Zygosity diagnosis in young twins by parental report

Young Netherlands Twin Register (Y-NTR): A Longitudinal Multiple Informant Study of Problem Behavior

The Netherlands Twin Register (NTR) was established around 1987 at the Vrije Universiteit in Amsterdam, the Netherlands. The current article summarizes the longitudinal genetic analyses of maternal and paternal ratings of twins' behavior as a function of the sex of the children for the traits of aggression (AGG), attention problems (AP), anxious/depression (ANX), internalizing behavior (INT) and externalizing behavior (EXT). We found that genetic influences are the most important factor in explaining individual differences in these traits. For most phenotypes, influences of genetic factors fluctuate throughout development, with the exception of AP, for which genetic influences remain of similar magnitude. Changes in genetic influences parallel those in shared environmental influences, while nonshared environmental influences remain relatively constant. Around 10% to 20% of the variance is accounted for by parent-specific shared environment, which includes rater bias. For all phenotypes, stability throughout childhood is accounted for by genetic and shared environmental factors, while nonshared environmental influences are mainly age/measurement specific. About 15% of the phenotypic stability is accounted for by rater-specific shared environmental influences, which include rater bias. In conclusion, between ages 3 and 12 genetic factors are the most important cause of individual differences in emotional and behavioral problems.

Influences on Achieving Motor Milestones: A Twin–Singleton Study

In order to determine if twinning impacted achievement of motor milestones the attainment of early motor milestones in twins was examined and compared to published data from singletons of the same age from the same culture and birth years. We examined the influence of twinning, sex, zygosity and birth cohort (1987–2001) on the motor development of twins aged 0 to 24 months. Data on the attainment of motor milestones (turn, sit, crawl, stand and walk) of twins were collected from maternal reports. All data were corrected for gestational age. Data from the twin sample were compared to normative data from singletons, which were available from Child Health Clinics (CHC). Analyses across twin data and the CHC singleton data revealed no differences between twins and singletons in achievement of motor milestones. Girls were able to sit without support slightly earlier than boys, otherwise there were no other sex differences. Birth-order analyses revealed minimal but significant differences in turning over from back to belly and for sitting without support between the first- and second-born. Dizygotic (DZ) twins were faster than monozygotic (MZ) twins in achieving the moment of sit, crawl, stand and walk. Twins born in earlier cohorts were faster in reaching the moment of crawl, sit and walk. It is concluded that there are no differences in time of reaching motor milestones between twins and singletons within the normal range. Sex has minimal to no effect on motor development in early childhood. DZ twins achieve motor milestones sooner than MZ twins. Attainment of gross motor milestones (crawl, stand and walk) is delayed in later birth cohorts.

A Comparison of Twin Birthweight Data From Australia, the Netherlands, the United States, Japan, and South Korea: Are Genetic and Environmental Variations in Birthweight Similar in Caucasians and East Asians?

Birthweight has implications for physical and mental health in later life. Using data from Caucasian twins collected in Australia, the Netherlands and the United States, and from East Asian twins collected in Japan and South Korea, we compared the total phenotypic, genetic and environmental variances of birthweight between Caucasians and East Asians. Model-fitting analyses yielded four major findings. First, for both males and females, the total phenotypic variances of birthweight were about 45% larger in Caucasians than in East Asians. The larger phenotypic variances were mainly attributable to a greater shared environmental variance of birth- weight in Caucasians (ranging from 62% to 67% of variance) than Asians (48% to 53%). Second, the genetic variance of birthweight was equal in Caucasians and East Asians for both males and females, explaining a maximum of 17% of variance. Third, small variations in total phenotypic variances of birthweight within Caucasians and within East Asians were mainly due to differences in nonshared environmental variances. We speculate that maternal effects (both genetic and environmental) explain the large shared environmental variance in birthweight and may account for the differences in phenotypic variance in birthweight between Caucasians and East Asians. Recent molecular findings and specific environmental factors that are subsumed by maternal effects are discussed.

A twin study of self-regulatory eating in early childhood: estimates of genetic and environmental influence, and measurement considerations

OBJECTIVE

Children differ greatly in their ability to self-regulate food intake for reasons that are poorly understood. This laboratory-based twin study tested the genetic and environmental contributions to self-regulatory eating and body fat in early childhood.

METHODS

A total of 69 4-7 year-old same-sex twin pairs, including 40 monozygotic and 29 dizygotic pairs, were studied. Self-regulatory eating was operationalized as the percentage compensation index (COMPX%), assessed by a 'preload' challenge in which lunch intake was measured following a low- (3 kcal) or high-calorie (159 kcal) drink. Body fat indexes also were measured. The familial association for COMPX% was estimated by an intraclass correlation, and biometric analyses estimated heritability.

RESULTS

Children ate more at lunch following the low- compared with high-energy preload (P<0.001), although variability in COMPX% was considerable. Compensation was significantly poorer among African American and Hispanic compared with European American children, and among girls compared with boys. There was a familial association for self-regulatory eating (ρ = 0.23, P = 0.03) but no significant genetic component. In all, 22% of the variance in COMPX% was due to shared environmental 'household' factors, with the remaining variance attributable to child-specific 'unique' or 'random' environments. Poorer self-regulatory eating was associated with greater percent body fat (r = -0.21, P = 0.04).

CONCLUSIONS

Self-regulatory eating was influenced by environmental factors, especially those differing among siblings. The absence of a significant genetic effect may reflect the age of the sample or could be artifactual due to measurement issues that need to be considered in future studies.

