Mental illness doesn’t come exclusively from the mother or the father. Both parents contribute genetic risk roughly equally through their DNA, and no single parent is “the cause” of a child’s psychiatric condition. That said, the picture is more nuanced than a simple 50/50 split. For certain disorders, research consistently finds that maternal transmission carries a somewhat higher statistical risk than paternal transmission, for reasons that go beyond genes alone.
Both Parents Pass On Genetic Risk
Every child inherits half their DNA from each parent. The genetic variants linked to psychiatric conditions are scattered across many genes on ordinary chromosomes, not bundled on one parent’s side. For disorders like schizophrenia and bipolar disorder, twin studies estimate heritability at around 80%, meaning genetics accounts for the large majority of what determines whether someone develops the condition. For major depression, heritability is lower, roughly 40% to 50%, with environment playing a bigger role.
These risk variants don’t come from a single gene. Researchers now use what are called polygenic risk scores, which add up the effect of hundreds or thousands of small genetic differences. A large study published in Molecular Psychiatry found that children whose parents and grandparents both had depression carried significantly higher polygenic risk scores for depression, bipolar disorder, and related traits compared to children with no family history. The genetic burden accumulated across generations from both sides of the family, not from one parent alone.
Why Maternal Transmission Often Looks Stronger
Despite the equal genetic contribution, studies repeatedly find that having a mother with a psychiatric condition raises a child’s risk more than having a father with the same condition. This pattern shows up across several disorders.
For ADHD, a large study in the Journal of Child Psychology and Psychiatry found that children of mothers with ADHD had a 21.1% chance of developing the condition, compared to 14.8% for children of fathers with ADHD. The relative risk was 8.4 times the general population rate for maternal ADHD versus 6.2 times for paternal ADHD. This held true for both sons and daughters, though daughters showed a particularly large gap: 10.4 times the baseline risk from maternal ADHD versus 6.7 times from paternal ADHD.
For schizophrenia, a nationwide Swedish study found a similar pattern. Women whose mothers had schizophrenia were about 9 times more likely to develop it themselves, compared to roughly 5.5 times more likely if their father had it. Men showed a smaller but still present difference: about 7.3 times the risk from a mother with schizophrenia versus 6 times from a father.
For bipolar disorder, offspring of mothers with the condition had higher risks of multiple psychiatric diagnoses, lower school grades, and more accidents compared to offspring of fathers with bipolar disorder. Having two parents with bipolar disorder carried the highest risk of all.
Three Reasons Beyond Shared Genes
Several biological mechanisms help explain why maternal transmission appears stronger:
- The prenatal environment. A mother’s physical and mental health during pregnancy directly shapes fetal brain development. When a mother’s immune system activates during pregnancy from infection or severe stress, it alters molecular signaling, synapse formation, and neurotransmitter balance in the developing brain. Meta-analyses have linked this prenatal immune activation to increased offspring risk for schizophrenia, depression, autism, and anxiety.
- Mitochondrial DNA. Mitochondria, the energy-producing structures inside every cell, carry their own small set of DNA that is inherited almost entirely from the mother. A single egg cell contains roughly 100,000 copies of mitochondrial DNA, compared to about 1,200 in a sperm cell. Mitochondrial dysfunction has been linked to schizophrenia, bipolar disorder, and depression, and researchers have documented families where these conditions appear to follow a maternal inheritance pattern consistent with mitochondrial transmission.
- Early caregiving environment. Mothers more often serve as primary caregivers, especially in early childhood. A parent struggling with untreated mental illness during those formative years can shape a child’s stress responses and emotional development through day-to-day interactions, not genetics.
How Fathers Contribute Risk in Unique Ways
Fathers aren’t off the hook. Beyond contributing half the child’s DNA, paternal factors carry their own distinct risks. Advanced paternal age is one of the most studied. A 10-year increase in a father’s age at conception is associated with a 21% higher risk of autism in the child. Fathers younger than 20 or older than 45 at conception have children with a 1.72-fold increased risk of developing poor social skills compared to fathers aged 25 to 29. This happens because sperm cells divide continuously throughout a man’s life, accumulating new mutations with each division. Older fathers pass on more of these spontaneous mutations.
Fathers also transmit epigenetic changes. Animal research has shown that a father’s diet, stress exposure, and substance use can alter small regulatory molecules in sperm. These molecules influence how genes are expressed in the resulting embryo without changing the DNA sequence itself. In mouse studies, stress-related changes in sperm were transmitted to offspring and influenced their behavior. While human evidence is still building, this means a father’s life experiences before conception may shape his child’s mental health risk in ways that don’t show up in standard genetic testing.
Environment Matters More Than Many People Realize
Genetics is only part of the story. The shared family environment, everything from household stress to neighborhood to parenting style, accounts for a meaningful portion of psychiatric risk. In studies of depression and anxiety in children, shared environment explained 26% to 34% of the overlap between the two conditions. For female adolescents, that number jumped to 65%. In rural settings, one study found that shared environment accounted for 86% of the variance in behavioral problems, with genetic factors dropping to nearly zero. The same genetic predisposition can express very differently depending on the environment a child grows up in.
This is actually encouraging. It means that even children with high genetic risk from both parents can have better outcomes in stable, supportive environments. And children with low genetic risk can still develop problems in chaotic or traumatic ones. The interplay between genes and environment is constant and bidirectional.
What This Means for Families
If you’re asking this question because mental illness runs in your family, the most useful takeaway is that risk is cumulative, not deterministic. Having one parent with a condition raises your risk. Having both parents affected raises it further. Having grandparents affected adds another layer. But none of these guarantee that a child will develop the same condition.
The disorders with the highest heritability, schizophrenia and bipolar disorder at around 80%, still leave room for 20% of the variation to come from environmental factors. For depression, roughly half the risk comes from non-genetic sources. Children who inherit elevated genetic risk but grow up with early support, stable relationships, and access to care when symptoms first appear tend to have significantly better trajectories than those who don’t.
Neither parent deserves blame. Both contribute genetic risk equally through their DNA, and the additional maternal effects from pregnancy and early caregiving are biological realities, not choices. Understanding where risk comes from helps families watch for early signs and act on them, which remains the most effective thing anyone can do.