Summary: A large Danish register-based study finds that most diagnosed cases of major mental illnesses—such as schizophrenia, bipolar disorder, and depression—occur in people who do not have an affected close relative. By analyzing data from over 3 million individuals across multiple generations, the researchers show that while family history raises the risk for the same disorder, the majority of people with these diagnoses have no known familial predisposition.
These results challenge common assumptions about mental illness inheritance, emphasize the complex interplay of many genetic variants and environmental factors, and support both individualized and population-level approaches to prevention and care.
Key facts:
- Family history uncommon: Most people diagnosed with schizophrenia do not have a close relative with the disorder.
- Complex origins: Mental disorders are influenced by many small genetic variations, environmental exposures, and chance.
- Reassuring perspective: Even when a close relative has a mental disorder, most relatives will not develop that disorder themselves.
Source: Aarhus University
It is widely believed that mental illness “runs in families.” The new study from the Danish Center for Register-Based Research at Aarhus University confirms that heredity is important, but it also makes clear that most cases of conditions such as schizophrenia, bipolar disorder, depression, and borderline personality disorder occur in people without a known family history.
The analysis covers more than 3 million Danes and aims to provide clear, evidence-based guidance that can reduce stigma and help families better understand risk.
“People often worry most about what they observe in close relatives,” says Professor Carsten Bøcker Pedersen, co-author of the study along with Professor Esben Agerbo. “But this work shows that everyone has some risk of developing a mental disorder, and that the vast majority of diagnosed individuals do not have an affected close relative.”
For example, the study reports that a large proportion of individuals diagnosed with schizophrenia have no history of the disorder among close family members. The same pattern is evident for depression: a substantial share of people diagnosed with depression have no relatives with that condition.
What does a parent’s illness mean for me?
The study addresses a practical question many people face: if a parent or sibling has a mental disorder, what does that mean for my own risk? The researchers emphasize that while having an affected relative increases relative and lifetime risk for the same disorder, absolute risks remain modest in many cases.
Mental disorders are heritable but highly polygenic—meaning they often arise from the combined effect of many small genetic variants rather than a single disease-causing gene. Environmental factors and random events also shape risk, so heredity is only one part of a multifaceted picture, explains Professor Esben Agerbo.
To give families clearer context, the study provides absolute risk estimates—how many people out of 100 might develop a specific disorder over their lifetime. These figures help translate statistical risk into a more understandable form. For instance, while a close family history raises the likelihood of depression, most people with an affected parent or sibling still do not develop the disorder themselves.
“If your father or sister has schizophrenia, it does not mean you will necessarily develop it,” says Carsten Bøcker Pedersen. “In fact, many relatives of affected individuals remain unaffected. That’s an important, reassuring message.”
Methods: linking generations through registers
The research used Denmark’s comprehensive national registries to link mental health diagnoses across family trees. Key data sources included the Civil Registration System, the Multigeneration Register, the Danish Psychiatric Central Research Register, and the National Patient Register. The study includes every person diagnosed in psychiatric care from January 1970 through December 2021, with all personal data pseudonymized so researchers never had access to identifiable information.
The Multigeneration Register, developed in collaboration with the Danish National Archives, connects modern civil registration with historical church records, allowing family relationships to be tracked back to the early 20th century. This linkage enabled the team to estimate lifetime and age-specific absolute risks, relative risks by degree of kinship, heritability, and the proportion of cases that occur without affected relatives.
Using these methods, the study shows that closer kinship corresponds to higher risk, but from a population perspective most mental disorders still arise in people without affected close relatives. That finding underpins the authors’ recommendation for a two-pronged approach to mental health: targeted, individualized interventions that consider genetics and personal circumstances, alongside broad population measures to reduce environmental stressors and promote mental well-being.
About the research
- Study design: prospective cohort study covering 1970–2021.
- Disorders examined: substance use disorders (including alcohol and cannabis), schizophrenia and related psychoses, mood disorders (including bipolar disorder and major depression), and personality disorders (including borderline and antisocial personality disorders).
- Collaborators: Danish National Archives, University of Copenhagen, Aarhus University Hospital.
- Funding: Novo Nordisk Foundation.
About this mental health and neuroscience research news
Author: Vibe Noordeloos
Source: Aarhus University
Contact: Vibe Noordeloos – Aarhus University
Image: Image credited to Neuroscience News
Original Research: Open access. “Absolute and relative risks of mental disorders in families: a Danish register-based study” by Carsten Bøcker Pedersen et al., Lancet Psychiatry. The study provides comprehensive absolute and relative risk estimates of familial aggregation for a range of mental disorders.
Abstract
Absolute and relative risks of mental disorders in families: a Danish register-based study
Background
While relative risks of familial aggregation have been reported for many mental disorders, absolute risk estimates—showing how many individuals develop a disorder in the absence of affected relatives—have been scarce. This study aimed to provide comprehensive absolute and relative risk estimates for familial aggregation of mental disorders.
Methods
The prospective cohort followed people of Danish origin from Jan 1, 1970, to Dec 31, 2021, using national registers to link individuals across extended family pedigrees and to psychiatric diagnoses. Disorders investigated included substance use disorders, schizophrenia and related disorders, mood disorders (including bipolar disorder and depression), and personality disorders. The study estimated lifetime risk up to age 60, age-specific absolute risk, relative risk by degree of kinship, heritability, and the proportion of non-familial cases. People with lived experience contributed to study design and implementation.
Findings
More than 3 million individuals were followed for over 80 million person-years. Individuals with an affected family member had higher lifetime and relative risks for the same disorder, with greater risk for closer relatives. For example, lifetime risk of depression was higher in those with affected first-degree relatives compared with the general population, while a substantial proportion of depression cases occurred without affected relatives. Heritability estimates varied by disorder, and many cases were non-familial.
Interpretation
Having family members with a mental disorder increases individual risk, but from a public-health perspective most disorders occur in people without affected close relatives. Prevention and policy should therefore combine targeted care for high-risk families with population-level strategies to reduce risk and promote mental health.
Funding
Novo Nordisk Foundation.
Translation
A Danish translation of the abstract is included in the study’s supplementary materials.