Breastfeeding, maternal education and cognitive function: a prospective study in twins

The effect of breastfeeding on cognitive abilities is examined in the offspring of highly educated women and compared to the effects in women with low or middle educational attainment. All offspring consisted of 12-year old mono- or dizygotic twins and this made it possible to study the effect of breastfeeding on mean cognition scores as well as the moderating effects of breastfeeding on the heritability of variation in cognition. Information on breastfeeding and cognitive ability was available for 6,569 children. Breastfeeding status was prospectively assessed in the first years after birth of the children. Maternal education is positively associated with performance on a standardized test for cognitive ability in offspring. A significant effect of breastfeeding on cognition was also observed. The effect was similar for offspring with mothers with a high, middle, and low educational level. Breast-fed children of highly educated mothers score on average 7.6 point higher on a standardized test of cognitive abilities (CITO test; range 500–550; effects size = .936) than formula-fed children of mothers with a low education. Individual differences in cognition scores are largely accounted for by additive genetic factors (80%) and breastfeeding does not modify the effect of genetic factors in any of the three strata of maternal education. Heritability was slightly lower in children with a mother with a middle-level education.

The heritability of general cognitive ability increases linearly from childhood to young adulthood

Although common sense suggests that environmental influences increasingly account for individual differences in behavior as experiences accumulate during the course of life, this hypothesis has not previously been tested, in part because of the large sample sizes needed for an adequately powered analysis. Here we show for general cognitive ability that, to the contrary, genetic influence increases with age. The heritability of general cognitive ability increases significantly and linearly from 41% in childhood (9 years) to 55% in adolescence (12 years) and to 66% in young adulthood (17 years) in a sample of 11 000 pairs of twins from four countries, a larger sample than all previous studies combined. In addition to its far-reaching implications for neuroscience and molecular genetics, this finding suggests new ways of thinking about the interface between nature and nurture during the school years. Why, despite life's 'slings and arrows of outrageous fortune', do genetically driven differences increasingly account for differences in general cognitive ability? We suggest that the answer lies with genotype-environment correlation: as children grow up, they increasingly select, modify and even create their own experiences in part based on their genetic propensities.

Attentional regulation in young twins with probable stuttering, high nonfluency, and typical fluency

PURPOSE

Using a sample of 20,445 Dutch twins, this study examined the relationship between speech fluency and attentional regulation in children. A secondary objective was to identify etiological overlap between nonfluency and poor attention using fluency-discordant twin pairs.

METHOD

Three fluency groups were created at age 5 using a parent questionnaire: (a) probable stuttering (PS; N=826; 4.0%), highly nonfluent (HNF; N=547; 2.7%), and typically fluent (TF; N=19,072; 93%). Multiple scales assessing attention, primarily self-regulation/inhibition, were obtained from both parents when children were ages 5 and 7 and from teachers when children were age 7.

RESULTS

When compared with the TF controls, both the PS and HNF children received higher (i.e., more problematic) scores on parental attention ratings at both ages (p<.002). Effect sizes were moderate for both groups. Teacher and parent ratings were generally comparable. The discordant co-twin analyses suggested that nonfluency and attention were influenced by potentially overlapping genetic and shared environmental factors.

CONCLUSIONS

The liability to express both high nonfluency and problems with self-regulation/inhibition may arise from a common set of pathogenic mechanisms. This supports emerging models of stuttering, which propose that poor fluency may be part of a broader network of impaired self-regulatory processes.

Identical but not the same: the value of discordant monozygotic twins in genetic research

Monozygotic (MZ) twins show remarkable resemblance in many aspects of behavior, health, and disease. Until recently, MZ twins were usually called "genetically identical"; however, evidence for genetic and epigenetic differences within rare MZ twin pairs has accumulated. Here, we summarize the literature on MZ twins discordant for Mendelian inherited disorders and chromosomal abnormalities. A systematic literature search for English articles on discordant MZ twin pairs was performed in Web of Science and PubMed. A total number of 2,016 publications were retrieved and reviewed and 439 reports were retained. Discordant MZ twin pairs are informative in respect to variability of phenotypic expression, pathogenetic mechanisms, epigenetics, and post-zygotic mutagenesis and may serve as a model for research on genetic defects. The analysis of single discordant MZ twin pairs may represent an elegant approach to identify genes in inherited disorders.

Heritability and genome-wide linkage scan of subjective happiness

Causes of individual differences in happiness, as assessed with the Subjective Happiness Scale, are investigated in a large of sample twins and siblings from the Netherlands Twin Register. Over 12,000 twins and siblings, average age 24.7 years (range 12 to 88), took part in the study. A genetic model with an age by sex design was fitted to the data with structural equation modeling in Mx. The heritability of happiness was estimated at 22% for males and 41% in females. No effect of age was observed. To identify the genomic regions contributing to this heritability, a genome-wide linkage study for happiness was conducted in sibling pairs. A subsample of 1157 offspring from 441 families was genotyped with an average of 371 micro-satellite markers per individual. Phenotype and genotype data were analyzed in MERLIN with multipoint variance component linkage analysis and age and sex as covariates. A linkage signal (logarithm of odds score 2.73, empirical p value 0.095) was obtained at the end of the long arm of chromosome 19 for marker D19S254 at 110 cM. A second suggestive linkage peak was found at the short arm of chromosome 1 (LOD of 2.37) at 153 cM, marker D1S534 (empirical p value of .209). These two regions of interest are not overlapping with the regions found for contrasting phenotypes (such as depression, which is negatively associated with happiness). Further linkage and future association studies are warranted.

