Maternal smoking has known adverse effects on fetal development. However, research on the association between maternal smoking during pregnancy and offspring intellectual disability (ID) is limited, and whether any associations are due to a causal effect or residual confounding is unknown.

Cohort study of all Danish births between 1995 and 2012 (1 066 989 persons from 658 335 families after exclusions), with prospectively recorded data for cohort members, parents and siblings. We assessed the association between maternal smoking during pregnancy (18.6% exposed, collected during prenatal visits) and offspring ID (8051 cases, measured using ICD-10 diagnosis codes F70–F79) using logistic generalised estimating equation regression models. Models were adjusted for confounders including measures of socio-economic status and parental psychiatric diagnoses and were adjusted for family averaged exposure between full siblings. Adjustment for a family averaged exposure allows calculation of the within-family effect of smoking on child outcomes which is robust against confounders that are shared between siblings.

We found increased odds of ID among those exposed to maternal smoking in pregnancy after confounder adjustment (OR 1.35, 95% CI 1.28–1.42) which attenuated to a null effect following adjustment for family averaged exposure (OR 0.91, 95% CI 0.78–1.06).

Our findings are inconsistent with a causal effect of maternal smoking during pregnancy on offspring ID risk. By estimating a within-family effect, our results suggest that prior associations were the result of unmeasured genetic or environmental characteristics of families in which the mother smokes during pregnancy.

Anorexia nervosa and bulimia nervosa are two severe eating disorders associated with high premature mortality, suicidal risk and serious medical complications. Transition between anorexia nervosa and bulimia nervosa over the illness course and familial co-aggregation of the two eating disorders imply aetiological overlap. However, genetic and environmental liabilities to the overlap are poorly understood. Quantitative genetic research using clinical diagnosis is needed.

We acquired a clinical diagnosis of anorexia nervosa (prevalence = 0.90%) and bulimia nervosa (prevalence = 0.48%) in a large population-based sample (N = 782 938) of randomly selected full-sisters and maternal half-sisters born in Sweden between 1970 and 2005. Structural equation modelling was applied to quantify heritability of clinically diagnosed anorexia nervosa and bulimia nervosa and the contributions of genetic and environmental effects on their overlap.

The heritability of clinically diagnosed anorexia nervosa and bulimia nervosa was estimated at 43% [95% confidence interval (CI) (36–50%)] and 41% (31–52%), respectively, in the study population, with the remaining variance explained by variance in unique environmental effects. We found statistically significant genetic [0.66, 95% CI (0.49–0.82)] and unique environmental correlations [0.55 (0.43–0.66)] between the two clinically diagnosed eating disorders; and their overlap was about equally explained by genetic and unique environmental effects [co-heritability 47% (35–58%)].

Our study supports shared mechanisms for anorexia nervosa and bulimia nervosa and extends the literature from self-reported behavioural measures to clinical diagnosis. The findings encourage future molecular genetic research on both eating disorders and emphasize clinical vigilance for symptom fluctuation between them.