Abstract

Birth order is a crucial environmental factor for child development. For example, later-born children are relatively unlikely to feel secure due to sibling competition or diluted parental resources. The positive effect of being earlier-born on cognitive intelligence is well-established. However, whether birth order is linked to social behavior remains controversial, and the neural correlates of birth order effects in adolescence when social cognition develops remain unknown. Here, we explored the birth order effect on prosociality using a large-scale population-based adolescent cohort. Next, since the amygdala is a key region for sociality and environmental stress, we examined amygdala substrates of the association between birth order and prosociality using a subset neuroimaging cohort. We found enhanced prosociality in later-born adolescents (N = 3160), and observed the mediating role of larger amygdala volume (N = 208) and amygdala-prefrontal functional connectivity with sex-selective effects (N = 183). We found that birth order, a non-genetic environmental factor, affects adolescent social development via different neural substrates. Our findings may indicate the later-born people’s adaptive survival strategy in stressful environments.

Highlights

  • Birth order is a crucial environmental factor for child development

  • A recent meta-analytic functional MRI (fMRI) study showed that cortical regions including middle cingulate cortex, dorsolateral prefrontal cortex (DLPFC), dorsal posterior cingulate cortex, and ventromedial prefrontal cortex are consistently engaged by ­PB32

  • We investigated the effect of birth order on Prosocial behavior (PB), using a large-scale population-based birth cohort sample of adolescents (N = 3160) recruited from the Tokyo TEEN Cohort (TTC) ­survey[44]

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Summary

Introduction

Birth order is a crucial environmental factor for child development. For example, later-born children are relatively unlikely to feel secure due to sibling competition or diluted parental resources. Since the amygdala is a key region for sociality and environmental stress, we examined amygdala substrates of the association between birth order and prosociality using a subset neuroimaging cohort. We found enhanced prosociality in later-born adolescents (N = 3160), and observed the mediating role of larger amygdala volume (N = 208) and amygdala-prefrontal functional connectivity with sex-selective effects (N = 183). We found that birth order, a non-genetic environmental factor, affects adolescent social development via different neural substrates. The positive effect of being earlier-born on cognitive intelligence was ­established[1,2,3] This phenomenon can be accounted for by the resource dilution model positing that parental resources are limited, the resource allocation to each child declines as sibling number and birth order ­increase[4]. Addressing sex differences when analyzing prosociality and MRI-based amygdala measurements is crucial

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