Bivariate genetic analyses of stuttering and nonfluency in a large sample of 5-year-old twins

PURPOSE

Behavioral genetic studies of speech fluency have focused on participants who present with clinical stuttering. Knowledge about genetic influences on the development and regulation of normal speech fluency is limited. The primary aims of this study were to identify the heritability of stuttering and high nonfluency and to assess the relative contribution of genetic and environmental factors to the correlation between these 2 fluency phenotypes.

METHOD

Information on 6 specific speech fluency behaviors was obtained by maternal report for over 10,500 5-year-old Dutch twin pairs.

RESULTS

Genetic analyses revealed that both fluency phenotypes were moderately heritable, with heritability estimates of 42% and 45% for probable stuttering and high nonfluency, respectively. Shared environmental factors were also significant, explaining 44% of the individual differences in probable stuttering and 32% in nonfluency. For both phenotypes, the magnitude of the genetic and environmental influences did not differ between boys and girls. The overlap between the 2 traits was substantial (tetrachoric correlation was .72). A bivariate genetic analysis showed that this overlap was due to both overlapping genetic and environmental influences.

CONCLUSIONS

These findings provide a foundation to justify further studies in normal fluency control, a scientific area that has received little cross-disciplinary attention.

Body size of twins compared with siblings and the general population: from birth to late adolescence

OBJECTIVES

We examined whether and when differences in body size disappear over time and whether twins attain normal final height and body mass index (BMI).

STUDY DESIGN

Height, weight, and BMI data of twins at ages 1, 4, and 18 years were compared with data from their nontwin siblings. Second, twin and sibling data were compared with population standards. In addition to height, weight, and BMI, data on body proportions at age 18 years were analyzed.

RESULTS

At the age of 18 years, twins were as tall as their siblings but were significantly leaner. Compared with children from the general population, adolescent twins attained the same height and BMI. Birth weight was shown to have a considerable effect on height in adolescent twins.

CONCLUSIONS

Twins attained normal final height compared with siblings and children from the general population. No differences in BMI were shown between 18-year-old twins and children from the general population, whereas the siblings of twins had increased BMI values compared with the general population.

Physical features observation: is it repeatable in zygosity determination of Chinese adult twins?

This study reports on the inter- and intrarater reliability of physical features observation. Study subjects were 176 Chinese adult persons, consisting of 89 males and 87 females. Three trained research assistants responded simultaneously and respectively to 12 items regarding the subject's physical features including 'hair', 'Mongoloid folds', left and right 'ear lobes', 'earwax', 'nostril shape', 'tongue rolling', left and right 'hitchhiker's thumb', 'mid-digital hair' and left and right 'simian crease' at the moment of interview. And 14 days later, these subjects received the same observation once again. The results showed that the inter- and intra-observer agreements of 'hair', 'earwax', 'tongue rolling', 'mid-digital hair' and 'simian crease' were almost perfect with most kappa (kappa) coefficients >or= .80, while 'Mongoloid fold' and 'nostril shape' showed poor inter-observer agreement and 'nostril shape' showed poor intra-observer agreement (kappa < .40). Two other physical features, 'hitchhiker's thumb' and 'ear lobes' showed moderate inter-observer agreement and three features, 'hitchhiker's thumb', 'ear lobes' and 'Mongoloid fold', showed moderate intra-observer agreement (.40<or= kappa <.80). In conclusion, this study suggests that as far as reliability is concerned, the five features which were 'hair', 'earwax', 'tongue rolling', 'mid-digital hair' and 'simian crease' could be considered in zygosity determination of Chinese adult twins, while the two features, 'Mongoloid fold' and 'nostril shape', should be abandoned.

Born to be happy? The etiology of subjective well-being

Subjective Wellbeing (SWB) can be assessed with distinct measures that have been hypothesized to represent different domains of SWB. The current study assessed SWB with four different measures in a genetically informative sample of adolescent twins and their siblings aged 13–28 years (N = 5,024 subjects from 2,157 families). Multivariate genetic modeling was applied to the data to explore the etiology of individual differences in SWB measures and the association among them. Developmental trends and sex differences were examined for mean levels and the variance-covariance structure. Mean SWB levels were equal in men and women. A small negative effect of age on mean levels of SWB was found. Individual differences in SWB were accounted for by additive and non-additive genetic influences, and non-shared environment. The broad-sense heritabilities were estimated between 40 and 50%. The clustering of the four different measures (quality of life in general, satisfaction with life, quality of life at present, and subjective happiness) was explained by an underlying additive genetic factor and an underlying non-additive genetic factor. The effect of these latent genetic factors on the phenotypes was not moderated by either age or sex.

Anesthesia and cognitive performance in children: no evidence for a causal relationship

Recent findings of an association between anesthesia administration in the first three years of life and later learning disabilities have created concerns that anesthesia has neurotoxic effects on synaptogenesis, causing later learning problems. An alternative hypothesis is that those children who are likely to undergo surgery early in life have significant medical problems that are associated with a vulnerability to learning disabilities. These two hypotheses were evaluated in a monozygotic concordant-discordant twin design. Data on anesthesia administration and learning abilities and disabilities were available for 1,143 monozygotic twin pairs (56% female) from the Netherlands Twin Registry. Parents of the twins reported on anesthesia use before age 3 and again between ages 3 and 12 years. Near age 12, educational achievement and cognitive problems were assessed with standardized tests and teacher ratings. Results showed that twins who were exposed to anesthesia before age 3 had significantly lower educational achievement scores and significantly more cognitive problems than twins not exposed to anesthesia. However, there was one important exception: the unexposed co-twin from discordant pairs did not differ from their exposed co-twin. Thus, there is no evidence for a causal relationship between anesthesia administration and later learning-related outcomes in this sample. Rather, there is evidence for early anesthesia being a marker of an individual's vulnerability for later learning problems, regardless of their exposure to anesthesia.

