The La Trobe Twin Study: a genetic approach to the structure and development of cognition in twin children

Twins are particularly useful for developmental behavior genetics, if adequate consideration is given to the possibilities of twin-singleton differences and of data from twins being unrepresentative of the entire population. As an alternative to existing analyses in the developmental behavior genetics of cognition, the covariance structure analysis method in biometrical genetics involving twins and other relatives has the potential for dissecting the determinants of the structure of cognitive abilities and how these change during childhood, as well as checking on the adequacy of twin data. Designed with these aims in mind, the La Trobe Twin Study is a mixed longitudinal analysis of 1,356 twins, their siblings, and cousins, who are followed between the ages of 3 and 15 years on a battery of physical and behavioral tests. The criteria for choosing behavioral tests in such a project are discussed, along with the comprehensive series of questionnaires necessary to examine the children's development in the context of the stresses a multiple birth may impose on a family. In discussion of methods of analysis, 2 points are emphasized: (1) Twin-singleton differences are specific to particular abilities rather than being general, and (2) the incidence of family changes and stresses in such that these should be considered among the environmental effects on development along with the more usual indices of family environment.     

Continuity in mental development from infancy

In this essay we document moderate continuity in mental development beginning in infancy and extending into childhood. Psychological opinion in the past has tended to favor discontinuity theories of cognitive development from infancy. In recent years, however, the foundations on which discontinuity positions were originally established have themselves come under question and new findings grounded in new assessment procedures have appeared, necessitating revision of opinion on this significant psychological and developmental issue. Our essay has several aims. We first review briefly the bases for contemporary discontinuity theories of mental development. Second, we present current findings that support the alternative proposition of continuity: Recent research demonstrates that infants who more efficiently encode visual stimuli or more efficiently recollect visual or auditory stimuli tend to perform more proficiently on traditional psychometric assessments of intelligence and language during childhood. Third, we scrutinize the assessment methods from which these continuity results derive. Fourth, we offer several models that help to explain the continuity findings. Fifth, we discuss critically the origins and the maintenance of continuity in mental development as it is coming to be conceptualized currently. Finally, we reflect on implications of continuity for the future of infant assessments specifically and for theories of early mental development generally.     

The development of interpersonal aggression during adolescence: the importance of parents, siblings, and family economics

Latent growth curve modeling employed data from a longitudinal study of 451 sibling families to examine parents, siblings, and family economics as factors in individual differences in the developmental course of interpersonal aggression during adolescence. Findings suggest that individual change in interpersonal aggression during adolescence can be predicted by the gender and aggression of one's sibling; predictions varied by the gender composition of the sibling dyad. Rates of parental hostility predicted levels of interpersonal aggression for both older (mean age = 12 years) and younger siblings (mean age = 15), and growth in aggression for younger siblings. Family economic pressure predicted interpersonal aggression of both siblings indirectly through parental hostility. Implications for future research and preventive interventions are discussed.     

Systems and Cascades in Cognitive Development and Academic Achievement

A large‐scale (N = 552) controlled multivariate prospective 14‐year longitudinal study of a developmental cascade embedded in a developmental system showed that information‐processing efficiency in infancy (4 months), general mental development in toddlerhood (18 months), behavior difficulties in early childhood (36 months), psychometric intelligence in middle childhood (8 years), and maternal education either directly or indirectly (or both) contribute to academic achievement in adolescence (14 years).     

The plasticity of intellectual development: insights from preventive intervention

Debates regarding the plasticity of intelligence are often fired by a confusion between 2 distinct realms of development, that is, between developmental functions (e.g., a group's average IQ over time) and individual differences (e.g., the relative rank ordering of individual IQs within a group). Questions concerning the stability of these 2 realms are statistically independent. Thus there are 2 kinds of intellectual plasticity, and there may be no developmental convergences between them. In the present study, data from an early intervention program were used to investigate the 2 kinds of plasticity separately and to examine certain possible convergences between them. The program involved children at risk for developmental retardation who were randomly assigned at birth to 2 rearing conditions (i.e., educational daycare vs. no educational intervention) and whose intellectual development was then studied longitudinally to 4 years of age. Our findings indicate that developmental functions are moderately alterable through systemic early education, particularly after infancy, whereas individual differences are moderately stable, again particularly after infancy. They also indicate that the 2 kinds of plasticity are independent; the alteration of developmental functions through daycare affects neither the stability nor the determinants of individual differences. We discuss the implications that these findings have for current models of mental development, for the nature-nurture debate, and for arguments concerning the efficacy of early intervention programs.     

Effect of Twinning on Chinese and English Vocabulary Knowledge

Vocabulary knowledge was tested in a native (Cantonese-Chinese) and foreign (English) language in 150 twins and 150 singletons aged 6–11 years, matched on age, gender, grade level, nonverbal intelligence, parents’ education, family income, and number of siblings and household members. The singletons clearly outperformed the twins on the native vocabulary, but this “twinning effect” was much less noticeable for the foreign vocabulary. The effect on English vocabulary was further reduced after exposure to English at home was controlled. Given that these participants learned most of their English in school rather than home, the present findings support the notion that the twinning effect is associated with increased competition for family interaction in twins compared with singletons.     

