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

Abstract

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.