental effects. Longitudinal analyses revealed genetic contributions to stability for perceived scholastic competence, athletic competence, physical appearance, and general self-worth. Social competence, on the other hand, showed nonshared environmental mediation across time. These findings highlight the importance of genetically influenced characteristics and unique experiences as correlates of individual differences in self-concept during adolescence.     

Genetic and Environmental Influences on Vocabulary IQ: Parental Education Level as Moderator

This article examines how parental education level moderates the genetic and environmental contributions to variation in verbal IQ. Data are from 1909 non-Hispanic Whites and African American sibling pairs from the National Longitudinal Study of Adolescent Health, which obtained nationally-based samples of identical (MZ) twins, fraternal (DZ) twins, full and half siblings, cousins (in the same household), and biologically unrelated siblings. In the whole sample, the variance estimate for heritability (h2 = .57, SE = .08) was greater than that for shared environment (c2 = .13, SE = .04). Both heritability and the shared environmental estimate were moderated, however, by level of parental education. Specifically, among more highly educated families, the average h2 = .74 (SE = .10) and the average c2 = .00 (SE = .05). Conversely, among less well-educated families, heritability decreased and shared environmental influences increased, yielding similar proportions of variance explained by genetic and environmental factors, average h2 = .26 (SE = .15), and average c2 = .23 (SE = .07).     

A biometrical analysis of perceptions of family environment: a study of twin and singleton sibling kinships

Biometrical genetic analysis was applied to sibling and twin kinship data on 2 dimensions of perceived home environment. Correlations on 1 dimension, Restrictiveness-Permissiveness, were equal and significant for all kinships: MZ twins, DZ twins, same-sex siblings, and opposite-sex siblings significant for all 4 kinships: MZ twins, DZ twins, same-sex siblings, and opposite-sex siblings (r greater than .40). An E2-E1 biometrical model fitted Restrictiveness-Permissiveness, implying that treatments common to siblings create agreement about perceived environment. As intrapair differences were the same for all 4 kinships under this model, the equal environments assumption of the twin method was supported. In contrast, the Acceptance-Rejection dimension fitted a G-E1 model that makes the assumption that sibling similarity is the result of genetic factors and postulates an absence of shared environmental influences. This finding suggests that this aspect of home environment may depend as much on the child's inherited traits as on actual treatments and is in accord with the genetic analysis of individual traits in that developmentally effective environmental factors do not appear to be common to siblings.     

Behavior problems among children from different family structures: the influence of genetic self-selection

To examine both genetic and environmental influences on children's behavior problems in households defined by marital status and sibling relatedness, this study applied behavioral genetic methodology to four groups totalling 1524 sibling pairs drawn from 796 households: (1) two-parent full siblings, (2) two-parent half siblings, (3) mother-only full siblings, and (4) mother-only half siblings. Model-fitting procedures found that within-group variation on four subscales from the Behavior Problems Index was best explained by a model including both genetic and shared environmental factors. This model was then fit to the behavior problems means of the four groups. Its successful fit to these mean structures suggested that mean-level differences between groups were explained with the same influences that accounted for within-group variation. Genetic influences accounted for 81% to 94% of the mean-level difference in behavior problems between the two-parent, full sibling and the mother-only, half sibling groups. In contrast, shared environmental influences accounted for 67% to 88% of the mean-level difference in behavior problems between the two-parent, full sibling and mother-only, full sibling groups. The genetic influences are interpreted in terms of genetic self-selection into family structures.     

Genetic and environmental etiologies of reading difficulties: DeFries-Fulker analysis of reading performance data from twin pairs and their nontwin siblings

Reading performance data from 254 pairs of identical (MZ) and 420 pairs of fraternal (DZ) twins, 8.0 to 20.0 years of age, were subjected to multiple regression analyses. An extension of the DeFries-Fulker (DF) analysis (DeFries & Fulker, 1985, 1988) that facilitated inclusion of data from 303 of their nontwin siblings was employed. In addition to providing estimates of heritability, this analysis yields a test of the difference between shared environmental influences for twins versus siblings (Astrom et al., 2011). Results suggest that proband reading deficits are due substantially to genetic factors (.67 ± .07, p < .001), and that shared environmental influences are significantly higher for members of twin pairs than for those of twins and their nontwin siblings (viz., .25 versus .17, p = .02).     

