Birth order and its link to prosociality investigated by the University of Tokyo

The research group composed of Professor Kiyoto Kasai of the University of Tokyo Graduate School of Medicine, Specially Appointed Associate Professor Naohiro Okada of the University of Tokyo International Research Center for Neurointelligence, Institutes for Advanced Study, Center Director Atsushi Nishida of the Tokyo Metropolitan Institute of Medical Science Research Center for Social Science and Medicine and others succeeded in showing that children born later in the birth order are more prosocial than early-born children in a cohort of about 3,000 10-year-olds. In addition, a magnetic resonance imaging (MRI) study of some 200 people showed that the volume of the amygdala mediates the association between late birth order and high prosociality. Furthermore, it was shown that there is a sex difference in the mediation effect of the functional network between the amygdala and the prefrontal area. This was the first study to reveal that the order of birth affects adolescent brain development as well as social development.

Several previous studies have reported the relationship between birth order and sociality; however, the effect of birth order on adolescent brain development was unknown. The research group first investigated the effect of birth order on prosociality in early adolescence using the Tokyo Teen Cohort Survey, which is a large-scale population-based adolescent cohort. Prosociality was evaluated by the parents of the participating adolescents who answered the "Children's Strength and Difficulty Questionnaire." Subsequently, the research group collected structural brain MRI and resting-state functional MRI data of some participants of the Tokyo Teen Cohort Survey to investigate the mediating effects of amygdala volume and amygdala functional networks on the relationship between birth order and prosociality, respectively.

As a result, it was shown that adolescents with late birth order were more prosocial than adolescents with early birth order. In addition, the research group found a relationship between "second and subsequent birth order" → "large amygdala volume" → "high prosociality." Furthermore, after the second birth, the functional network between the amygdala and the prefrontal cortex became relatively large, and a significant sex difference was found in the relationship between "second and subsequent birth order" → "large functional network between the amygdala and prefrontal cortex" → "high prosociality". This relationship between "second and subsequent birth order" → "large functional network between the amygdala and prefrontal cortex" → "high prosociality" is suggested for girls, while the opposite tendency was recognized in boys.

Associate Professor Okada said, "If the adolescent has a high degree of prosociality, it will be easier for them to ask for help if they have troubles or problems. In the future, based on the results of this research, we would like to develop educational efforts to foster adolescents' flexibility, including prosociality."

■ Amygdala: It is one of the parts of the brain and is anatomically located inside the temporal lobe. It is known to be related to emotions.

■ Prefrontal cortex: It is one of the areas of the brain. Anatomically, it refers to the anterior region of the frontal lobe located in the anterior part of the cerebral hemisphere. It is known to be related to executive functions (update of information, suppression of behavior, switching of behavior) required when acting toward a goal.