A groundbreaking Swedish study has revealed how fathers’ genetic vulnerabilities can shape their children’s mental health outcomes, even when those fathers never raised them. The research, published in JAMA Psychiatry in November 2025, examined over 2.5 million offspring and their fathers across multiple family structures, offering fresh insights into the complex interplay between heredity and environment.

Tracking Families Across Generations

Researchers from Virginia Commonwealth University and Lund University tracked Swedish families from 1955 to 2018, analysing how paternal genetic susceptibility influences offspring development of psychiatric and substance misuse conditions. The genetic risk adoption study examined major depression, anxiety disorders, alcohol use disorder, drug use disorder, bipolar disorder, and schizophrenia.

The findings paint a nuanced picture of familial mental health transmission. When fathers with elevated genetic risk raised their biological children, the impact was strongest for substance use disorders. Alcohol use disorder showed a 25% increased risk in offspring, whilst major depression demonstrated a 19% elevation and bipolar disorder similarly showed 19% higher risk.

Isolating Genetic from Environmental Effects

What makes this research particularly revealing is its examination of different father-child relationships. By comparing biological fathers who raised their children, biological fathers who never lived with offspring, stepfathers, and adoptive fathers, researchers could isolate genetic effects from environmental influences.

The research team, led by Dr Kenneth Kendler, discovered that genetic vulnerability alone—without any rearing involvement—still predicted offspring mental health outcomes across all conditions studied. Biological fathers who never lived with their children still passed on measurable risk through genes, with genetic risk associations ranging from 13% to 17% depending on the disorder.

The Discovery of Indirect Genetic Effects

Perhaps most intriguingly, the study identified what researchers call “indirect genetic effects.” Stepfathers and adoptive fathers with high genetic susceptibility to mental health conditions influenced their non-biological children’s risk for depression and substance misuse, despite sharing no genetic material. This suggests that a father’s genetic predisposition shapes his behaviour and parenting style, which subsequently affects children raised in his care.

However, this environmental transmission pathway appeared selective. Whilst genetic risk in non-biological fathers predicted offspring development of internalising conditions like depression and anxiety, as well as substance use disorders, it showed no association with severe psychiatric illnesses such as bipolar disorder or schizophrenia. These findings suggest different transmission mechanisms operate for various mental health conditions.

Evolution in Psychiatric Research Methods

The genetic risk adoption study methodology represents an evolution in psychiatric research. Rather than simply tracking whether parents with diagnosed conditions have affected children, this approach used family genetic risk scores—sophisticated calculations based on illness patterns across extended family networks. This method captures hereditary vulnerability even in relatives who never developed clinical symptoms themselves.

For substance use disorders specifically, the environmental effects proved particularly pronounced. When adoptive or stepfathers carried high genetic susceptibility for alcohol or drug problems, their non-biological children showed elevated risk for developing similar conditions. The association was statistically significant and clinically meaningful, with risk increases of 8% for alcohol use disorder in rearing-only relationships.

Large-Scale Analysis Provides Robust Evidence

The research team examined 2,584,384 offspring with a mean age of 41.7 years at follow-up, providing long-term developmental perspective. Male offspring comprised 51.5% of the sample, and the extended follow-up period through December 2018 allowed researchers to capture conditions that often emerge in early adulthood.

These findings carry important implications for understanding family mental health patterns. They suggest that genetic vulnerability operates through both direct biological transmission and indirect pathways mediated by parenting behaviours and household environment. A father’s own genetic risk—whether he develops symptoms or not—can shape the emotional climate and coping patterns children learn.

Different Disorders Show Varying Environmental Sensitivity

The study also highlights why some psychiatric conditions appear more environmentally sensitive than others. Substance misuse and depression showed clear rearing effects, whilst severe psychotic disorders like schizophrenia demonstrated primarily genetic transmission. This distinction may reflect different developmental windows of vulnerability or varying degrees of environmental modifiability across diagnostic categories.

From a prevention standpoint, understanding these transmission pathways offers potential intervention points. If parental genetic susceptibility influences children through modifiable parenting behaviours rather than inevitable biological destiny, then targeted family support could potentially reduce risk transmission. Early identification of at-risk families, combined with evidence-based parenting programmes and mental health support, might interrupt intergenerational cycles.

Cross-Disorder Transmission Patterns Identified

The research team noted that genetic risk in non-biological fathers was “not diagnostically specific”—meaning that paternal vulnerability to one condition could elevate offspring risk for different but related conditions. A stepfather’s genetic predisposition to alcohol problems, for instance, might increase risk for both substance misuse and depression in children he raises. This cross-disorder transmission pattern underscores the interconnected nature of mental health vulnerabilities.

Whilst the study provides robust evidence from Swedish national registries, researchers acknowledge that findings may vary across cultural contexts with different family structures, substance use patterns, or mental health service provision. The Swedish healthcare system’s comprehensive registries enabled this large-scale genetic risk adoption study, but replication in diverse populations would strengthen generalisability.

Complex Feedback Loops Between Genes and Environment

These findings join growing evidence that mental health transmission across generations operates through multiple, intersecting pathways. Neither pure genetic determinism nor pure environmental causation adequately explains familial patterns. Instead, genes and environment engage in complex feedback loops, with parental genetic characteristics shaping the environments children experience, which in turn influences whether genetic vulnerabilities manifest as clinical conditions.