A twin study of the genetics of high cognitive ability selected from 11,000 twin pairs in six studies from four countries

Although much genetic research has addressed normal variation in intelligence, little is known about the etiology of high cognitive abilities. Using data from 11,000 twin pairs (age range = 6-71 years) from the genetics of high cognitive abilities consortium, we investigated the genetic and environmental etiologies of high general cognitive ability (g). Age-appropriate psychometric cognitive tests were administered to the twins and used to create g scores standardized within each study. Liability-threshold model fitting was used to estimate genetic and environmental parameters for the top 15% of the distribution of g. Genetic influence for high g was substantial (0.50, with a 95% confidence interval of 0.41-0.60). Shared environmental influences were moderate (0.28, 0.19-0.37). We conclude that genetic variation contributes substantially to high g in Australia, the Netherlands, the United Kingdom and the United States.

Familial twinning and fertility in Dutch mothers of twins

We studied twinning and fertility indices in mothers with spontaneous monozygotic (MZ) and dizygotic (DZ) twins and in mothers who conceived their twins after the use of assisted reproduction techniques (ART). Participants in this study consisted of 8,222 and 5,505 women with spontaneous DZ and MZ offspring and 4,164 and 250 women with ART DZ and MZ twin pairs, respectively. Women were compared with respect to the number of sibs and offspring, the presence of other relatives with twins and the time it took to conceive the twins. We also compared familial twinning between a younger and an older age group. Women with spontaneous DZ twins more often reported female relatives with twins than those with spontaneous MZ twins. The proportion of DZ versus MZ twin offspring in relatives was also larger in women with spontaneous DZ offspring than in women with MZ offspring. The first group of women reported a shorter time to conceive. Women with ART twins had fewer sibs and offspring and less often reported relatives with twins. We did not observe that DZ twinning was more familial in women who had their twins before age 36 years compared to older women. Familial DZ twinning is clearly present in mothers of spontaneous DZ twins. The mechanisms underlying spontaneous and non-spontaneous DZ twinning are different and fertility treatment should be taken into account in any study of twinning. Twinning is not more familial in women who have their twins at a younger age.

An exploration of gene-environment interaction and asthma in a large sample of 5-year-old Dutch twins

A consistent finding from twin studies is that the environment shared by family members does not contribute to the variation in susceptibility to asthma. At the same time, it is known that environmental risk factors that are shared by family members are associated with the liability for asthma. We hypothesize that the absence of a main effect of shared environmental factors in twin studies can be explained by gene-environment interaction, that is, that the effect of an environmental factor shared by family members depends on the genotype of the individual. We explore this hypothesis by modeling the resemblance in asthma liability in twin pairs as a function of various environmental risk factors and test for gene-environment interaction. Asthma data were obtained by parental report for nearly 12,000 5-year-old twin pairs. A series of environmental risk factors was examined: birth cohort, gestational age, time spent in incubator, breastfeeding, maternal educational level, maternal smoking during pregnancy, current smoking of parents, having older siblings, and amount of child care outside home. Results revealed that being a boy, born in the 1990s, premature birth, longer incubator time, and child care outside home increased the risk for asthma. With the exception of premature birth, however, none of these factors modified the genetic effects on asthma. In very premature children shared environmental influences were important. In children born after a gestation of 32 weeks or more only genetic factors were important to explain familial resemblance for asthma.

Genetic and environmental influences on the stability of withdrawn behavior in children: a longitudinal, multi-informant twin study

We examined the contribution of genetic and environmental influences on the stability of withdrawn behavior (WB) in childhood using a longitudinal multiple rater twin design. Maternal and paternal ratings on the withdrawn subscale of the Child Behavior Checklist (CBCL) were obtained from 14,889 families when the twins were 3, 7, 10 and 12 years old. A longitudinal psychometric model was fitted to the data and the fit of transmission and common factor models were evaluated for each variance component. WB showed considerable stability throughout childhood, with correlation coefficients ranging from about .30 for the 9-year time interval to .65 for shorter time intervals. Individual differences in WB as observed by the mother and the father were found to be largely influenced by genetic effects at all four time points, in both boys (50-66%) and girls (38-64%). Shared environmental influences explained a small to modest proportion (0-24%) of the variance at all ages and were slightly more pronounced in girls. Non-shared environmental influences were of moderate importance to the variance and slightly increased with age, from 22-28% at age 3 to 35-41% at age 12 years. The stability of WB was largely explained by genetic effects, accounting for 74% of stability in boys and 65% in girls. Shared environmental effects explained 7% (boys) and 17% (girls) of the behavioral stability. Most shared environmental effects were common to both raters, suggesting little influence of rater bias in the assessment of WB. The shared environmental effects common to both raters were best described by a common factor model, indicating that these effects are stable and persistent throughout childhood. Non-shared environmental effects accounted for the remaining covariance over time.