Genetic and Environmental Influences on Perceptions of Self-Worth and Competence in Adolescence: A Study of Twins, Full Siblings, and Step-Siblings

Although it is generally assumed that the origins of adolescents' perceptions of self-competence lie in shared family environmental influences, the contributions of nonshared environmental or genetic influences have not been explored. We investigated sibling resemblance for perceived competence and self-worth in 720 adolescent pairs aged 10 to 18 years, using a twin, full sibling, and step-sibling design. Our goals were to assess the magnitude of shared and nonshared environmental influences and to disentangle resemblance due to shared genetic heritage from that due to shared environmental experiences. Shared environment was not significant for any of the scales. 4 of the subscales showed significant genetic influence: scholastic, social, physical, and athletic competence. We also explored possible sources of genetic influences on perceived competence. Bivariate models revealed common genetic variance between scholastic competence and vocabulary and social competence and sociability. These measures, however, did not account for all of the genetic variance in perceived social and scholastic competence.     

Examining genetic and environmental effects on social aggression: a study of 6-year-old twins

Using a genetic design of 234 six-year-old twins, this study examined (a) the contribution of genes and environment to social versus physical aggression, and (b) whether the correlation between social and physical aggression can be explained by similar genetic or environmental factors or by a directional link between the phenotypes. For social aggression, substantial (shared and unique) environmental effects but only weak genetic effects were found. For physical aggression, significant effects of genes and unique environment were found. Bivariate modeling suggests that social and physical aggression share most of their underlying genes but only very few overlapping environmental factors. The correlation between the two phenotypes can also be explained by a directional effect from physical to social aggression.     

Mathematical ability of 10-year-old boys and girls: genetic and environmental etiology of typical and low performance

The genetic and environmental etiologies of 3 aspects of low mathematical performance (math disability) and the full range of variability (math ability) were compared for boys and girls in a sample of 5,348 children age 10 years (members of 2,674 pairs of same-sex and opposite-sex twins) from the United Kingdom (UK). The measures, which we developed for Web-based testing, included problems from 3 domains of mathematics taught as part of the UK National Curriculum. Using quantitative genetic model-fitting analyses, similar results were found for math disabilities and abilities for all 3 measures: Moderate genetic influence and environmental influence were mainly due to nonshared environmental factors that were unique to the individual, with little influence from shared environment. No sex differences were found in the etiologies of math abilities and disabilities. We conclude that low mathematical performance is the quantitative extreme of the same genetic and environmental factors responsible for variation throughout the distribution.     

Adolescent, parent, and observer perceptions of parenting: genetic and environmental influences on shared and distinct perceptions

Agreement among reporters on features of family life, whether family members or outside observers, is considered to be low. This study, which involved a national sample of 720 families comprised of identical and fraternal twins, full siblings, half siblings, and biologically unrelated stepsiblings, examined the issue of low interrater agreement by decomposing the common and unique variance among parent, child, and observer reports of parenting behaviors (warmth and negativity) into genetic and environmental factors. Quantitative genetic analyses were employed to decompose the "Social" level of perception (common variance among parents, children, observers), the level of "Family" subculture (common variance only among parents and children), and the unique "Individual" level into genetic and environmental components. It was predicted that genetic factors would account for substantial portions of the variance at the Social and Family levels; nonshared environmental factors were expected to influence variance unique to child reports; and shared environmental factors were expected to influence variance unique to parent reports. A second and related aim of the study was to examine the subjective-objective dimension of genetic effects on measures of the environment. Results of model-fitting analyses generally supported the predictions for parental warmth and negativity at the Family and Individual levels. At the Social level, genetic factors were predominant for parental negativity and shared environmental factors for parental warmth. The findings are discussed in terms of genetically influenced child effects on parenting and methodological difficulties in constructing latent variables.     

