Summary: A major Danish study has found that the majority of mental health disorders including schizophrenia, bipolar disorder, and depression arise in individuals without any direct family history of these conditions. Researchers analyzed data from more than three million people and found that while heredity increases the risk, most people who are diagnosed do not have family members affected.
These findings break down stigma and demonstrate that mental disorders are the result of complex genetic and environmental factors. They emphasize the need for personalized, population-specific approaches to mental health.
Important facts:
- Rare family history: Around 89% of people with schizophrenia have no close relatives with the condition.
- Complex causes: Mental illnesses often result from a mix of many small genetic factors and environmental influences.
- Reassuring fact: Even when there is a family history, most people never develop a mental disorder.
Source: Aarhus University
It is often thought that mental illnesses are hereditary. While heredity plays a role, most cases of mental illnesses—such as schizophrenia, bipolar disorder, depression, and borderline personality disorder—actually occur in people who have no genetic predisposition.
This is the result of a large study conducted by researchers at the Danish Center for Register-Based Research at Aarhus University, which was recently published in the scientific journal Lancet Psychiatry.
Typically, people are primarily concerned about what they see in their loved ones. Few people live with the fear of developing schizophrenia unless they have a loved one with the disease. But this study clearly shows that we are all at risk of developing mental illness, says Professor Carsten Becker Pedersen, co-author of the study with Professor Esben Egerbo.
We see a similar pattern with depression: 60% of people with depression have no family members who suffer from the disorder.
What does my mother’s illness mean to me?
The research raises an important question: If most mental health conditions occur in people who have no family members with the same condition, how do they arise?
Researchers cannot give a clear answer to this question.
Mental disorders are hereditary, but also highly multifactorial. They are often the result of many small genetic mutations, not a single pathological gene. In addition, environment and opportunity also play a role, explains Professor Esben Egerbo.
But this study is innovative because it defines absolute risk: how many people out of every 100 will develop a specific mental disorder in their lifetime? He describes it as functioning like an atlas a detailed guide that helps people navigate and understand the statistical impact of factors such as having a mother with a mental disorder. It serves as a reference point, mapping out how family history can influence the likelihood of developing similar conditions.
Researchers hope that these data can help paint a clearer picture of families dealing with highly hereditary illnesses like schizophrenia.
Carsten Becker-Pedersen emphasizes that having a close family member with schizophrenia does not make the condition inevitable. In fact, 92% of people with a first‑degree relative who has schizophrenia never develop the disorder themselves.
He adds that while heredity does play a role, it is far from a straightforward predictor. The genetic contribution to mental illness is highly complex and unpredictable.
If you have a parent, sibling, or child with depression, your risk of developing it is about 15 percent, compared to less than 5 percent if you have no close family members with the condition. This also means that you have an 85 percent chance of not having the condition, even if it is in your immediate family. This is an important message.
Insights from Old Church Records
Researchers drew on extensive Danish national registers to track mental health diagnoses across multiple generations. By linking records from the Civil Register, Multigenerational Register, Danish Central Register for Psychiatric Examinations, and the National Patient Register, they were able to build a comprehensive picture of how psychiatric conditions appear within families over time.
The study covered all individuals who underwent a psychiatric evaluation between January 1970 and December 2021. Importantly, all information was pseudonymized, ensuring that researchers could analyze patterns and trends without ever accessing personally identifiable details. This approach safeguarded privacy while enabling large‑scale, long‑term mental health research.
Carsten Becker-Pedersen and Esben Egerbo are key figures in the Multigenerational Register, which they are developing in collaboration with the Danish National Archives. The register combines kinship data from civil records with historical information from church registers, thereby mapping Danish family relationships since 1920.
The multi-ethnic register provides a solid basis for future research into the influence of heredity and family structure on health in Denmark, for example, on the development of mental disorders.
The study highlights the need for a two-pronged approach to treating mental health conditions. The first is based on personalized medicine, taking into account an individual’s genetics and environment. The second is based on population-level measures, such as reducing social stress, reducing alcohol consumption, and promoting mental well-being, explains Esben Agerbo.
They concluded that “if we want to prevent and understand mental illness, both are necessary, both in individuals who have family members with mental illness and in the majority of people who have the illness without a family history.”
About research
- This research was conducted as a prospective cohort study, tracking participants over time to explore links between various mental health and substance use disorders. The conditions examined included drug dependence, cannabis use, alcoholism, schizophrenia and related disorders, schizoaffective psychosis, affective disorders, bipolar disorder, depression, personality disorders, borderline personality disorder, and antisocial personality disorder.
- The project was a collaboration between the National Archives of Denmark, the University of Copenhagen, and Aarhus University Hospital, with external funding provided by the Novo Nordisk Foundation.
Abstract
Absolute and relative risks of mental disorders in families: A Danish register study
Background
While relative risk figures for family clustering of many mental disorders are well documented, far less is known about the absolute risk — the actual likelihood that a person will develop a disorder when a family member is affected. Even more striking is the lack of data on non‑familial cases, where individuals develop a mental disorder despite having no relatives with the same condition.
This study set out to fill that gap by producing a comprehensive assessment of how mental disorders cluster within families. By examining both familial and non‑familial cases, the research aims to provide a clearer picture of the true burden of inherited and shared environmental risk, offering insights that could guide prevention, early detection, and targeted interventions.
Methods
In this prospective cohort study, we followed individuals of Danish descent between January 1, 1970, and December 31, 2021. We used Danish population registers to link individuals and their mental health in extended family lines.
These registries include the Danish Civil Registry, the Danish Multigenerational Register, the Danish Central Register for Psychiatric Research, and the Danish National Patient Register. The mental disorders studied were: substance abuse, cannabis use, alcoholism, schizophrenia and related disorders, schizophrenia, schizoaffective disorder, mood disorder, bipolar disorder, single and recurrent depressive disorder (depression), personality disorder, borderline personality disorder, and antisocial personality.
Lifetime risk (up to age 60), absolute risk by age, and relative risk were estimated for each mental disorder and type of affected family member (e.g., first-, second-, or third-degree relative). Heritability estimates and percentages of nonfamilial cases were calculated. Individuals with relevant life experience participated in the design and conduct of the study.
Recommendations
A total of 3,048,583 individuals (1,486,132 [48.75%] women and 1,562,451 [51.25%] men) were followed for 80,425,971 person-years. Individuals who had a family member with a specific mental disorder had an increased risk of developing that mental disorder later in life. The risk was higher the closer the affected family member lived to them.
For example, the lifetime risk of depression was 15.48% (95% CI: 15.31–15.65) in affected first-degree relatives, 13.50% (13.25–13.75) in affected second-degree relatives, 7.80% (7.76–7.4) in the general population, and 7.76–7.8% (4.65–4.71) in individuals with unaffected first- and second-degree relatives.
The heritability of depression was 45.4% (95% CI: 44.8–46.0) and the proportion of non-familial cases was 60.0% (95% CI: 59.8–60.2).
Interpretation
People who have a family member with a mental health disorder are at higher risk of developing the disorder themselves. In terms of population, most mental health disorders occur in people who do not have an affected close relative, which emphasizes the need for population-based prevention strategies.
Finding
Novo Nordisk Foundation.
Translation
For a Danish translation of the abstract, see the Supplementary Materials section.
