The association between childhood income and mental health: can sibling comparison designs help resolve this research question?

Background In previous studies, null results from sibling comparison designs have been used to infer that no causal association exists between childhood family income and subsequent risk of mental disorders. The aim of our study is to replicate the findings from previous studies and propose three alternative explanations based on i) variability in income and mental health between siblings, ii) life course models, and iii) detailed descriptive statistics. Methods We followed a nationwide Danish cohort born from 1986 to 1996 (n = 643,814, including 404,179 siblings) from age 15 until diagnosis of severe mental disorders or censoring. Adjusted hazard ratios (aHR) were estimated between childhood family income and subsequent mental disorders, accompanied by comprehensive descriptive statistics and triangulation analyses that help in interpreting the findings. Results We observed an association between a $15,000 increase in family income and severe mental disorders (0.78; 0.76-0.81), but the sibling comparison design showed null results (1.02; 0.94-1.11). Siblings were typically born three years apart, with a difference in monthly average income at age 14 of $496 (interquartile range: $150-$641). A pseudo-sibling cohort also indicated null results (0.93; 0.85-1.01), although siblings were not family members. Family income measured yearly demonstrated comparable estimates across ages 1-14 (from 0.67 to 0.82). Conclusion Our investigation suggests that the null associations between childhood income and mental health estimated through sibling comparison designs may be explained by the fact that siblings are close together in age and residing within the same family income environment with only minor income fluctuations. Additionally, a sibling comparison design is useful to test a critical/sensitive period, but our analyses suggest that no age posits a critical risk age for this research question. This raises caution on the current practice of relying on sibling comparison designs to estimate the association between childhood income and mental health. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was supported by a grant from Independent Research Fund Denmark (grant 1030-00085B). Oleguer Plana-Ripoll is also supported by a Lundbeck Foundation Fellowship (R345-2020-1588) and Independent Research Fund Denmark (grant 2066- 00009B). Carsten B. Pedersen is supported by the Big Data Centre for Environment and Health funded by the Novo Nordisk Foundation Challenge Programme (grant NNF17OC0027864). Christian Hakulinen was supported by the European Union (ERC, MENTALNET, 101040247). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study is approved by the Danish Data Protection Agency. No further ethical approval is required for registry-based research in Denmark.cf. LBK nr 1338 af 01/09/2020, sektion 10 Bekendtgoerelse af lov om videnskabsetisk behandling afsundhedsvidenskabelige forskningsprojekter og sundhedsdatavidenskabelige forskningsprojekter [Act no. 1338 of 1 September 2020, section 10 on research ethics for administration of health scientific research projects and health data scientific research projects]. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes The individual-level data used for this study are not publicly available but can be obtained by application to Statistics Denmark and the Danish Health Data Authority.