Two Peas in a Pod? Development of Twin Relationships in Light of Twins' Temperament Differences

This study examines the hypothesis that temperamental (dis)similarity is associated with twin relationship quality. In a longitudinal study that followed 322 monozygotic twins (who share close to 100% of their genes) and 1199 dizygotic twins (who on average, share 50% of their segregating genes) throughout childhood, mothers (N = 1547) and fathers (N = 536) reported on their twins' relationships on at least one of four measurement points when the twins were between 3 and 8-9 years of age. Mothers also reported on the twins' temperament. Negative associations were found between reports by both parents on the twins' closeness and their temperament difference throughout childhood, while positive associations were found between twins' conflict and their temperament difference in late childhood. Latent growth modeling indicated that the association between temperament differences and the twins' mother-reported closeness was evident beyond the effect of zygosity. A different pattern was found for twin conflict: the more the twins differed in their temperament (specifically negative emotionality) with age, the more the conflict between them increased. Our findings support the hypothesis that personality similarities can contribute to positive relationships from early childhood, and vice versa, beyond the effect of genetic similarity.