When it comes to family, many might assume “the more, the merrier” — but for some, that might not be the case, according to a recent study.

Researchers from The Ohio State University found that teens with a greater number of siblings reported poorer mental health than those who came from smaller families.

“The association between the number of siblings and mental health was negative in two large datasets in different countries (U.S. and China),” Doug Downey, lead author of the study and professor of sociology at The Ohio State University, told Fox News Digital.

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The large-scale study, published last month in the Journal of Family Issues, included more than 9,400 eighth graders in China and more than 9,100 children of the same age in the U.S.

The participants in both countries answered various questions about their mental health. 

Based on the responses, among the Chinese teens, the ones with no siblings were found to have the best mental health. 

In the U.S., teens with no siblings or just one sibling had the best mental health.

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The age differences between siblings also appeared to be a factor, the study found.

Those who had older siblings and siblings who were born within a year of one another were shown to have the worst mental well-being.

One theory about the study’s findings is what Downey calls the “resource dilution” explanation.

“If you think of parental resources like a pie, one child means they get all the pie — all the attention and resources of the parents,” he said in an OSU press release.

“But when you add more siblings, each child gets fewer resources and [less] attention from the parents, and that may have an impact on their mental health.”

“If you think of parental resources like a pie, one child means they get all the pie – all the attention and resources of the parents.”

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The researchers were surprised by the findings, given that previous research had shown hints of positive outcomes related to siblings, such as better social skills and lower probability of divorce, noted Downey.

‘Cannot be sure it is causal’

The study did have some limitations, the researchers acknowledged.

“We found an association between [the] number of siblings and poorer mental health — but we cannot be sure that it is causal,” Downey said.

The researchers also did not analyze the quality of sibling relationships, which could have a direct impact on mental health.

At this point, Downey said, it is too early to recommend any changes in human behavior based on these findings.

“Scholars are only beginning to understand the consequences of fertility change,” he told Fox News Digital.

“As fertility decline continues, understanding the consequences of growing up with fewer siblings becomes increasingly important,” he added. “In this case, the evidence appears to be positive.”

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Kim Arrington, a clinical psychologist at Hackensack Meridian Health in New Jersey, was not involved in the study but offered her input on the findings.

“With this study, we are witnessing the results of the evolution of modern societies away from agrarian societies where having multiple children could be an asset to working farms,” she told Fox News Digital. 

“Now, with greater specialization in jobs and higher economic demands in our culture resulting in many households requiring two working parents, there are fewer resources in the form of attention from parents to go around, no doubt having an effect on child development.”

In Arrington’s opinion, modern cultures are also less likely to have multi-generational households and communities that, in the past, served as additional sources of childcare, when it was easier for families to pool resources. 

“Prior studies showed advantages to having more siblings, so overall the data is mixed.”

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Alex Dimitriu, M.D., a psychiatrist and founder of Menlo Park Psychiatry & Sleep Medicine in California, also was not part of the OSU research, but he shared his thoughts on the concept of “research dilution.”

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“In psychology, ‘resource dilution’ suggests that the availability of parental resources, such as time, attention and finances, decreases as the number of children in a family increases,” he told Fox News Digital. 

“The authors also mention that prior studies showed advantages to having more siblings, so overall the data is mixed,” he added.

Dimitriu said he would expect there to be a “sweet spot” for the number of siblings.

“It appears 0 to 1 may be optimal, per this study — however, I would also expect numerous factors to impact this, including socioeconomic factors, parental divorce and parenting styles.”

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