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.     

The stability of attachment security from infancy to adolescence and early adulthood: general discussion

For over three decades, critics of the developmental and psychometric paradigms have argued that individual differences are neither stable, coherent, nor clinically significant. The present studies extend a long line of research demonstrating the coherence of individual development in attachment security. They make it clear that attachment security can be stable from infancy through early adulthood and that change in attachment security is meaningfully related to changes in the family environment. The task now is to better understand the roles of cross-age consistency in caregiver behavior and the structure of mental representations of early experience in stability and change.     

Risk of maltreatment for siblings: Factors associated with similar and different childhood experiences in a dyadic sample of adult siblings

Siblings share the same environment and thus potentially a substantial number of risk factors for child maltreatment. Furthermore, the number of siblings and the sibling constellation itself might pose a risk for child maltreatment. Little is known about the likelihood that more than one child in a family is maltreated and which factors increase the risk. This study sought to investigate similarities and differences in maltreatment in siblings and risk factors associated with the maltreatment of more than one child from the same family. Data on maltreatment during childhood and adolescence, family background, and sibling constellation were collected from 870 pairs of siblings. In the dyadic analyses, siblings reported similar maltreatment experiences, especially when any type of maltreatment was considered. Parents’ mental health problems were significant predictors for maltreatment of at least one sibling. Father’s mental health problems were predictive of maltreatment of both or only the younger sibling, mother’s mental health problems of both or only the older sibling. Closeness in age and same gender of siblings did not emerge as a consistent predictor. The increasing number of siblings was a risk factor for any type of maltreatment of both siblings. The results highlight the need for preventive measures for families with a large number of children and with parents with mental health problems as well as a repeated risk assessment of all siblings in a family when one sibling was maltreated.     

Opportunities for Neurodevelopmental Plasticity From Infancy Through Early Adulthood

Multiple and rapid changes in brain development occur in infancy and early childhood that undergird behavioral development in core domains. The period of adolescence also carries a second influx of growth and change in the brain to support the unique developmental tasks of adolescence. This special section documents two core conclusions from multiple studies. First, evidence for change in brain‐based metrics that underlie cognitive and behavioral functions are not limited to narrow windows in development, but are evident from infancy into early adulthood. Second, the specific evident changes are unique to challenges and goals that are salient for a respective developmental period. These brain‐based changes interface with environmental inputs, whether from the child's broader ecology or at an individual level.     

Sibling differentiation: sibling and parent relationship trajectories in adolescence

Studied here were the links between sibling differences in trajectories of change in the qualities of parent-child relationships and the qualities of sibling relationships across a 2-year period in adolescence. Participants were first- and second-born siblings (M age = 14.94 years for firstborns and M age = 12.46 years for secondborns) from 185 predominantly White, working and middle-class families. In home interviews, siblings reported on their dyadic family relationships. For reports of parent-child warmth but not parent-child conflict, results were consistent with sibling differentiation theory: Increasing differences between siblings over time in parent-child warmth were linked to trajectories of increasing warmth and decreasing conflict in the sibling relationship as reported by firstborns, and increasing warmth in the sibling relationship as reported by secondborns. The findings support the view that sibling differentiation may be a strategy for managing sibling conflict and rivalry.     

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.     

Individual Differences in Young Children's Pretend Play with Mother and Sibling: Links to Relationships and Understanding of Other People's Feelings and Beliefs

50 33-month-old children were observed at home with their siblings and mothers. Observational measures of pretend play, observer ratings of the child's, mother's, and sibling's behavior, and measures of family discourse about feelings were collected. At 40 months each child was assessed on Bartsch and Wellman's false beliefs task and Denham's affective perspective taking task. Results revealed individual differences in the amount and sophistication of young children's social pretend play and suggested that these individual differences are related to experiences in the relationships that young children have with their mothers and siblings. Results also indicated that early social pretend play was significantly related to the child's developing understanding of other people's feelings and beliefs. The data are interpreted as providing support for the notion that early experience in social pretense is associated with children's mastery of the relation between mental life and real life. The importance of considering the relationship context of social pretense is also discussed.     

Evolutionary developmental psychology and socio-emotional development

Abstract

Evolutionary developmental psychology is relevant to understanding many aspects of socio-emotional development in infancy, childhood and adolescence. This article focuses on parental investment and paternity certainty, child abuse, grandparenting, and parent-offspring conflict; sibling relationships and theory of mind; children's peer groups, especially dominance, aggression, sex differences and pretend and rough-and-tumble play; and on a model linking early family experiences to peer behaviour, age of puberty and reproductive behaviour. The review illustrates some of the potential of evolutionary developmental psychology in providing a comprehensive framework for understanding many aspects of social behaviour, and of providing testable hypotheses. Evolutionary explanations should be seen as compatible with cultural and environmental explanations, that may provide the proximal mechanisms by which the more distal functions of these behaviours are typically realised in actual developmental process and experience     

