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

Abstract

Several related and complementary theoretical frameworks have been proposed to explain the existence of prosocial behavior, despite its potential fitness cost to the individual. These include kin selection theory, proposing that organisms have a propensity to help those to whom they are genetically related, and reciprocity, referring to the benefit of being prosocial, depending on past and future mutual interactions. A useful paradigm to examine prosociality is to compare mean levels of this behavior between monozygotic (MZ) and dizygotic (DZ) twins. Here, we examined the performance of 883 6.5-year-old twins (139 MZ and 302 DZ same-sex 6.5-year-old full twin pairs) in the Differential Productivity Task. In this task, the twins' behaviors were observed under two conditions: working for themselves vs. working for their co-twin. There were no significant differences between the performances of MZ and DZ twins in the prosocial condition of the task. Correlations within the twin dyads were significantly higher in MZ than DZ twins in the self-interested condition. However, similar MZ and DZ correlations were found in the prosocial condition, supporting the role of reciprocity in twins' prosociality towards each other.

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

“…The first report from LIST on this topic found that 3-year-old MZ twins were perceived as closer and more co-twin dependent than DZ twins who in turn were closer and more co-dependent than a matched sample of nontwin siblings (Fortuna et al, 2010). However, using an experimental measure of intertwin prosocial behavior at the age of 6.5 years, we did not find a difference between MZ and DZ twins in their prosocial behavior toward each other (Yirmiya et al, 2018), and the two kinds of twins were similarly positively correlated with each other, suggesting that this behavior is driven by reciprocity (i.e. the likelihood of being prosocial, depending on the past and future mutual interactions), rather than relative genetic similarity between the twins.…”

“Side by side”: Development of twin relationship dimensions from early to middle childhood and the role of zygosity and parenting

“…Finally, understanding the complexities of twins' relationships can bring insights into relationships in general (Mark et al, 2017;Yirmiya et al, 2018). The results of the current study can open a window to the complex associations between the dimensions of sibling relationships through childhood.…”

The Twin Relationship Questionnaire

“…Prosocial behaviors between the twins were assessed using the Differential Productivity Task (N. L. Segal, 1984; Yirmiya et al, 2018), which is a variation of a task used by Madsen (1967) to study children’s tendencies toward cooperation, competition, and altruism. At the beginning of the current task, the child received a sheet of paper lined with 320 schematic small trees and was asked to outline as many trees as possible during one minute with a red marker.…”

“…All twins participated in both conditions, counterbalanced for order. Detailed information about this task appears in Yirmiya et al (2018).…”

“…Finally, to study the external validity of the scales, we tested the associations between the TRQ subscales and other measures of twins’ relationship. Prosocial behavior between the twins as measured in an experimental task at the age of 6.5 (Yirmiya, Segal, Bloch & Knafo-Noam, 2018) was expected to relate positively to the positive aspect of twin relationships, namely closeness, and negatively to the negative aspects of the relationship, namely conflict and rivalry (concurrent validity). In addition, we expected twin’s closeness as reported by parents in the TRQ to relate positively to twins’ reports on their own closeness, using a different measure, at age 11 (predictive validity).…”