Genetic and environmental contributions to stability in loneliness throughout childhood

Heritability estimates based on two small cross-sectional studies in children indicate that the genetic contribution to individual differences in loneliness is approximately 50%. A recent study estimated the genetic contribution to variation in loneliness in adults to be 48%. The current study aims to replicate and expand these findings by conducting longitudinal analyses in order to study causes of individual differences in stability of loneliness throughout childhood. Univariate and multivariate longitudinal analyses are conducted in a large sample of young Dutch twins. Information on loneliness comes from maternal ratings on the Child Behavior Checklist. Using an average score of loneliness over ages 7, 10, and 12, results from the two previous studies are replicated and a heritability estimate of 45% is found. The remaining variance is accounted for by shared environmental influences (12%), and nonshared environmental influences (43%). The longitudinal analyses, however, show that heritability is 58% at age 7, 56% at age 10, but drops to 26% at age 12. A parallel increase in influences of shared family environment is observed, explaining 6% of the variance at age 7, 8% at age 10 and 35% at age 12. The remaining variance is explained by relatively stable influences of nonshared environmental factors. Stability in loneliness is high, with phenotypic correlations in the range of 0.51-0.69. This phenotypic stability is mainly caused by genetic and nonshared environmental influences. The results indicate the importance of both innate as well as nonshared environmental factors for individual differences in loneliness. Further, different results between causes of individual differences for the average score of loneliness and results for age 12 from the longitudinal analyses, indicate the importance of longitudinal analyses with data at well-defined ages.

Genetic and environmental covariation between autistic traits and behavioral problems

Our objective was to examine the overlap between autistic traits and other behavioral problems in a general population sample, and explore the extent to which this overlap is due to genetic or environmental factors. Youth Self Report (YSR) data were collected in a general population sample of 424 twin pairs at 18 years of age, and their nontwin siblings. In 197 of these twin families, self-report ratings on the Autism-spectrum Quotient (AQ) were collected. Stepwise backward regression analyses revealed that of all 8 YSR syndrome scales, the Withdrawn Behavior (WB) and Social Problems (SOC) scale were the most important predictors of AQ scores, and together with sex, explained 23% of the variance in AQ scores. Genetic structural equation modeling showed that the overlap between AQ and WB and SOC was mainly due to genetic effects. About half of the genetic variance in AQ scores was specific to the AQ, with the remaining half shared with genetic variance in WB and SOC. Endorsement of autistic traits in a general population sample is associated with social and withdrawn behavioral problems and these problems partly share a common genetic etiology with autistic traits. However, most of the variance in AQ scores remains unexplained by YSR scores, and half of the genetic variance in AQ is unshared with WB and SOC. These results indicate that autistic traits have specific characteristics that are substantially genetically independent from other common but related behavioral domains such as social problems and withdrawn behavior.

Non-additive and additive genetic effects on extraversion in 3314 Dutch adolescent twins and their parents

The influence of non-additive genetic influences on personality traits has been increasingly reported in adult populations. Less is known, however, with respect to younger samples. In this study, we examine additive and non-additive genetic contributions to the personality trait of extraversion in 1,689 Dutch twin pairs, 1,505 mothers and 1,637 fathers of the twins. The twins were on average 15.5 years (range 12-18 years). To increase statistical power to detect non-additive genetic influences, data on extraversion were also collected in parents and simultaneously analyzed. Genetic modeling procedures incorporating age as a potential modifier of heritability showed significant influences of additive (20-23%) and non-additive genetic factors (31-33%) in addition to unshared environment (46-48%) for adolescents and for their parents. The additive genetic component was slightly and positively related to age. No significant sex differences were found for either extraversion means or for the magnitude of the genetic and environmental influences. There was no evidence of non-random mating for extraversion in the parental generation. Results show that in addition to additive genetic influences, extraversion in adolescents is influenced by non-additive genetic factors.

Genetic and environmental influences on the relation between attention problems and attention deficit hyperactivity disorder

Objective The assessment of symptoms of ADHD in children is usually based on a clinical interview or a behavior checklist. The aim of the present study is to investigate the extent to which these instruments measure an underlying construct and to estimate the genetic and environmental influences on individual differences in ADHD. Methods Maternal ratings were collected on 10,916 twins from 5,458 families. Child Behavior Checklist (CBCL) ratings were available for 10,018, 6,565, and 5,780 twins at the ages 7, 10, and 12, respectively. The Conners Rating Scale (4,887 twins) and the DSM interview (1,006 twins) were completed at age 12. The magnitude of genetic and environmental influences on the variance of the three measures of ADHD and the covariance among the three measures of ADHD was obtained. Results Phenotypic correlations range between .45 and .77. Variances and covariances of the measurements were explained mainly by genetic influences. The model that provided the best account of the data included an independent pathway for additive and dominant genetic effects. The genetic correlations among the measures collected at age 12 varied between .63 and 1.00. Conclusions The genetic overlap between questionnaire ratings and the DSM-IV diagnosis of ADHD is high. Clinical and research implications of these findings are presented.

Genetic regulation of growth in height and weight from 3 to 12 years of age: a longitudinal study of Dutch twin children

Human growth is a complex and poorly understood process. We studied the effect of genetic and environmental factors on height and body mass index (BMI, kg/m(2)) based on maternal reports at 3, 4, 5, 7, 10 and 12 years of age in a large longitudinal cohort of Dutch twins (7755 complete twin pairs at age 3). Several multivariate variance component models for twins were fitted to the data using the Mx statistical package. The first-born twin was taller until age 10 and heavier until age 12 than the second-born co-twin. Heritability estimates were high for height (a(2) = .58-.91) and BMI (a(2) = .31-.82), but common and unshared environmental factors were also important. The phenotypic correlations across the ages for height and BMI were mainly explained by correlated additive genetic factors (r(a) = .77-.96 for height and .43-.92 for BMI), but common (r(c) = .40-.84 and .09-.78, respectively) and specific environmental correlations (r(e) = .50-.81 and .42-.80, respectively) were also significant. Additive genetic factors decreased with increasing age difference for both height and BMI. However, the full Cholesky model, which does not make any assumptions regarding the underlying genetic structure, had the best fit. High genetic correlations across the ages, especially for height, may help further molecular genetic studies of human growth. Environmental factors affecting height and BMI during growth period are also important, and further studies are needed to identify these factors and test whether they interact with genetic factors.

