- Design
- population-based cohort study using linked national registries, with simulation against a random-mating scenario
- Population
- 4,244,585 Danish residents born 1969-2017, including 549,541 parental couples and 822,209 offspring
- Primary outcome
- parental correlation for psychiatric and somatic diagnoses, and its effect on prevalence and heritability
- Effect
- r = 0.28 overall (SE 0.003), schizophrenia 0.38; heritability inflated 12.5%, prevalence 1.8%
Danish national registry data covered 4,244,585 residents born between 1969 and 2017, from which 549,541 parental couples and 822,209 offspring were identified. Parental correlations for psychiatric and somatic diagnoses were calculated and compared with what a random-mating scenario would produce.
Partner similarity was substantial: r = 0.28 for psychiatric disorders overall, highest for schizophrenia at 0.38 and lowest for anorexia nervosa at 0.11. Cross-disorder correlations, where one partner has one diagnosis and the other a different one, ran from 0.01 to 0.26 — so the resemblance is not confined to the same illness. Correlations for somatic illness were weak (0.03 to 0.14) and were almost entirely explained by comorbid psychiatric diagnoses.
The consequence is methodological and clinical at once. Simulation showed this non-random pairing inflated the observed prevalence of any psychiatric disorder by 1.8% and family-based heritability by 12.5%. Heritability figures quoted from family studies therefore carry a component that is not genetic transmission but who partners with whom.
Clinically it means familial risk clusters more tightly than a single-parent history suggests. When a parent presents with a psychiatric illness, the other parent is more likely than chance to have one too, and the children of such couples concentrate risk from both sides. That is an argument for asking about both parents when taking a family history, and for noticing the whole household rather than the index patient.
- Ask about both parents' mental health when taking a family history, not only the side that prompted the referral
- Expect cross-disorder resemblance — a partner's diagnosis need not match to add to familial risk
- Treat quoted family-based heritability figures as partly reflecting partner similarity, not purely genetic transmission
- Where both parents have a psychiatric diagnosis, consider whether the children are known to any service
- Do not convert these correlations into a risk figure for an individual family; they are population-level estimates
Why it matters
A family history taken from one parent systematically underestimates what the children carry.
Don't overread it
These are population correlations from one national registry and cannot be turned into an individual family's risk.
The statistics, in plain English
A correlation of 0.28 is a moderate association across a population, not a prediction about any couple — plenty of partners share no diagnosis. Standard errors here are tiny (0.003 for the overall figure) because the registry is enormous, so the estimates are precise; precision is not the same as clinical importance. The 12.5% inflation of heritability comes from simulation comparing observed data with a modelled random-mating scenario, so it depends on that model's assumptions rather than being directly observed.
Read the rest in the app
You have read your two free briefings this month. The app carries all 27 specialties, every morning, free — and this finding is waiting in it.
Scan to keep reading on your phone. No account needed to start.
Tomorrow morning, before your first patient
One edition a day for psychiatry, written by the desk, every claim tied to its paper. Six minutes.
Get the app — free