Genetics, Environment, and the Complex Origins of Mental Health Conditions

Not everyone is equally vulnerable to mental health conditions, and part of this difference comes from our genetic makeup. As Professor Boris Chaumette—psychiatrist, researcher at Université Paris Cité and Inserm, and member of FondaMental—explains, psychiatric disorders are among the conditions with the strongest genetic component in humans. In some cases, this genetic influence is even greater than what is observed in cardiovascular diseases or certain cancers.

But genetic does not mean predetermined, and it certainly does not mean hereditary in a simple, linear way.

Genetic Vulnerability Is Not Destiny

Genetic factors undeniably play a role, yet they do not impose a fixed outcome. A genetic vulnerability does not guarantee that symptoms will appear in future generations. In fact, in about 95% of cases, genetic predisposition alone is not enough to trigger a mental health condition.

These disorders are complex and multifactorial. They arise from the interplay of many influences, including:

- Environmental factors

- Stressful life events

- Substance use (for example, cannabis consumption)

- Social and psychological stressors

According to Professor Chaumette, it is most likely the combination of genes and environment that leads to the development of a disorder. Neither acts alone; they interact continuously, shaping risk and resilience.

Understanding Heritability

To better understand the weight of genetics, researchers use a statistical concept called heritability. Heritability measures how much of the variation observed between individuals for a given condition can be explained by genetic differences, as opposed to environmental ones.

Professor Chaumette highlights several estimates:

- Depression: around 40% heritability

- Bipolar disorder: around 60%

- Schizophrenia: around 80%

These numbers do not mean that a person with a family history will develop the condition. Instead, they indicate how strongly genetics contribute to the overall risk within a population.

A More Nuanced Understanding of Risk

This perspective invites a more compassionate and realistic understanding of mental health:

- Genetics can increase vulnerability, but they do not dictate fate

- Environment, life experiences, and personal history matter profoundly

- Prevention, support, and early intervention can make a meaningful difference

- Mental health conditions are not the result of personal weakness or moral failure

Recognizing the complexity of these disorders helps reduce stigma and encourages a more holistic approach to care—one that honors both biological and environmental influences.