Teens from larger families have poorer mental health than those with fewer siblings, according to a large analysis of children in the United States and China.
The details of the pattern vary depending on factors such as the spacing of sibling ages and the age of the siblings.
But the fact that the overall pattern was found in both countries is striking, said Doug Downey, lead author of the study and professor of sociology at The Ohio State University.
"Our results couldn't have been easily predicted before we did the study," Downey said.
"Other studies have shown that having more siblings is associated with some positive effects, so our results were not a given."
Downey conducted the study with Rui Cao, a doctoral student in sociology at Ohio State. Their results were published recently in the Journal of Family Issues.
Their Chinese analysis draws on more than 9,400 eighth graders from the China Education Panel Study. In the United States, they analyzed over 9,100 American eighth graders from the Early Childhood Longitudinal Study -- Kindergarten Cohort of 1988.
The average youth in China has nearly .7 fewer siblings than the average American youth (.89 compared to 1.6).
Consistent with what was expected because of China's One Child Policy, about one-third of Chinese children are only children (34%), compared to just 12.6% of American children.
In both countries, researchers asked students (average age of 14) a variety of questions about their mental health, although the questions were different in China and the United States.
In China, teens with no siblings showed the best mental health, while in the United States, those with no or one sibling had similar mental health.
Some issues could only be analyzed using the U.S. data.
Results in the U.S. showed that half and full siblings are both linked to poorer mental health.
And having older siblings and siblings closely spaced in age tended to have the worst impacts on well-being, the U.S. data found. Siblings born within one year of each other had the strongest negative association with mental health.
Why are more siblings linked with poorer mental health?
Downey said the overall findings fits with the "resource dilution" explanation.
"If you think of parental resources like a pie, one child means that they get all the pie -- all the attention and resources of the parents," he said.
"But when you add more siblings, each child gets fewer resources and attention from the parents, and that may have an impact on their mental health."
The fact that closely spaced siblings have the most negative impact bolsters that explanation. Children who are near the same age will be competing for the same types of parental resources, he said.
Another possibility, though, is that the families that have many versus few children are different in other ways that may reduce mental health for their kids -- the so-called selectivity explanation.
The differences between China and the U.S. do provide some support for the selectivity explanation. In each country, children from families associated with the most socioeconomic advantage had the best mental health.
In China, that was children in one-child families, while in the U.S. it was children with zero or one sibling.
But the overall results still suggest that selectivity explanation falls short in accounting for what is happening.
"What we found is that when you add all the evidence up, the effect of siblings on mental health is more on the negative side than the positive side," Downey said.
Downey noted that the data doesn't get at the quality of sibling relationships. It is likely that higher-quality sibling relationships will be more beneficial to children and may have more positive effects on mental health.
While this study shows a negative impact of siblings, other research has shown that having more brothers and sisters is associated with better social skills among kindergarteners and a lower likelihood of divorce among adults.
"This combination of results is not easily explained. We still have more to learn about the impact of siblings," Downey said.
"This is particularly important now as the U.S. and other countries have lower fertility rates. Understanding the consequences of growing up with fewer or no brothers and sisters is an increasingly important social issue."
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