Genetic Influences on Perceptions of Childhood Family Environment: A Reared Apart Twin Study

This study estimates the extent to which heredity influences perceptions of childhood family environment in a sample of 58 monozygotic and 46 dizygotic pairs of adult twins who were reared apart. The measures used to assess family environments were the Family Environment Scale (FES) and Block Environmental Questionnaire (BEQ). A principal component factor analysis with a VARIMAX rotation of the FES and BEQ yielded 2 major factors—Support, and Organization and Cultural Orientation. Single and multiple indicator model-fitting techniques were applied to the reared apart twin data on the 2 factors. Perceived support in childhood family environments was fitted best by a model incorporating additive genetic and unshared environmental factors. Perceived organization was fitted most adequately by a model which includes only unshared environmental factors. Maximum-likelihood estimates of heritability from model-fitting analyses suggest that genetic factors explain 44% of the variance of perceptions of support dimension in childhood family environments.     

The genetic-environmental etiology of cognitive school readiness and later academic achievement in early childhood

Using a genetic design of 840 60-month-old twins, this study investigated the genetic and environmental contributions to (a) individual differences in four components of cognitive school readiness, (b) the general ability underlying these four components, and (c) the predictive association between school readiness and school achievement. Results revealed that the contribution of the shared environment for cognitive school readiness was substantial. Genetic effects were more important for the core abilities underlying school readiness than for each specific skill, although shared environment remained the largest factor overall. Genetic, shared, and nonshared environmental factors all accounted for the predictive association between school readiness and early school achievement. These results contribute to a better understanding of the early determinants of school readiness.     

Genetic and environmental influences on early literacy

Prereading and early reading skills of preschool twin children in Australia, Scandinavia and the United States were explored in a genetically sensitive design (max. N=627 preschool pairs and 422 kindergarten pairs). Analyses indicated a strong genetic influence on preschool phonological awareness, rapid naming and verbal memory. Print awareness, vocabulary and grammar/morphology were subject primarily to shared environment effects. There were significant genetic and shared environment correlations among the preschool traits. Kindergarten reading, phonological awareness and rapid naming were primarily affected by genes, and spelling was equally affected by genes and shared environment. Multivariate analyses revealed genetic and environmental overlap and independence among kindergarten variables. Longitudinal analyses showed genetic continuity as well as change in phonological awareness and rapid naming across the 2 years. Relations among the preschool variables of print awareness, phonological awareness and rapid naming and kindergarten reading were also explored in longitudinal analyses. Educational implications are discussed.     

Evidence for Common Etiological Influences on Early Literacy Skills in Kindergarten

Understanding how the etiology of print awareness and phonological awareness are related to the etiology of decoding can provide insights into the development of word reading. To address this issue, we examined the degree of overlap among etiological influences of prereading skills in 1,252 twin pairs in kindergarten. Genetic, shared environmental, and nonshared environmental factors were significant for all three literacy phenotypes. The majority of genetic and shared environmental influence on decoding was due to common factors that included print awareness and phonological awareness. Notably, only a single genetic factor contributed to all three literacy phenotypes, but there was additional shared environmental influence common to phonological awareness and decoding. Findings suggest commonalities in the etiology of prereading literacy skills that could inform work on the development of reading skill.     

Genetic and environmental influences on writing and their relations to language and reading

Identical and fraternal twins (N?=?540, age 8 to 18 years) were tested on three different measures of writing (Woodcock-Johnson III Tests of Achievement—Writing Samples and Writing Fluency; Handwriting Copy from the Group Diagnostic Reading and Aptitude Achievement Tests), three different language skills (phonological awareness, rapid naming, and vocabulary), and three different reading skills (word recognition, spelling, and reading comprehension). Substantial genetic influence was found on two of the writing measures, writing samples and handwriting copy, and all of the language and reading measures. Shared environment influences were generally not significant, except for Vocabulary. Non-shared environment estimates, including measurement error, were significant for all variables. Genetic influences among the writing measures were significantly correlated (highest between the speeded measures writing fluency and handwriting copy), but there were also significant independent genetic influences between copy and samples and between fluency and samples. Genetic influences on writing were significantly correlated with genetic influences on all of the language and reading skills, but significant independent genetic influences were also found for copy and samples, whose genetic correlations were significantly less than 1.0 with the reading and language skills. The genetic correlations varied significantly in strength depending on the overlap between the writing, language, and reading task demands. We discuss implications of our results for education, limitations of the study, and new directions for research on writing and its relations to language and reading.