The effect, detailed this month in the journal Economics and Human Biology, is only correlational and so the scientists can’t say whether one causes the other. But they do offer a mechanism, suggesting that in this region, a younger sibling — particularly a younger brother — takes parental attention from older children while burdening the older kids with extra responsibility.
“If there are more siblings in the family, the parents’ resources are limited. The older brother will see the younger brother as a competitor and it will kind of stress them out,” Zeng said.
Little brothers are a stressor, not because they are competitors, but because keeping them out of trouble is a constant worry.