The equal environments assumption of classical twin studies may not hold

The classical twin method - comprising comparisons of monozygotic (MZ) and dizygotic (DZ) twins - in the domain of cognitive abilities and attainments has led to wide acceptance of results suggesting a large amount of additive genetic variance, with far-reaching implications both for the nature of future studies on the causes of cognitive variance and for intervention policies, as in education. However, this interpretation is only valid if the method observes a number of conditions, which have to hold. Here, we show that the most crucial of these, namely, the equal environments assumption (EEA), may not hold. Consequently, differences in twin correlations might be at least partly explained by treatment effects from parents, teachers, peers, and so on. In addition, well-known interactions at various levels confound the model of simple additive effects on which the classical twin method is predicated and results are interpreted. For example, at a socio-cognitive level, DZ twins may respond to treatments differently from MZ twins. This interaction may further explain MZ-DZ correlation differences. There is abundant evidence for such interactive effects in published twin data. We suggest that there is a need for a more thorough examination of these problems.

Genetic and environmental influences on cross-gender behavior and relation to behavior problems: a study of Dutch twins at ages 7 and 10 years

The aim of this study was to investigate the prevalence of cross-gender behavior during childhood, to estimate the influence of genotype and environment on variation in cross-gender behavior, and to explore the association of cross-gender behavior with maternal ratings of behavior problems as indexed by the Internalizing and Externalizing scales of the Child Behavior Checklist (CBCL). Cross-gender behavior was assessed by two items from the CBCL: "behaves like opposite sex" and "wishes to be of opposite sex." As part of an ongoing longitudinal study of the Netherlands Twin Registry, mothers were asked to complete the CBCL for their twins when they were 7 (n approximately 14,000 twins) and 10 years old (n approximately 8,500 twins). The prevalence of cross-gender behavior (as measured by maternal report of behaving like or wishing to be the opposite sex) was 3.2% and 5.2% for 7-year-old boys and girls, respectively, and decreased to 2.4% and 3.3% for 10-year-old boys and girls. Surprisingly, the prevalence rate of cross-gender behavior of girls with a male co-twin was lower than of girls with a female co-twin. At both ages, the similarity for cross-gender behavior was greater in monozygotic than in dizygotic twins pairs. Genetic structural equation modeling showed that 70% of the variance in the liability of cross-gender behavior could be explained by genetic factors, at both ages and for both sexes. Cross-gender behavior was associated with higher scores on Internalizing and Externalizing problems, both in boys and in girls.

Genetic analyses of maternal and teacher ratings on attention problems in 7-year-old Dutch twins

The goal of the present study is to examine genetic and environmental influences on maternal and teacher ratings of Attention Problems (AP) in 7-year-old children. Teachers completed the Teacher Report Form (N=2259 pairs), and mothers the Child Behavior Checklist (N=2057 pairs). Higher correlations were found in twins rated by the same teacher than in twins rated by different teachers. This can be explained by rater bias or by a greater environmental sharing in twins, who are in the same classroom. We further found that 41% of the variation in maternal and teacher ratings is explained by a common factor. The heritability of this common factor is 78%. The heritabilities of the rater specific factors of mothers and teachers are 76% and 39%, respectively. Because Attention Problems that are persistent over situations may indicate more serious behavior problems than context dependent Attention Problems, we believe that gene finding strategies should focus on this common phenotype.

Prevalence and genetic architecture of Child Behavior Checklist-juvenile bipolar disorder

BACKGROUND

No consensus has been reached yet on how best to characterize children with juvenile bipolar disorder (JBD). Several groups have shown that children on the attention problems (AP), aggressive behavior (AGG), and anxious-depressed (AD) syndromes of the Child Behavior Checklist (CBCL) are likely to meet criteria for DSM-JBD. We aimed to use a large population-based twin sample to evaluate the prevalence and genetic architecture of the CBCL-JBD (deviant on AP, AGG, and AD) phenotype and compare these data to children who are deviant on just the CBCL-AP syndrome.

METHODS

Structural equation modeling (SEM) was applied to CBCL data from 5418, 3562, and 1971 Dutch twin pairs at ages 7, 10, and 12 years.

RESULTS

The CBCL-JBD phenotype occurs in approximately 1% of children at each age. Among the children who meet criteria for the CBCL-AP phenotype ( approximately 5%), between 13 and 20% also meet criteria for CBCL-JBD. The best SEM for CBCL-JBD includes additive genetic, shared and unique environmental factors. The best SEM for CBCL-AP includes dominant and additive genetic and unique environmental factors.

CONCLUSIONS

These data suggest that CBCL-JBD is common, and even more common among children who have severe attention problems. CBCL-JBD shows familial aggregation due to both genetic and shared environmental factors.

The genetic and environmental contributions to oppositional defiant behavior: a multi-informant twin study

OBJECTIVE

To estimate the genetic and environmental contributions to oppositional defiant behavior (ODB) from mother, father, and teacher report using the Conners Revised Short Forms in a large twin sample.

METHOD

ODB data were collected from 1,595 mothers, 1,114 fathers, and 793 teachers of 7-year-old twin pairs from the Netherlands Twin Registry in the 1990-1992 cohort with an 80% response rate. Models were fit for each informant to determine the genetic, environmental, gender, and informant influences on ODB.

