Summary: A massive Danish study shows that most mental illnesses—like schizophrenia, bipolar disorder, and depression—occur in people with no close family history of the condition. Analyzing data from over 3 million individuals, researchers found that while heredity increases risk, most diagnosed individuals do not have affected relatives.
This challenges stigma and clarifies that mental disorders arise from complex genetic and environmental factors. The findings highlight the need for both personalized and population-level approaches to mental health.
Key Facts:
- Family History Rare: 89% of people with schizophrenia have no close relative with the condition.
- Complex Causes: Mental illness stems from many small genetic variations plus environment.
- Positive Outlook: Even with family history, most people never develop a mental disorder.
Source: Aarhus University
It is a common belief that mental illness ‘runs in families’. And while it is true that heredity plays a role, in reality, the vast majority of cases – such as schizophrenia, bipolar disorder, depression, and borderline personality disorder – occur in people without any known hereditary predisposition.
This is the conclusion of a major new study by researchers at the Danish Center for Register-Based Research, Aarhus University, recently published in the scientific journal Lancet Psychiatry.
The study is based on data from over 3 million Danes, and the researchers hope it will help alleviate concerns and reduce stigma.
“Typically, people are most worried about what they see among close relatives. Very few go around fearing they’ll develop schizophrenia unless they have a relative with the condition. But this study clearly shows that all of us have a risk of developing mental illness,” says Professor Carsten Bøcker Pedersen, who co-authored the study with Professor Esben Agerbo.
“Eighty-nine percent of people diagnosed with schizophrenia have no history of the disorder among close relatives. So it’s far from something that only occurs in affected families,” he adds.
A similar pattern is seen with depression: 60% of individuals who experience depression have no relatives with the disorder.
What does my mother’s illness mean for me?
The study raises an important question: If most mental illnesses occur in people without a relative with the same condition, how do they arise?
The researchers can’t give a precise answer.
“Mental disorders are hereditary, but they are also highly polygenic. They often result from many small genetic variations, not a single ‘disease gene.’ Additionally, environment and chance also play a role,” explains Professor Esben Agerbo.
“But the study is groundbreaking because it describes the absolute risk – how many out of 100 people will develop a specific mental disorder in their lifetime. This functions like an atlas, helping individuals understand, for example, the statistical implication of having a mother with a mental illness,” he says.
The researchers hope the data can help nuance the narrative for families affected by highly heritable conditions like schizophrenia.
“If your father or sister has schizophrenia, it does not mean you will necessarily develop it. In fact, the study shows that 92% of people with a first-degree relative with schizophrenia do not develop the disorder themselves,” says Carsten Bøcker Pedersen.
So while the study supports that heredity is a factor, it also makes clear that heredity is unpredictable and complex, he adds:
“If you have a parent or sibling who has had depression, your risk of developing depression is around 15% – while your risk is under 5% if you have no close relatives with the condition. But that also means you still have an 85% chance of not developing the illness, even if it appears in your immediate family. That’s an important message.”
Digging into old church records
The researchers used Danish national registries to link individuals and their mental health diagnoses across generations. The study draws on the Civil Registration System, the Multigeneration Register, the Danish Psychiatric Central Research Register, and the National Patient Register.
Everyone diagnosed in psychiatric care between January 1970 and December 2021 is included in the study – but all data is pseudonymized, meaning that researchers never accessed personally identifiable information.
Carsten Bøcker Pedersen and Esben Agerbo are key figures behind the Multigeneration Register, which they are developing in collaboration with the Danish National Archives. The register links family relationship data from the Civil Registration System with historical information from church records, tracing Danish family links back to 1920.
The Multigeneration Register provides a powerful foundation for further studies on how heredity and family structure influence health in Denmark – such as the development of mental illnesses.
“This study points to the need for a dual-track approach to addressing mental illness. One track involves personalized medicine, considering individual genetics and environment. The other involves population-wide measures, such as reducing societal stressors, lowering alcohol consumption, and promoting mental well-being,” says Esben Agerbo.
“Both are essential if we are to prevent and understand mental illness – in both individuals with mentally ill relatives and the majority for whom the illness arises without a family history,” he concludes.
About the research
- The study is a prospective cohort study.
- Investigated disorders: substance abuse, cannabis abuse, alcohol abuse, schizophrenia and related disorders, schizoaffective psychosis, affective disorders, bipolar disorder, depression, personality disorders, borderline personality disorder, and antisocial personality disorder.
- Collaborators: Danish National Archives, University of Copenhagen, Aarhus University Hospital
- External funding: Novo Nordisk Foundation
About this mental health and neuroscience research news
Author: Vibe Noordeloos
Source: Aarhus University
Contact: Vibe Noordeloos – Aarhus University
Image: The image is 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. LancetPsychiatry
Abstract
Absolute and relative risks of mental disorders in families: a Danish register-based study
Background
Relative risk estimates of familial aggregation of many types of mental disorders are available, but absolute risk estimates of familial aggregation of mental disorders remain sparse.
The proportion of individuals who develop a mental disorder in the absence of the same disorder in a relative (non-familial cases) has not been examined. We aimed to create comprehensive risk estimates of the familial aggregation of mental disorders.
Methods
In this prospective cohort study, we followed people of Danish origin between Jan 1, 1970, and Dec 31, 2021. We used Danish population-based registers to link individuals and their mental health across extended family pedigrees.
These registers include the Danish Civil Registration System, the Danish Multi Generation Register, the Danish Psychiatric Central Research Register, and the Danish National Patient Register. Mental disorders investigated were substance use disorder, cannabis use disorder, alcohol use disorder, schizophrenia and related disorders, schizophrenia, schizoaffective disorder, mood disorders, bipolar disorder, single and recurrent depressive disorders (depression), personality disorder, borderline personality disorder, and antisocial personality disorder.
We estimated lifetime risk (risk up to age 60 years), age-specific absolute risk, and relative risk for each mental disorder and type of affected relative (eg first, second, or third-degree relatives). We calculated heritability estimates and the proportion of non-familial cases. We involved people with related lived experience in the study design and implementation.
Findings
A total of 3 048 583 individuals (1 486 132 [48·75%] females and 1 562 451 [51·25%] males) were followed up for 80 425 971 person-years. Individuals with a family member with a specific type of mental disorder had higher lifetime and relative risks of developing the same type of mental disorder. Both lifetime and relative risks were higher the closer the affected kinship.
For example, the lifetime risk of depression was 15·48% (95% CI 15·31–15·65) in individuals with affected first-degree relatives, 13·50% (13·25–13·75) in individuals with affected second-degree relatives, 7·80% (7·76–7·84) in the general population, and 4·68% (4·65–4·71) in individuals without affected first-degree and second-degree relatives.
The heritability for depression was 45·4% (95% CI 44·8–46·0) and the proportion of non-familial cases constituted 60·0% (95% CI 59·8–60·2).
Interpretation
Individuals with family members with a mental disorder face increased risks of the same disorder. From a population perspective, most mental disorders occur in individuals without affected close relatives, thus highlighting the need for prevention strategies which target the entire population.
Funding
Novo Nordisk Foundation.
Translation
For the Danish translation of the abstract see Supplementary Materials section.
