Abstract
BACKGROUND: In working ages, sickness absence is strongly related to persons' health condition. We studied how birth order was associated with receipt of sickness allowance, distinguishing between 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. RESULTS: Results of stratified Cox regressions revealed that each increase in birth order was associated with a slightly higher risk of sickness absence from any cause. For mental disorders, associations were stronger; the hazard ratio as compared to first borns was 1.03 (95% CI: 0.98-1.08) of second borns, 1.10 (0.99-1.22) of third borns, and 1.52 (1.25-1.85) of fourth or higher borns. Corresponding numbers for musculoskeletal disorders were 1.12 (1.07-1.17), 1.19 (1.09-1.30) and 1.15 (0.96-1.38), and for injuries 1.06 (1.01-1.12), 1.09 (1.21-1.14) and 0.96 (0.77-1.20), respectively. CONCLUSIONS: Birth order effects were generally stronger for women than men, and to some extent influenced by educational level, occupation, income, and family composition. Possible latent mechanisms behind the associations may relate to within-family dynamics at childhood.