A comparison of twins and singletons with child psychiatric disorders: an Item Sheet study

Twins and Their Friendships: Differences Between Monozygotic, Dizygotic Same-Sex and Dizygotic Mixed-Sex Pairs

This study examined the influence of twin status (monozygotic [MZ], dizygotic same-sex [DZss] and dizygotic opposite-sex [DZos]) on friendship patterns. It examined the friendships of 60 children from 30 twin pairs, mean age 8 years, 4 months. The study sought to establish how number of friendships and degree to which these are shared vary according to twin status. Additionally, it sought to assess the children's meaning of shared friendship and to examine whether there were group differences according to twin status. Results indicated that, while number of friends did not vary, the degree to which friendships were shared was significantly associated with twin status. MZ children shared approximately 50% of their friends, DZss 25% and DZos 5%. Group patterns emerged from interviews with the children concerning shared friendships. MZ twins were characteristically positive or accepting about shared friends; DZos pairs shared fewer friends and were, therefore, less challenged by the twin situation in negotiating friendships; and DZss pairs were more diverse in their reaction with some expressing ambivalent or negative views about the twin situation and sharing of friendship. The results are presented in the light of their implications for twin children's developmental and scholastic progress.

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.

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.

Depression scale scores in 8-17-year-olds: effects of age and gender

BACKGROUND

The excess of unipolar depression in females emerges in adolescence. However, studies of age effects on depression scale scores have produced divergent estimates of changes from childhood to adolescence.

METHOD

We explored possible reasons for this discrepancy in two large, longitudinal samples of twins and singletons aged 8-17.

RESULTS

There were no differences between twins and singletons in their scores on the Short Mood and Feelings Questionnaire (SMFQ), a 13-item self-report depression scale. SMFQ scores for boys fell over this age-range, while those for girls fell from age 9 to age 11 and then increased from age 12 to age 17. The mean scores of girls under 12 and those 12 and over differed by only around one-fifth of a standard deviation. However, given the non-normal distribution of the scores, a cut point that selected the upper 6% of scores created the expected female:male ratio of 2:1.

CONCLUSIONS

Implications for future research on adolescent depression are discussed.

Associations between behaviour problems and verbal and nonverbal cognitive abilities and disabilities in early childhood

BACKGROUND

We investigated associations between behaviour problems and verbal and nonverbal cognitive abilities at 2, 3 and 4 years of age both for the entire distribution and for the lowest 5% and 10% of the verbal and nonverbal cognitive disabilities.

METHODS

A community sample of 4,000 pairs of twins born in England and Wales in 1994 and 1995 was assessed by their parents at 2, 3 and 4 years using the Revised Rutter Parent Scale for Preschool Children (RRPSPC, behaviour problems), the MacArthur Communicative Development Inventory (MCDI, verbal development), and the Parent Report of Children's Abilities (PARCA, nonverbal cognitive development).

RESULTS

For the entire sample, behaviour problem scores were modestly associated with lower MCDI and PARCA scores - correlations were less than .30. Similarly modest effect sizes were found for relationships between behaviour problem scores and the lowest 5% and 10% of the MCDI and of the PARCA distributions. Associations were stronger for nonverbal than for verbal development, increased from 2 to 3 to 4 years, and, at the extremes of the distributions, were stronger for boys than for girls. Multivariate genetic analyses indicated that both genetic and shared environmental factors mediate the links between behaviour problems and cognitive development both for the total distribution and for the extremes. Genetic links may be stronger for the extremes than for the total sample.

CONCLUSIONS

We conclude that, in this community sample of young children, associations between behaviour problems and verbal and nonverbal cognitive development are generally modest for the entire distribution and are no greater at the extremes than expected on the basis of the associations for the entire distribution.

Differences in younger, middle, and older children admitted to child psychiatric inpatient services

Sociodemographic, family-environmental, clinical, and service use differences among younger (3-6 years), middle (7-10 years), and older (11-16 years) children admitted to a child psychiatric inpatient service were examined. Data was collected retrospectively on 327 children using a 152-item chart abstraction form from the children's medical charts in years 1993, 1995, 1996, and 1997. The results suggest that younger children compared to their older counterparts come from poorer functioning families and that family-environmental, rather than clinical, diagnostic variables differentiate these groups. Implications of the findings for future research and practice are discussed.