RESULTS

Genetic analyses of the ODB quantitative scale showed additive genetic (A) by mother (55%), by father (57%), and by teacher (21% girls, 38% boys) unique environmental (E) (mother, 22%; father, 29%; teacher, 48% girls, 39% boys) and shared environmental (C) (mother, 14%; father, 23%; teacher, 31% girls, 23% boys) influences.

CONCLUSIONS

Additive genetic and unique environmental factors account for the majority of the influences on ODB for boys and girls by all informants.

A study of genetic and environmental influences on maternal and paternal CBCL syndrome scores in a large sample of 3-year-old Dutch twins

BACKGROUND

There is increasing evidence that behavioral problems are common in very young children, yet little is known about the etiology of individual differences in these problems. It is unclear to what degree environmental and genetic factors influence the development of early child psychopathology. In this paper, we focus on the following issues. Firstly, to what degree do genetic and environmental factors influence variation in behavioral problems? Secondly, to what degree are these underlying etiological factors moderated by sex and informant? We investigate these issues by analyzing Child Behavior Checklist (CBCL) data on 9689 3-year-old twin pairs.

METHODS

Rater Bias and Psychometric Models were fitted to CBCL/2-3 data obtained from mothers and fathers to determine the genetic and environmental contributions to the five CBCL syndromes:aggressive, oppositional, overactive, withdrawn, and anxious/depressed behavior.

RESULTS

Parental ratings are influenced by aspects of the child's behavior that are experienced in the same way by both parents and by aspects of the child's behavior that are experienced uniquely by each parent. There is evidence for high genetic contributions to all CBCL syndromes. Shared and non-shared environmental influences play significant roles as well. One exception is overactive behavior, which is influenced by genetic and non-shared environmental influences only.

CONCLUSIONS

Variation in behavior problems in the very young shows high heritability. Individual raters offer unique perspectives that can have an impact on estimates of problem behavior and genetic architecture. Therefore, multi-informant approaches in the assessment of the very young will be useful to clinicians and researchers alike.

Etiology of sex differences in the prevalence of ADHD: an examination of inattention and hyperactivity-impulsivity

Several studies have examined the predictions of two models (i.e., the polygenic multiple threshold (PMT) model and the constitutional variability (CV) model) developed to explain sex differences in the prevalence of attention-deficit/hyperactivity disorder (ADHD), and the results of these studies are conflicting. Although there is substantial evidence that two distinct, moderately correlated symptom dimensions (inattention and hyperactivity-impulsivity) underlie ADHD symptoms, all of these studies have examined ADHD as a unidimensional construct. Therefore, we tested the PMT and CV models for the two ADHD symptom dimensions separately. Dizygotic twins or siblings of girls with ADHD had a higher number of ADHD symptoms than twins or siblings of boys with ADHD. This evidence for the PMT model and against the CV model was found for both inattention and hyperactivity-impulsivity, but became weaker for hyperactivity-impulsivity as symptom severity increased.

Insulin resistance in healthy prepubertal twins

OBJECTIVES

To evaluate insulin sensitivity (S(I)) in prepubertal twins and to examine the relation to reduced birth weight, prematurity, and peroxisome proliferator-activated receptor-gamma (PPAR gamma) polymorphism.

STUDY DESIGN

Fifty twins (birth weight SDS, -0.7 +/- 0.2; gestation, 33.5 +/- 0.5 weeks; and body mass index SDS, -0.04 +/- 0.2) were studied at 8.2 +/- 0.3 years. S(I) was measured by Bergman's minimal model from a 90 minutes frequently sampled intravenous glucose test. Twenty control children (height SDS, -1.7 +/- 0.3; birth weight SDS, -0.3 +/- 0.2; and gestation of 39.2 +/- 0.7 weeks) were also evaluated at 7.0 +/- 0.4 years. The PPAR gamma T-variant polymorphism was evaluated in 41 twins. Values are expressed as mean +/- SEM, or 95% confidence intervals.

RESULTS

S(I) was reduced in twins compared with control subjects, (12.7 [11-15] versus 23.0 [16.8-31.4] 10(-4) min(-1) microU/mL, respectively, P=.005). The reduction in S(I) was independent of prematurity and birth weight and zygosity (P<.0001). There was no difference in S(I), even in twin pairs with >20% difference in birth weight (P=.9). The PPAR gamma heterozygote T-variant polymorphism was present in 7 of 41, with a further reduction in S(I) (P=.03) and a later gestation (P=.03). These twins also had increased fat mass (P=.02) but with similar fat free mass (P=.14).

CONCLUSIONS

Twin children, independent of prematurity or birth weight, had a marked reduction in S(I). To use twins as a model to study the fetal origins of adult diseases for glucose homeostasis is not valid.

A study of parent ratings of internalizing and externalizing problem behavior in 12-year-old twins

OBJECTIVE

Studies on 3-, 7-, and 10-year-old twins' internalizing and externalizing problems have emphasized the importance of understanding sources of agreement and disagreement between maternal and paternal ratings. A psychometric model that assumes that each parent assesses rater-specific aspects of the child's behavior provided the best explanation for parental disagreement. This study investigates two models that have been used to explain the agreement and disagreement between mothers and fathers in the ratings of their children.

METHOD

Child Behavior Checklists filled in by mothers and fathers were collected for a sample of 1,481 twelve-year-old twin pairs. Genetic and environmental influences on internalizing and externalizing problems were estimated using models that corrected for rater bias, rater-specific effects, and unreliability.

RESULTS

The psychometric model fitted the data significantly better than a rater bias model. Significant influences of genetic, shared, and nonshared environmental factors were found for internalizing and externalizing behavior. Parent-specific views, rater bias, and unreliability were significant.

