Prosocial and self-interested intra-twin pair behavior in monozygotic and dizygotic twins in the early to middle childhood transition

The dual journey: The development of twins' relationships throughout childhood

Twin bonds, likely the most enduring of human relationships, provide both solace and rivalry for twins. Using an evolutionary psychology perspective, this chapter scrutinizes twins' bonds from prenatal stages to childhood to better understand their unique relationships. Twins' interactions, which begin in the womb, establish patterns of cooperation and competition. The initial years pose parenting challenges that shape the twins' experiences of dependency and rivalry. As twins grow, five dimensions-closeness, dependence, conflict, rivalry, and dominance-emerge, evolving distinctly between monozygotic twins (MZ: sharing close to 100% of their genes) and dizygotic twins (DZ: sharing on average 50% of their genetic variance). The chapter notes the closer relationship MZ twins share compared to DZ twins. While the closeness and dependence among DZ twins decline throughout childhood, these elements remain stable in MZ twins. The effect of zygosity on conflict and rivalry is less clear. For both MZ and DZ twins, conflict stays steady, while rivalry intensifies with school entry, probably driven by external comparisons, but lessens as twins develop into late childhood. Unlike singletons, where birth order dictates dominance dynamics, in twins, this dynamic is more variable and becomes more defined by around 6.5 years of age. Several factors are presented as impacting the nature of the twins' relationships: the evolvement of 'twin language', the parenting style and the differential parenting they receive. This exploration into the development of twins' relationships underlines the importance of tailored caregiving and invites further research into the genetic and environmental factors that shape close bonds.

The Twin Relationship Questionnaire

Abstract. Twins’ relationships evoke critical dilemmas for parents and teachers regarding raising and educating their twins. The current study provides comprehensive psychometric information about the Twin Relationship Questionnaire (TRQ; Fortuna, Goldner, & Knafo, 2010 ) as examined in nine samples, using mothers’ and fathers’ reports on 1,560 pairs of twins, tested in five measurement points from age 3 to age 9. On half of the subsamples, we conducted principal component analyses (PCA), whereas on the other subsamples, we conducted confirmatory factor analyses (CFA). Five factors were found in the PCA and confirmed by the CFA: conflict, closeness, dependence, dominance, and rivalry. All scales showed adequate internal consistencies and corrected item-total correlations. Correlation matrixes between scales demonstrate a triad of conflict, rivalry, and dominance scales, as opposed to the dyad of closeness and dependence. As evidence for the TRQ’s reliability and validity, we report the degree of agreement between mothers and fathers, the 10-month stability of mothers’ reports, and the associations of the TRQ subscales with experimentally assessed prosocial behaviors between the twins at age 6.5, and with the twins’ reports on their closeness at age 11. The replication of the findings across ages and raters reinforces the reliability and validity of the questionnaire.

The Longitudinal Israeli Study of Twins (LIST) Reaches Adolescence: Genetic and Environmental Pathways to Social, Personality and Moral Development

The Longitudinal Israeli Study of Twins (LIST) focuses on the developmental, genetic and environmental contributions to individual differences in children's and adolescents' social behavior. Key variables have been empathy, prosocial behavior, temperament and values. Another major goal of LIST has been to study gene-environment correlations, mainly concerning parenting. LIST includes 1657 families of Hebrew-speaking Israeli twins who have participated at least once in the study. Children's environment and their development are assessed in a multivariate, multimethod fashion, including observed, parent-reported and self-reported data. The current article summarizes and updates recent findings from LIST. For example, LIST provided evidence for the heritability of human values with the youngest sample to date, and the first genetic investigation of adolescents' identity formation. Finally, future aims of LIST are discussed.