Contrast effects in Buss and Plomin’s EAS questionnaire: a behavioral-genetic study on early developing personality traits assessed through parental ratings

Temperament was assessed through separate mother and father ratings in a sample of 354 German mono- and dizygotic twin pairs aged 2–14 years. We administered the EAS Temperament Survey (Buss and Plomin, 1984), measuring Emotionality, Activity, Sociability and Shyness to investigate the usefulness of the 20-item inventory for behavior genetic studies of early developing personality traits in children. Most of the studies that report zero or even negative correlations for dizygotic twins in parental ratings assessed temperament in very young children only. The present study covered a larger age range to examine whether this pattern of results was also found in parental ratings of older children when the same inventory was used. Our findings clearly demonstrate that similarities of dizygotic twins were substantially less than half the MZ twin correlations. For all EAS dimensions except Emotionality, DZ twin correlations were close to zero or negative. Hardly any systematic correspondence emerged between age and twin similarity. The design of our study made it possible to analyse contrast effects, which might be responsible for DZ twin dissimilarity, within and across raters. We found evidence that contrast effects were not only observed in separate mother or father ratings but were also found when cross-correlations were analysed. Thediscussion focuses on the nature of contrast effects in parental rating data and the usefulness of parental ratings in temperament questionnaires given the growing evidence that true similarities of dizygotic twins are underestimated.