Birth order and sickness absence: register-based evidence from Finland

Background In working ages, sickness absence is strongly related to persons' health condition. We studied how birth order was associated with receipt of sickness absence, distinguishing mental disorders, musculoskeletal disorders and injuries. Methods A follow-up study based on the entire Finnish population was conducted for sibling groups born 1969-1982, in which each sibling was observed from age 35 years in the period 2004-2018. Focus was on within-family variation in first-time sickness allowance receipt. Stratified Cox regressions were estimated using Stata 16. Results Each increase in birth order was associated with a higher risk of sickness absence. For mental disorders, the hazard ratio as compared to first borns was 1.03 (95% CI: 0.98, 1.08) of second borns, 1.10 (95% CI: 0.99, 1.22) of third borns, and 1.52 (95% CI: 1.25, 1.85) of fourth or higher borns. Corresponding numbers for musculoskeletal disorders were 1.12 (95% CI: 1.07, 1.17), 1.19 (95% CI: 1.09, 1.30) and 1.15 (95% CI: 0.96, 1.38), and for injuries 1.06 (95% CI: 1.01, 1.12), 1.09 (95% CI: 1.21, 1.14) and 0.96 (95% CI: 0.77, 1.20), respectively. These associations were notably stronger for women than men, but modestly influenced by mother's age at birth, educational level, occupation, income and family composition. Conclusions The study was the first to provide evidence about an interrelation between birth order and sickness absence, utilising diagnose information and family fixed effects methods. A clear birth order pattern was observed for these main causes, but not for a residual group that consisted of other miscellaneous causes. Higher birth order was found associated with a higher risk of sickness absence, and particularly so for mental disorders and muscoloskeletal disorders in women. The underlying mechanisms behind these associations constitute an avenue for future research. Key messages * Using sibling fixed effects, the study was the first to analyse if sickness absence by main cause is associated with birth order in working ages. * First-born siblings were the least likely to be sick because of mental or musculoskeletal disorders, and the associations were notably more accentuated for women than men.