Number of siblings and social capital among parents rearing schoolchildren: Results from the A-CHILD study.BACKGROUND
 Having siblings may foster sociality; however, little is known about whether sibling number determines social capital, the resources obtained through social networks.  We examined the association between sibling number and social capital among Japanese parents rearing schoolchildren.


 We used cross-sectional data from the 2018 and 2019 Adachi Child Health Impact of Living Difficulty (A-CHILD) study, targeting all primary and junior high school students and their parents in Adachi, Tokyo, Japan (n = 8,082).  Individual-level social capital was evaluated by assessing caregivers' social cohesion, social support, and group affiliation.  An inverse U-shaped association was found between sibling number and social capital.  Adults who grew up with 1 or 2, but not ≥3, siblings had greater social support (β = 0.23, 95% confidence interval [CI], 0.06-0.40; β = 0.46, 95% CI, 0.29-0.64, respectively) than those who grew up as an only child after covariate adjustment.  Adults who grew up with 2 or 3, but not 1 or ≥4, siblings had greater group affiliation (β = 0.09, 95% CI, 0.03-0.16; β = 0.09, 95% CI, 0.01-0.18, respectively) than those who grew up as an only child after covariate adjustment.  Sibling number was not associated with social cohesion.


 CONCLUSIONS
 Growing up with 1 to 3 siblings was associated with higher social capital in adulthood than being an only child.  Having siblings may provide an opportunity to foster social capital.
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