The midlife health of only children: chronic disease indicators and biomarkers by sibship size in three nationally representative UK cohorts.

Background: Despite persistent concerns about only children's disadvantage relative to individuals with siblings, existing health-related evidence is inconsistent. Recent evidence from Nordic countries about only children having poorer health outcomes may not apply elsewhere because selection processes differ across contexts. We investigate the midlife health of only children in the UK where one-child families tend to be socio-economically advantaged relative to large families.

Methods: Using the 1946, 1958 and 1970 British birth cohort studies, we examine various biomarkers and self-reported measures of chronic disease by sibship size when respondents are aged in their mid-40s, mid-50s and mid-60s. We estimate separate linear probability models for each cohort, age and outcome, adjusting for childhood and early adulthood circumstances.

Results: We found no evidence of only children differing from those with one, two or three or more siblings, at any age, in any of the cohorts, on: heart problems, hypertension, high triglycerides, high glycated haemoglobin or high C-reactive protein. However, compared with only children, the probability for cancer (0.019, 95% confidence interval [CI]: 0.002, 0.035; age 46/1970) and poor general health (0.060, CI: 0.015, 0.127; age 55/1958; and 0.110, CI: 0.052, 0.168; age 63/1946) was higher among those with three or more siblings.

Conclusions: There is no consistent pattern of only child health disadvantage for midlife chronic disease outcomes across ages or cohorts in the UK. Research should focus on better understanding how sibship size differentials are contingent on context.