79. Saudino, K.J., Cherny, S.S. & Plomin, R. (2000). Parent ratings of temperament in twins: Explaining the "too low" DZ correlations. Twin Research, 3, 224-233.

Twin studies of child temperament using objective measures consistently suggest moderate heritability for most dimensions; however, parent rating measures produce unusual patterns of results. Correlations for identical (MZ) twins are typically high, whereas fraternal (DZ) correlations are much lower than would be predicted. The "two low" DZ correlations can be explained by parent rating biases that either exaggerate the differences between DZ twins (contrast effects) or that inflate the similarity of MZ twins (assimilation effects), or by the presence of nonadditive genetic variance. These data are best described by a model that includes contrast effects. This finding suggests the need to learn more about the factors that affect the parent-rating process and to use more objective measures to support the great deal that has been learned from parent-report data.