CONCLUSIONS

The best-fitting model implies that disagreement between parents is due to the fact that mothers and fathers provide information from their own perspective. This information should be seen as important and adding to the diagnostic formulation rather than as a point of disagreement. The finding that internalizing and externalizing problems are influenced by genetic and environmental factors fosters the understanding that it is the interaction of nature and nurture that puts children at risk for common behavioral disorders.

Genetic and environmental contributions to stability and change in children's internalizing and externalizing problems

OBJECTIVE

To estimate genetic, shared environmental, and nonshared environmental contributions to stability and change in internalizing and externalizing problems.

METHOD

Maternal Child Behavior Checklist ratings were obtained for 3,873 twin pairs at age 3 and 1,924 twin pairs at age 7. For 1,575 twin pairs, ratings were available at both ages.

RESULTS

For Internalizing/Externalizing ratings, genetic, shared, and nonshared environmental factors explained about 59/51%, 10/30%, and 31/19% of the variance at age 3, and 40/52%, 31/32%, and 29/16% of the variance at age 7. The phenotypic correlation of r = 0.38/0.54 between problems assessed at 3 and 7 years of age was explained for 66/55% by genetic factors, for 23/37% by shared environmental factors, and for 11/8% by nonshared environmental factors. The genetic, shared environmental, and nonshared environmental correlations between ages 3 and 7 were 0.51/0.57, 0.47/0.66, and 0.13/0.24, respectively.

CONCLUSIONS

Genetic and shared environmental factors were most important for the stability of Internalizing and Externalizing Problems between ages 3 and 7. Nonshared environmental factors were mainly age-specific. For Internalizing Problems, shared environment may become more important from early to middle childhood.

Maternal adjustment, parenting and child behaviour in families of school-aged twins conceived after IVF and ovulation induction

BACKGROUND

Previous studies that have examined the long-term effects of infertility and assisted reproductive technology on parenting and child behaviour in families with twins have suffered from methodological problems. This study compared measures of parental adjustment, parenting and child behaviour in families with 5-year-old twins who were conceived after in vitro fertilisation (IVF) or ovulation induction (OI) with families whose twins were naturally conceived (NC).

METHODS

The families who conceived via IVF/OI (N = 121) were identified from an epidemiological study of twins and matched to families who were conceived naturally (N = 121) on the basis of eleven child and family variables. Mothers were interviewed in their homes for the study.

RESULTS

No significant differences were observed between the IVF/OI families and the NC families on measures of parental adjustment or parent and teacher ratings of the twins' behaviour. IVF/OI mothers and their partners agreed with each other about discipline more than NC couples, but otherwise no other differences in parenting were found.

CONCLUSIONS

Overall, this study provides evidence that families who conceive twins following IVF/OI are functioning well and that the experience of fertility treatment does not lead to long-term difficulties for parents or children.

Heritability of attention problems in children: I. cross-sectional results from a study of twins, age 3-12 years

Multiple twin studies of attention problems (AP) from the Child Behavior Checklist or ADHD from the DSM criteria have reported on the genetic and environmental influences on these behaviors. The majority of these have studied AP and ADHD symptoms in twin samples combined across wide age spans, combined rater information and both genders. Thus, it is possible that the results are complicated by developmental, informant, and gender differences. The purpose of this study was to assess for the genetic and environmental contributions to overactive behavior (a syndrome highly related to AP in 7-, 10-, and 12-years olds) in 3-years olds (3,671 twin pairs), and attention problems in 7- (3,373 twin pairs), 10- (2,485 twin pairs), and 12-years olds (1,305 twin pairs) while controlling for developmental, gender and rater contrast contributions. Using a cross-sectional twin design, contributions from genetic additive, genetic dominance, unique environmental and rater contrast effects were estimated for CBCL maternal reports. We found that genetic influences on overactive behavior and attention problems are high across an age span that covers pre-school and elementary school age. Although girls display less problem behavior compared to boys, heritability estimates were found equal for both genders at each age. Environmental experiences that are unique to the individual accounted for the remaining influence. At the age of 3 years, a rater contrast effect was detected. We hypothesize that the contrast effect represents a maternal rater bias effect that is dependent on the age of the twins. The implications of these findings are discussed with reference to the clinical setting and in the context of future research.

Emotional and behavioral problems among female twins: an evaluation of the equal environments assumption

OBJECTIVE

To evaluate the equal environments assumption (EEA) of the twin method for mother-reported symptoms of child and adolescent emotional and behavioral problems.

METHOD

Four emotional and behavioral problem scales (symptoms of separation anxiety disorder, attention-deficit/hyperactivity disorder, oppositional defiant disorder, and conduct disorder) and four environmental similarity measures (sharing friends, sharing classes, dressing alike, and perceived zygosity) were assessed by telephone interviews with the biological mothers of 1,948 female adolescent and young adult twin pairs. The effect of environmental similarity on the magnitude of the monozygotic and dizygotic twin correlations and on the parameter estimates from genetic model-fitting was examined for each symptom scale.

RESULTS

The measures of environmental similarity were not strongly or consistently related to the similarity of twins for mother-reported emotional and behavioral problems. On average, controlling for environmental similarity did not substantially affect estimates of genetic and environmental influences.

CONCLUSIONS

These results lend support for the validity of the EEA and suggest that estimates of genetic and environmental influences obtained from twin studies of mother-reported child and adolescent emotional and behavioral problems are not unduly biased by the greater environmental similarity of monozygotic than dizygotic twins.