Teen study: The more the siblings, the poorer the mental health

For teens, growing up in large families can take a toll on their mental health.

In a large study from China and the US, teens without or had one sibling reported the highest levels of mental well-being. [J Fam Issues 2023;doi:10.1177/0192513X231220045]

Additional data from the US cohort showed that factors such as having a substantial age difference with older siblings and a narrow age gap between siblings were the most detrimental for adolescent well-being. This was true regardless of whether the siblings shared both or only one biological parent.

The Chinese cohort included 9,417 eighth graders from the China Education Panel Study, while the US cohort comprised roughly 9,191 eighth graders from the Early Childhood Longitudinal Study -- Kindergarten Cohort of 1988.

Notably, the teenagers in China were more likely to have 0.7 fewer siblings compared with their peers in the US (0.89 vs 1.6). About one-third of Chinese children were an only child in their family (34 percent), consistent with China’s One Child Policy, as opposed to 12.6 percent of American children.

The findings came as a surprise for the researchers, with several studies showing that a larger number of siblings may exert some positive effects, including better social skills among kindergarteners and a lower likelihood of divorce among adults.

“Our results couldn’t have been easily predicted before we did the study,” said lead researcher Prof Douglas Downey of Ohio State University in Columbus, Ohio, US. “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 acknowledged that the study did not capture data on the quality of sibling relationships, pointing out that higher-quality sibling relationships would likely be more beneficial to children and mental health.

Resource dilution and selectivity

According to Downey, the study lend support to the “resource dilution” theory, wherein resources that are already limited in the first place, such as parent’s time or attention, are spread thin when divided among more individuals.

“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. 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,” he explained.

Conversely, families with different numbers of children may have other inherent differences that play a role in their children's mental health outcomes—the so-called “selectivity” explanation, Downey noted, adding that there is more to learn about the impact of siblings.

“There are reasons to believe that siblings could influence mental health differently at later ages, however. It could be that adolescence is an unusual life stage and that later in adulthood siblings become sources of social support. If so the association between siblings and mental health might change at later stages of life,” Downey pointed out.