How many siblings teens have may affect their mental health, study finds

The number of siblings that teenagers have is beyond their control, but new research suggests it may influence their mental health. 

Teens that come from larger families tend to have worse mental health than those with fewer siblings, according to a study involving more than 18,500 teenagers in the United States and China.

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In the U.S., single children and teens with just one sibling had the best mental health, Ohio State University researchers found. Additionally, having older siblings, or being close in age to a sibling, had the worst effect on well-being. The strongest negative associations with mental health were seen in siblings born within one year of each other. 

In China, where most households were limited to having one child from 1980 to 2016, teens without siblings had the best mental health. Due to that policy, 34% of Chinese children do not have siblings. In the U.S., only 12.6% are only children.

So why might having more brothers or sisters adversely affect mental health? Researchers pointed to 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," said Doug Downey, the study's lead author and a sociology professor at Ohio State. "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."

This also may explain why siblings close in age have worse mental health. Siblings of similar ages compete for the same types of resources from their parents.

It's also possible that larger and smaller families differ in other ways that may influence mental health. For instance, children with the most socioeconomic advantages had the best mental health outcomes in both the U.S. and China, the study found. In the U.S., this was families with one or two children; In China, it was one-child families. 

The study, published in the Journal of Family Issues, draws on data from more than 9,100 eighth grade students in the U.S. and more than 9,400 eight graders in China. The teens – who had an average age of 14 – were asked a variety of questions about their mental health, though specific questions differed by country.

Though this study found that the effect of siblings on mental health is "more on the negative side," according to Downey, previous research has shown that having siblings can be associated with positive effects. Having more brothers and sisters has been linked with better social skills among kindergartener students and a lower likelihood of divorce among adults. 

Overall, experts say more research is needed to determine the impact of siblings on mental health, especially as fertility rates in the U.S. are steadily declining and the longer term trend has been toward smaller families. In 2014, 18% of U.S. women had had just one child by the end of their childbearing years, up from 10% in 1976, according to the Pew Research Center.

"Understanding the consequences of growing up with fewer or no brothers and sisters is an increasingly important social issue," Downey said.