There's some evidence of a health disadvantage for people without siblings - but mainly in Sweden, where 1-child families tend to be socioeconomically disadvantaged. So findings might not apply elsewhere because selection processes differ across contexts.

We contribute by analysing 3 British cohort studies to examine multiple biomarkers and self-reported measures of cardiovascular problems and cancer in midlife (mid-40s, mid-50s and mid-60s). Overall, we find a lack of robust evidence of only child distinctiveness in the UK, for these indicators.