Statistics suggest that the size of families in many countries is shrinking and a growing number of parents worldwide either willingly or unwillingly end up only having one child. While many psychology studies have explored the differences between individuals who have siblings and those who don’t, the effects of not having any brothers or sisters on people’s brains and behavior are not yet fully understood.

Past research has yielded varying and sometimes contradictory results, which sometimes hinted at negative effects of being an only child and other times highlighted its positive implications. In addition, these negative and positive effects were found to be inconsistent across studies, with some studies suggesting that only children tend to do better at school, are more pro-social and less problematic, while others showed the opposite.

Researchers at Tianjin Medical University General Hospital and other institutes in China recently carried out a study aimed at better understanding how being an only child affects people’s brain and behavior during adulthood. Their findings, published in Nature Human Behaviour, highlight specific patterns in the brain’s development and activity, as well as behavioral tendencies, that are commonly observed in adults who grew up without siblings.

“With the worldwide increase in only-child families, it is crucial to understand the effects of growing up without siblings (GWS) on the adult brain, behavior and the underlying pathways,” Jie Tang, Jing Zhang and their colleagues wrote in their paper.

“Using the CHIMGEN cohort, we investigated the associations of GWS with adult brain structure, function, connectivity, cognition, personality and mental health, as well as the pathway from GWS to GWS-related growth environments to brain and to behavior development, in 2,397 pairs of individuals with and without siblings well matched in covariates.”

The researchers analyzed data collected as part of the Chinese Imaging Genetics (CHIMGEN) study, which was carried out at 32 research centers in China. They arranged the participants who met their study’s selection criteria into pairs, which contained one only child and one person with siblings who had similar demographics and backgrounds.

By comparing the brains and behaviors of these pairs of participants, the researchers were able to attain new insight into how a lack of siblings affects the brain and people’s behaviors in adulthood. Interestingly, they uncovered recurring patterns among only children in the integrity of white matter, volume of the brain, neuronal activity, memory and mental health, while also unveiling characteristic behavioral phenotypes (i.e., cognitive, personality-related and behavioral tendencies).

“We found associations linking GWS to higher language fiber integrity, lower motor fiber integrity, larger cerebellar volume, smaller cerebral volume and lower frontotemporal spontaneous brain activity,” wrote Tang, Zhang and their colleagues.

“Contrary to the stereotypical impression of associations between GWS and problem behaviors, we found positive correlations of GWS with neurocognition and mental health. Despite direct effects, GWS affects most brain and behavioral outcomes through modifiable environments, such as socioeconomic status, maternal care and family support, suggesting targets for interventions to enhance children’s healthy growth.”

This recent study suggests that while being an only child can have direct effects on people’s brain and behavior, most of its effects are related to the environment in which only children grow up and the stimuli they are or are not exposed to. This suggests that carefully designed interventions aimed at broadening the experiences and opportunities available to only children could help to improve their brain functions, mental health and behavior as adults.

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