The developmental trajectory of Borderline personality disorder and peer victimisation: Australian family carers’ perspectives

Victimisation is a traumatic experience linked to development of Borderline personality disorder (BPD). However, there is limited research investigating the developmental journey prior to BPD diagnosis. School environments offer an opportunity for BPD prevention and early intervention. A survey with 19 Australian family carers of people with BPD asked what they noticed during the person’s infancy, childhood and adolescence, and their experiences of seeking help during that time. Sensitivity was most noticeable during infancy; difficulty making friends, school refusal/truancy and being bullied were prominent concerns once the child was at school; and concerns about anger, moodiness and impulsivity were the strongest prompts to seek help during adolescence, though doctor or teacher recommendation to do so was uncommon. BPD family carers’ experiences suggest that improved focus on addressing bullying and communication is needed.     

Time With Peers From Middle Childhood to Late Adolescence: Developmental Course and Adjustment Correlates

This study examined the developmental course and adjustment correlates of time with peers from age 8 to 18. On seven occasions over 8 years, the two eldest siblings from 201 European American, working‐ and middle‐class families provided questionnaire and/or phone diary data. Multilevel models revealed that girls' time with mixed‐/opposite‐sex peers increased beginning in middle childhood, but boys' time increased beginning in early adolescence. For both girls and boys, time with same‐sex peers peaked in middle adolescence. At the within‐person level, unsupervised time with mixed‐/opposite‐sex peers longitudinally predicted problem behaviors and depressive symptoms, and supervised time with mixed‐/opposite‐sex peers longitudinally predicted better school performance. Findings highlight the importance of social context in understanding peer involvement and its implications for youth development.     

Developmental behavioral genetics

Developmental behavioral genetics is the study of genetic and environmental influences on individual differences in behavioral development. The interdiscipline offers exciting possibilities for research in both child development and behavioral genetics. In this prelude to a special section on developmental behavioral genetics, the potentialities of the interdiscipline are discussed and an overview of the articles that follow is presented.     

Why Talk about Mental States? The Significance of Children's Conversations with Friends, Siblings, and Mothers

Natural language data from 38 47-month-olds recorded at home in unstructured observations were analyzed and comparisons made of characteristics of mental state term use in child-friend, child-sibling, and child-mother dyads. Significantly more references to mental states were made by the children in conversations with siblings and friends than with mothers. Frequent use of mental state terms by both partners was related to cooperative interaction in both child-friend and child-sibling dyads and several associations were found with measures of language fluency, gender, and maternal education, although these varied across the 2 dyads. Children's use of mental state terms in conversations with siblings and friends was correlated with their performance on two false belief measures. Results highlight the importance of extending investigations into the social implications of the development of children's "theories of mind."     

A Longitudinal Study of Consistency and Change in Self-Esteem from Early Adolescence to Early Adulthood

In a longitudinal study of 47 girls and 44 boys, developmental change in self-esteem (SE) was examined from early adolescence through late adolescence to early adulthood. Males tended to increase and females tended to decrease in SE over time. There was appreciable rank-order consistency in SE over time. Within each gender, the considerable individual differences in developmental trajectories were coherently related to personality characteristics independently assessed in early adolescence. Boys and girls with high SE possessed quite different personality characteristics in early adolescence; by early adulthood, although important differences remained, the personality characteristics associated with high SE were similar for the 2 sexes. Discussion focuses on the implications of our findings for the "consistency versus change" debate, the influence of gender-role socialization on SE development, and the importance of examining normative, gender-specific, and individual developmental change in SE.     

Family size,birth order,and intelligence in a large South American sample

The confluence theory, which hypothesizes a relationship between intellectual development birth order, and family size, was examined in a colombian study of more than 36,000 college applicants. The results of the study did not support the confluence theory. The confluence theory states that the intellectual development of a child is related to average mental age of the members of his family at the time of his birth. The mental age of the parents is always assigned a value of 30 and siblings are given scores equivalent to their chronological age at the birth of the subject. Therefore, the average mental age of family members for a 1st born child is 30, or 60 divided by 2. If a subject is born into a family consisting of 2 parents and a 6-year old sibling, the average mental age of family members tends, therefore, to decrease with each birth order. The hypothesis derived from the confluence theory states that there is a positive relationship between average mental age of a subject's family and the subject's performance on intelligence tests. In the Colombian study, data on family size, birth order and socioeconomic status was derived from college application forms. Intelligence test scores for each subject was obtained from college entrance exams. The mental age of each applicant's family at the time of the applicant's birth was calculated. Multiple correlation analysis and path analysis were used to assess the relationship. Results were 1) the test scores of subjects from families with 2,3,4, and 5 children were higher than test scores of the 1st born subjects; 2) the rank order of intelligence by family size was 3,4,5,2,6,1 instead of the hypothesized 1,2,3,4,5,6; and 3) only 1% of the variability in test scores was explained by the variables of birth order and family size. Further analysis indicated that socioeconomic status was a far more powerful explanatory variable than family size.