Mental health disorders like depression and anxiety are common conditions that affect millions of people in the United States. Importantly, not everyone has the same risk of developing such disorders. A well-known risk factor is having one or several family members who already suffer from a mental health disorder. For example, when both parents suffer from depression, a child has a higher chance of also being diagnosed with depression.
A new study on the familial risk of developing a mental health disorder
A new large-scale study from Denmark focused on the question: how the risk of developing a mental health disorder looks in people with family members who are diagnosed with the same disorders compared to people who have no cases in their family (Pedersen and co-workers, 2025). The study, published in the journal The Lancet Psychiatry, was led by Professor Carsten Bøcker Pedersen from the National Centre for Register-Based Research at Aarhus University in Denmark. The research team analysed data from a so-called prospective cohort study. In this type of clinical psychological study, scientists follow a group of people (called a cohort in research) over a long period of time to see whether they develop a condition or not and which factors influence the onset of a disorder. The data in the study were collected over 50 years, from 1970 to 2021. The scientists used data from Danish population registers to link data on mental health disorders between family members. Mental health disorders investigated in the study were depression, different forms of addiction, schizophrenia, schizoaffective disorder, mood disorders, bipolar disorder, personality disorder, borderline personality disorder, and antisocial personality disorder. Overall, data from over 3 million people were analysed, making the statistics in this study highly robust and trustworthy.
What were the results of the study?
In general, the scientists could replicate what previous studies had shown: People who have relatives who suffer from a mental health disorder have a higher chance of also getting a diagnosis of the same disorder, compared to people who have no one in their family living with that disorder. Also, the closer the affected relative, the higher the increase in risk. For example, for depression, the risk for people with an affected close relative was about 15%. For people with an affected distant relative, it was 13.5%. In contrast, for people without an affected relative, it was only about 5%.
The researchers made another important discovery: While the risk of developing a mental health disorder was higher in people with an affected relative, most people who developed a disorder did not have any affected relatives. The range of these non-familial cases was between 53% for addiction (47% familial cases) and 96% for schizoaffective disorder (only 4% familial cases). For depression, the rate of non-familial cases was 60%. The reason for this is that, compared to the size of the general population, most disorders are uncommon. While people without an affected family member have a lower risk of developing a disorder, there are many more unaffected people than affected people. Thus, even with a lower general risk, the larger size of this group leads to a high percentage of non-familial cases.
Most mental health disorders affect people without familial risk
Therefore, the study has an important takeaway: While people with affected family members have a higher risk of developing a disorder, the overall percentage of patients without familial risk is higher. This needs to be considered by psychotherapists, doctors, and other professionals in the health-care system.
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