In the nature-nurture debate, twins have always been very interesting and popular subjects. In this longitudinal study, twins are, however, not examined to gain insight into genetic dispositions, but are asked to evaluate their relationships using the Brunswick Dyadic Identity Scale for Siblings (BDISS). The answers of 65 sets of twins (24 monozygotic, 21 same-sex and 20 opposite-sex dizygotic) were compared with those of 22 same-sex and 20 opposite-sex pairs of siblings (age difference max. 2 years). At the time of the first interview, the children were between 9 and 13 years old (average age = 11.2; SD = 1.4), follow-up interviews were conducted after one and two years. Due to the correlated data sets, correspondence and multilevel analyses were used to examine whether twins and nontwins differ in how both children evaluate their relationship in agreement and individually. Results show that primarily monozygotic twins differ from the other groups of siblings. They more often agree on how they evaluate their relationship, which surprisingly is not associated with stronger emotional attachment. Effects on identity development and the chosen means of analysis are discussed.