Genetics and developmental psychopathology: 2. The main effects of genes and environment on behavioral problems in the Virginia Twin Study of Adolescent Behavioral Development

Little is known about the contribution of genetic and environmental factors to risk for juvenile psychopathology. The Virginia Twin Study of Adolescent Behavioral Development allows these contributions to be estimated. A population-based, unselected sample of 1412 Caucasian twin pairs aged 8-16 years was ascertained through Virginia schools. Assessment of the children involved semi-structured face-to-face interviews with both twins and both parents using the Child and Adolescent Psychiatric Assessment (CAPA). Self-report questionnaires were also completed by parents, children, and teachers. Measures assessed DSM-III-R symptoms of Attention Deficit Hyperactivity Disorder (ADHD). Conduct Disorder, Oppositional Defiant Disorder, Overanxious Disorder, Separation Anxiety, and Depressive Disorder. Factorially derived questionnaire scales were also extracted. Scores were normalized and standardized by age and sex. Maximum likelihood methods were used to estimate contributions of additive and nonadditive genetic effects, the shared and unique environment, and sibling imitation or contrast effects. Estimates were tested for heterogeneity over sexes. Generally, monozygotic (MZ) twins correlated more highly than dizygotic (DZ) twins, parental ratings more than child ratings, and questionnaire scales more highly than interviews. DZ correlations were very low for measures of ADHD and DZ variances were greater than MZ variances for these variables. Correlations sometimes differed between sexes but those for boy-girl pairs were usually similar to those for like-sex pairs. Most of the measures showed small to moderate additive genetic effects and moderate to large effects of the unique individual environment. Measures of ADHD and related constructs showed marked sibling contrast effects. Some measures of oppositional behavior and conduct disorder showed shared environmental effects. There were marked sex differences in the genetic contribution to separation anxiety, otherwise similar genetic effects appear to be expressed in boys and girls. Effects of rater biases on the genetic analysis are considered. The study supports a widespread influence of genetic factors on risk to adolescent psychopathology and suggests that the contribution of different types of social influence may vary consistently across domains of measurement.

Twin sibling differences in parental reports of ADHD, speech, reading and behaviour problems

Differences between twins and siblings in behaviour problems were investigated in a non-selected sample of 1938 families with children aged 4-12 years. Families were sent a questionnaire based on DSM-III-R criteria for Attention Deficit Hyperactivity Disorder (ADHD), Oppositional Defiant Disorder (ODD), Conduct Disorder (CD) and Separation Anxiety (SA), which was validated by formal clinical interview. The questionnaire also included measures of speech and reading problems. There were significant differences between twins and siblings for ADHD symptoms, but not for symptoms of ODD, CD or SA. Twins and siblings differed significantly for gestational age, birth weight, speech and reading problems. While there was little evidence for birth weight or gestational age contributing to the difference in ADHD symptoms, there was a strong association between ADHD symptoms and speech and reading problems.

Parental ratings of behaviour problems: a twin and general population comparison

Generalization from twin studies relies on the assumption that liability to develop the disorder which is studied is influenced by similar factors in twins and singletons. The present study examines the impact of pre- and perinatal factors on parental reports of behaviour problems in 1832 twins, and compares level of attention problems, internalizing and externalizing behaviour in the twins with 723 children and adolescents from a general population sample. The modest influence of relative birth weight and birth order on level of attention problems in twins did not contribute to significant differences between twins and the general population sample. The levels of attention problems and externalizing behaviour was similar and there was a modest age dependent difference in internalizing behaviour. An increased variance was found for externalizing behaviour in twins.

A twin-singleton comparison of problem behaviour in 2-3-year-olds

Twin-singleton differences in problem behaviours in 2-3-year-olds were studied. Maternal ratings of children's problem behaviours were obtained with the CBCL/2-3. The twin sample consisted of 1363 twin pairs (456 MZ, 907 DZ), the sample of singletons consisted of 420 children from the general population. Results indicated that the general level of problem behaviours in twins was broadly comparable to that in singletons. Four of the seven syndromes showed lower scores for twins. These differences, however, were small and mainly caused by lower scores for DZ twins in comparison to MZ twins and singletons. Part of the difference could be attributed to the higher maternal age in the twin groups. Higher means for boys were found for the total problem score, and the Aggressive and Overactive syndromes.

Antisocial behaviour and spelling disability in a population sample of 13 year old twins

The aetiology of the co-morbidity of specific spelling retardation and antisocial behaviour was investigated in a general population sample of 213 pairs of 13 year old same-sex twins. The rate of co-morbidity was 2.45 greater than that expected by chance. Using multiple regression it was shown that the heritability (h_g²) of spelling disability was high whilst that for antisocial behaviour was not significant. A bivariate h_g² analysis showed that the co-morbidity was not due to common genetic factors. The data were not consistent with either one condition leading to the other nor with each resulting from a common origin linked with hyperactivity. The co-morbid children resembled the children with antisocial behaviour alone on family and social characteristics and the heritability of spelling disability is as high when co-morbid with antisocial behaviour as it is when occurring alone. It is concluded that co-morbidity arises by the genetic factors leading to spelling disability being associated with environmental factors leading to antisocial behaviour. The mechanisms are discussed whereby fathers genetically predisposed to spelling disability might create an adverse environment leading in some cases to antisocial behaviour in their children.