Longitudinal genetic analysis of problem behaviors in biologically related and unrelated adoptees.

The genetic and environmental influences on problem behaviors at two assessment points, three years apart, and their stability were studied in a sample of international adoptees, initially aged 10 to 15 years. Parents of 111 pairs of adopted biological siblings, 221 pairs of adopted nonbiological siblings and 1484 adopted singletons completed the Child Behavior Checklist (75 pairs, 154 pairs and 1080 singletons respectively at second assessment). At first assessment, genetic factors accounted for more than 50% of the variance in the Externalizing, Aggressive Behavior, Attention Problems and Social Problems scales. Shared environmental influences explained 40% of the variance in the Total Problem scale and less for all other scales. Nonshared environmental influences were most important for the Internalizing scale and its subscales, and for the Thought Problems and Delinquent Behavior scales. At the second assessment, genetic factors explained most of the variance in the Total Problem, Externalizing and Aggressive Behavior scales, while nonshared environmental influences explained most of the variance in all other scales. Shared environmental influences explained 33% of the variance in the Internalizing scale and less for the other scales. The stability of the Externalizing scale over time was caused mostly by genetic factors, while nonshared environmental factors mostly caused the stability of the Internalizing scale.

Quantitative genetic analysis of internalising and externalising problems in a large sample of 3-year-old twins.

For a quantitative genetic study of pre-school problem behaviours, we have collected data with the Child Behavior Checklist for 2 and 3-year-old children (CBCL 2/3). Questionnaires were completed by mothers of 3620 twin pairs: 633 monozygotic males, 581 dizygotic males, 695 monozygotic females, 519 dizygotic females and 1192 dizygotic opposite sex twin pairs. The genetic and environmental influences on the Externalising and Internalising Problem scales were estimated, simultaneously with sex differences and sibling interaction effects. Genetic factors explained most of the observed variance for both Externalising and Internalising Problems. Cooperative sibling interactions were found for Externalising Problems, indicating that twins reinforce each other's behaviour. Sex differences in genetic architecture were found for Externalising Problems. Genetic factors explained 75% of the variance in girls and 50% in boys. Shared environmental influences were only of importance in boys. For both problem scales, non-shared environmental factors accounted for 25 to 32% of the variance. The observed variances of Internalising Problems could be adequately explained by genetic and nonshared environmental factors, with genetic factors accounting for 68% of the variance.

Using shared and unique parental views to study the etiology of 7-year-old twins' internalizing and externalizing problems.

In a sample of 1,940 Dutch 7-year-old twin pairs we studied the etiology of individual differences in Internalizing and Externalizing behavioral problems. For the majority of twins in the sample, both maternal and paternal ratings of behavioral problems were obtained from the Child Behavior Checklist. This made it possible to take into account processes underlying agreement and disagreement between maternal and paternal ratings. For both problem behaviors, a Psychometric model fitted the data better than a Rater Bias model, implying that parents, in addition to the behaviors they similarly observed, also assessed unique aspects of their children's behaviors. Relatively large genetic influences were found for Externalizing problems, explaining over 50% of the variance in both boys and girls. For internalizing problems, the heritability was over 30% in both sexes. Shared environmental factors were nearly as important as genetic influences in explaining the variation in behavioral problems. For both Externalizing and Internalizing problems, around 30% of the variance was accounted for by the shared environmental factors.

Using parental ratings to study the etiology of 3-year-old twins' problem behaviors: different views or rater bias?

Child Behavior Checklist questionnaires (Achenbach, 1992), filled in separately by mothers and fathers, were collected for an effective sample of 3,501 Dutch 3-year-old twin pairs. To disentangle the child's phenotype from that of the rater, two contrasting models were fitted to the data. One model, called a Rater Bias model, is based on the assumption that both parents assess exactly the same behaviors in the child. A weaker alternative of this model, called a Psychometric model, assumes that apart from these common behavioral views, each parent also assesses a unique aspect of the child's behavior. A Psychometric model fitted the data of both Internalizing and Externalizing scales significantly better than a Rater Bias model. This implied that each parent provided unique information from his or her own perspective, apart from the common behavioral view. Using this best fitting model, the etiology of both the Internalizing and Externalizing scales was studied. Common factors (influencing behaviors similarly assessed by both parents) were more important than unique factors (influencing behaviors uniquely assessed by one parent). Common genetic factors explained about 50% of the variance of both scales, indicating a possible inborn vulnerability to childhood psychopathology. Common environmental factors not shared between twins (free of unreliability and error) explained around 14% of both scales, suggesting the importance of pure idiosyncratic experiences even for children as young as 3 years. Common environmental factors shared between twins (unconfounded by rater bias) were only found for the Externalizing scale, explaining 18% of the variance. Rater bias and unreliability, if present in the data, were included in the estimates of the unique factors. Unique genetic, shared, and nonshared environmental factors each explained around 8% of the variance for both scales. These small effects could be detected because of the large sample of twin pairs used.

Zygosity diagnosis in young twins by parental report.

This study reports on zygosity determination in twins of childhood age. Parents responded to questionnaire items dealing with twin similarity in physical characteristics and frequency of mistaking one twin for another by parents, relatives and strangers. The accuracy of zygosity diagnosis was evaluated across twins aged 6, 8, and 10 and across parents. In addition, it was examined whether the use of multiple raters and the use of longitudinal data lead to an improvement of zygosity assignment. Complete data on zygosity questions and on genetic markers or blood profiles were available for 618 twin pairs at the age of 6 years. The method used was predictive discriminant analyses. Agreement between zygosity assigned by the replies to the questions and zygosity determined by DNA markers/blood typing was around 93%. The accuracy of assignment remained constant across age and parents. Analyses of data provided by both parents and collected over multiple ages did not result in better prediction of zygosity. Details on the discriminant function are provided.