GENETIC INFLUENCES ON CHILDHOOD HYPERACTIVITY - CONTRAST EFFECTS IMPLY PARENTAL RATING BIAS, NOT SIBLING INTERACTION

Background. Previous twin studies of hyperactivity have supported a 'c ontrast effect', whereby the more hyperactive the rating of one twin, the less the rating of the other. It has not been clear whether contra st effects occur in the twins' behaviour or in the ratings made of the ir behaviour but the implications for hyperactivity are different unde r the two models. Method. We use hyperactivity ratings from mothers an d teachers for 1644 twin pairs in the Virginia Twin Study of Adolescen t Behavioral Development (VTSABD) to explore the origin of contrast ef fects, making use of independent teacher reports in a proportion of tw ins. Models were fitted separately for the two informants and jointly to ratings combined through a latent variable, using structural equati on modelling. Results. Models for maternal data confirm the contrast e ffect previously reported. Teacher ratings show a different form of bi as, with both twin confusion and correlated errors representing altern ative but not mutually exclusive explanations of the data. Latent vari able modelling of the joint responses allowed comparison of a model in which the contrast effect was placed on maternal ratings, representin g bias, versus one in which the contrast occurred in the underlying 't rue' phenotype. The fit of the former model was significantly better. Conclusions. Support is provided for the notion of contrast effects as a form of rater bias in maternal hyperactivity ratings. Different bia s in teacher reports highlight that no one report can be considered a gold standard. The extent to which such biases may distort information for other data sources such as sib-pair studies of concordance/discor dance is discussed.