Article: What clinicians should know about the contribution of modern behavioral genetics to psychiatric problems
Authors: Robert Plomin and Evangelos Vassos
Affiliation: Institute of Psychiatry, Psychology & Neuroscience, King’s College London
Understanding why some people develop mental health difficulties while others do not has long been a central question in psychiatry and psychology. For many years, debates focused on the relative influence of nature (genetics) versus nurture (environment). In their editorial, behavioural geneticists Robert Plomin and Evangelos Vassos explain how modern genetic research has reshaped this debate and what practising clinicians should understand about these developments.
Over the past several decades, research using twin and adoption studies demonstrated that both genetic and environmental factors contribute to psychiatric problems. However, advances in DNA technology since the early 2000s have accelerated discoveries in the field, making it possible to identify specific genetic variations associated with mental health conditions. These developments are increasingly relevant to clinical practice.
This article outlines several key insights from modern behavioural genetics and discusses how they may influence the future of psychiatric diagnosis, prevention, and treatment.
Genetic Influences on Mental Health
One of the most robust findings in behavioural genetics is that genetic differences between individuals account for approximately half of the variation in psychiatric problems across the population.
This estimate, known as heritability, means that around 50% of the differences observed between people in mental health traits or disorders are associated with inherited DNA variation. Importantly, this does not mean that a person’s mental health is determined solely by their genes. Rather, it reflects the average contribution of genetics within a population.
Heritability also varies across conditions. For example:
- Disorders such as schizophrenia and bipolar disorder show relatively high heritability.
- Conditions such as major depression tend to have somewhat lower estimates.
Even so, genetic influence in psychiatry is unusually large compared with many other factors studied in mental health research.
The authors stress that genetic influence does not equal determinism. Environmental factors and interventions, including therapy, can still play a powerful role in improving outcomes. Genetics simply indicates that some individuals may have a greater underlying vulnerability or resilience to particular psychological difficulties.
Rethinking Environmental Influences
Traditional thinking often assumed that shared family environments were a major cause of mental health differences between individuals. However, decades of twin research have revealed a surprising result.
While genetics explains roughly half of the variation in psychiatric traits, the remaining variation is largely attributed to non-shared environmental influences rather than the family environment.
Non-shared environmental factors include experiences that differ between individuals even within the same household, such as:
- Different peer relationships
- School experiences
- Illnesses or accidents
- Unique life events
These experiences can make siblings, including identical twins, quite different from each other. Research attempting to identify specific non-shared environmental factors has often struggled to find consistent explanations, suggesting that many such influences may involve complex and unpredictable life circumstances.
Recognising this randomness may also help reduce stigma surrounding mental illness. If life experiences are partly shaped by chance rather than personal fault, individuals may be less likely to blame themselves or others.
The Genetic Influence on “Environmental” Measures
Another surprising discovery from behavioural genetics is that many environmental factors themselves show genetic influence.
For instance, measures commonly treated as environmental, such as parenting style, life events, or social support, often show measurable genetic components. This occurs because individuals are not passive recipients of their environment. Instead, they actively shape their experiences.
People may:
- Select environments that fit their personality or temperament
- Influence how others respond to them
- Interpret experiences differently depending on their characteristics
This process is sometimes described as gene x environment correlation. In practice, it means that observed relationships between environmental factors and mental health may partly reflect underlying genetic influences.
For clinicians, this insight suggests that patients may benefit from learning how their predispositions interact with their environments. For example, someone with a genetic vulnerability to addiction might reduce risk by avoiding situations that increase exposure to alcohol or drugs.
The Polygenic Nature of Psychiatric Disorders
Modern genomic research has shown that most psychiatric conditions are polygenic, meaning they are influenced by thousands of genetic variants, each with a very small effect.
This finding overturns earlier expectations that individual “genes for” specific disorders might be discovered. Instead, mental health traits arise from the combined influence of many small genetic differences.
Some rare genetic mutations can have strong effects on individuals, but these account for only a small proportion of cases in the general population.
The polygenic nature of psychiatric conditions means that genetic risk operates in terms of probability, not certainty. Genes influence susceptibility rather than determining outcomes.
Polygenic Scores and Future Clinical Use
Researchers have begun combining thousands of genetic variants into polygenic scores, which estimate an individual’s genetic risk for certain conditions.
Currently, these scores can predict a modest proportion of the risk for several psychiatric disorders. For example, existing research suggests they can explain approximately:
- 8% of risk for schizophrenia
- 8% for bipolar disorder
- 6% for major depression
- 5% for ADHD
- 2% for autism
Although these levels of prediction are not yet sufficient for routine clinical use, the predictive power is expected to improve as genetic datasets grow.
Polygenic scores may eventually allow clinicians to:
- Identify individuals at higher risk earlier in life
- Develop preventive interventions
- Tailor treatments to genetic profiles
Because DNA does not change across the lifespan, genetic information could potentially serve as an early warning system, enabling prevention before symptoms emerge.
Genetic Overlap Between Psychiatric Disorders
Genetic research has also revealed that many psychiatric disorders share substantial genetic overlap.
For example:
- Schizophrenia and bipolar disorder share a large proportion of genetic risk factors.
- Major depression, anxiety, PTSD, and ADHD also show significant genetic correlations.
These findings challenge the assumption that psychiatric diagnoses represent clearly distinct diseases.
Instead, the results suggest that many disorders lie on overlapping dimensions of psychological difficulty, rather than forming completely separate categories.
Researchers have proposed a broader underlying factor known as the p factor, representing a general vulnerability to mental health problems.
This perspective may eventually encourage more transdiagnostic approaches to treatment, focusing on common mechanisms rather than strictly separate diagnoses.
Implications for Psychiatric Diagnosis
Another implication of modern genetic research is that psychiatric disorders may not represent distinct categories in the way traditional medical illnesses do.
Genetic evidence suggests that most mental health conditions are better understood as continuous dimensions. Individuals diagnosed with a disorder typically have higher levels of risk factors that exist throughout the population, rather than possessing unique biological features absent in others.
For example, genetic risk scores for disorders are distributed continuously across the population rather than forming two distinct groups of “cases” and “controls”.
This dimensional perspective could eventually influence how mental health conditions are classified and treated.
Modern behavioural genetics has significantly reshaped our understanding of mental health. Research indicates that genetic differences play a substantial role in psychiatric problems, accounting for roughly half of the variation between individuals. At the same time, environmental influences—particularly non-shared experiences—remain important.
The discovery that psychiatric disorders are highly polygenic and genetically overlapping challenges traditional diagnostic boundaries and may lead to more flexible, transdiagnostic approaches to treatment.
Although genetic findings are still developing, tools such as polygenic scores hold promise for improving early detection, prevention, and personalised care in psychiatry.
For clinicians, understanding these developments is increasingly important as genetic information becomes more integrated into healthcare. As Plomin and Vassos argue, the growing influence of genetics in psychiatry means that clinicians are likely to encounter genetic data more frequently and should be prepared to interpret and communicate its implications responsibly.
Summary by Miguel Mealha Estrada