Preamble

Title: Growing Siblings Child: Potentially Confounding Socioeconomic

Background

Authors: Bolong Haoran Zhang Xiang-Zhen 1,2,3* Affiliations:

1 Department

Psychology Behavioral Sciences, Zhejiang University, Hangzhou, China.

2 The

State Brain-Machine Intelligence, Zhejiang University, Hangzhou, China.

3 Department

Psychiatry Hospital, Zhejiang University School Medicine, Hangzhou, China.

Correspondence Author: Xiang-Zhen Kong, Ph.D. Yuhangtang Road, Hangzhou, Zhejiang, China, 0571-88273337,

great interest study colleagues, which investigate growing without siblings (GWS) affects adult brain structure, function, behavior using large-scale Their findings challenge long-standing stereotypes about children reporting positive associations between sibling absence neurocognitive outcomes mental health indicators.

Importantly, their beyond simple association

analysis

incorporating proximal environmental exposures (PEEs), including multiple socioeconomic family environmental factors, which showed mediated observed brain-behavioral associations. multidimensional design represents major forward understanding social structure interacts neurodevelopment.

However, argue interpretation these findings requires careful consideration historical demographic context sample, specifically, heterogeneity enforcement China one-child policy (1979-2015).

Rather representing random distribution family sizes, sibling status China during period strongly structured socioeconomic geographic factors.

Urban families, particularly those employed government state-owned sectors faced strict enforcement, making likely child contrast, rural families, informal labor populations greater flexibility practice, especially firstborn female result, being child during period one-child policy simply family preference social circumstance, instead often outcome shaped state regulation

socioeconomic opportunity.

themselves reported robust group differences between children those siblings family indicators, including parental education occupation, income-to-needs ratios, family resources, particularly urbanicity scores (pre-adult urban living) These factors formed components latent variables (family city-level shown significantly mediate associations between brain behavioral outcomes. fact, their causal mediation

analysis

demonstrated large proportion variance initially attributed explained SES-linked environments. peripheral result, instead central interpretation their study.

SES-related environments simply mediators sibling effects shape likelihood being child first place policy-dependent enforcement.

Thus, cannot treated exclusively intermediate pathway; function antecedent determinant sibling status (Fig.

Schematic models sibling status family relation brain behaviour.

Sibling status indicates whether child child siblings. mediates effect sibling status.

Sibling status mediates effect sibling status reciprocal direct effects.

acknowledge limitation: unable collect participants children. prevents disentangling policy-related, socioeconomic, cultural, personal motivations underlying family size.

Acknowledging limitation important, shifts interpretation their findings individual familial choice towards historically constrained demographic structure. isolate developmental effects rigorously simple SES-related influences future studies could benefit several methodological extensions. example, stratified analyses across urban rural cohorts, across birth cohorts differing policy exposure, would reduce confounding creating comparable subgroups.

Natural

experiment

approaches exploiting regional variation policy enforcement implementation intensity could further strengthen causal inference. addition, sensitivity analyses testing robustness effects after jointly residualizing multiple indicators would assess extent which

results

depend adjustment strategies. Finally, while authors discussed potential policy implications, caution needed translating these findings contemporary demographic policy.

China, facing declining birth rates, shifted decades birth restriction encouraging larger families. context, findings about only-child development misinterpreted evidence against value sibling relationships Instead, argument support different message: developmental outcomes shaped sibling number alone,

socioeconomic caregiving environments which children Future research should continue disentangle these intertwined influences inform family education policy responsibly.

Competing interests authors declare competing interests. contributions statement B.W., wrote paper.

Acknowledgements supported National Natural Science Foundation China (32171031 32571219), 2030-Major Projects (2021ZD0200409; 2025ZD0218801), Information Technology Center, Zhejiang University.

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