論文

　　慶應義塾大学首都圏ふたごプロジェクト（研究代表：安藤寿康 慶應義塾大学教授）では，『ふたご「の」研究』『ふたご「による」研究』『ふたご「のための」研究』を三本の柱として行っています（プロジェクトの全体像については，Ando, et al.2）をご参照ください）．本講演では，首都圏ふたごプロジェクトがこれまでに行ってきた三本の柱の研究の中から，乳幼児期の双生児に関するものを中心に研究結果を紹介しました．

This study investigated the reciprocity of prosocial behavior among 3- and 4-year-old Japanese preschool children during free-play time. Matrix correlation tests revealed positive correlations between the frequencies of object offering given and received within dyads and between the frequencies of helping given and received within dyads. These results suggest that young children reciprocate prosocial behavior spontaneously. Positive correlations were also found between the frequencies of object offering and helping behavior exchanged within dyads, suggesting that children exchanged the two types of prosocial behaviors (i.e., "interchanged"). The interchange was independent of both reciprocity within object offering and reciprocity within helping behavior in 4-year-olds. Friends reciprocated object offerings more frequently than non-friends, suggesting that friendship affects the quantitative aspect of reciprocity. These data provide refined evidence of reciprocity among children and also suggest that reciprocity becomes more complicated as children grow older. © 2008 Sage Publications.

Using social network analysis, we investigated the characteristics of social networks composed of positive relationships (positive network: PN) and negative relationships (negative network: NN) in classrooms of Japanese 3- and 4-year-olds. Analysis of "density" showed that PNs were denser than NNs among 4-year-olds but that this was not the case among 3-year-olds. The difference between the probability of dyads of girls forming cliques, between PNs and NNs, was larger than that for dyads of boys or mixed-sex dyads. Four-year-olds formed cliques more often in PNs than in NNs, compared to 3-year-olds. This study showed that both sex combination of dyads and age affect the quantified properties of social networks among preschoolers. © 2009 The International Society for the Study of Behavioural Development.

This study examined the link between sibling relationships and children's social adjustment by comparing twin siblings and siblings with different ages (singleton siblings), and clarified the role of reciprocity in sibling relationships on children's social development. Mothers of 58 monozygotic twin pairs, 48 dizygotic twin pairs, and 86 singleton sibling pairs reported their children's sibling relationships and social adjustment. This study showed that the effects of sibling relationships on the prosocial behaviors and conduct problems of each child are stronger for twins than for singleton siblings. Moreover, positivity toward one's sibling increased peer problems only among monozygotic twins. The opposite tendency was present among dizygotic twins and singleton siblings. This study suggests the importance for children's social development of having many interactions with siblings and establishing reciprocity in sibling relationships. Moreover, our results suggest that the quality of sibling relationships among monozygotic twins may be different from those among dizygotic twins and singleton siblings. © The Authors 2012.