It’s easy to think of psychiatric disorders as categories with distinct boundaries. Schizophrenia is separate from depression, which is different than anxiety. At a genetic level, though, it’s not at all clear that this is the case.
We know that most mental health conditions have a significant genetic component. If you have a family history of major depression, you’re at higher risk for major depression. But a more complicated question is how much the genetic components of different disorders overlap: if you have a family history of major depression, are you at higher risk for schizophrenia too?
In other words, it’s possible that there’s no clear dividing line between genes for different psychiatric conditions. A given set of genes could increase someone’s risk for multiple different mental health disorders.
One study that suggests this may be the case was published in the journal Nature in 2013. In the study, researchers looked at a specific type of genetic variation called an SNP and estimated the genetic correlation between five different disorders: schizophrenia, bipolar disorder, major depression, autism spectrum disorder and ADHD.
They found several different correlations between pairs of these disorders. For example, there was a strong genetic correlation between schizophrenia and bipolar disorder and a moderate genetic correlation between major depression and ADHD.
More recently, a study published in Psychological Medicine looked at whether people’s genetic risk for schizophrenia and for the personality trait neuroticism could predict whether they had other psychiatric disorders. People who score higher on the personality trait neuroticism tend to experience more anxiety, apprehension, anger and negative emotions.
The study found that people with higher genetic risk for schizophrenia were also more likely to have anxiety and depression, to use nicotine, and to have a family history of psychiatric disorders in general. Meanwhile, people who had a higher genetic risk for neuroticism had higher rates of anxiety, depression, phobia and panic disorders.
Interestingly, they also had higher genetic risk for heart disease. In other words, it’s not just that genes for various psychiatric disorders overlap. To the extent that mental and physical health are intertwined, genes that affect mental and physical health could also overlap.
All this means that while we can’t definitively answer the question of how much different psychological conditions share the same genes, we know that at a biological level, mental health disorders with distinct labels aren’t entirely separate from each other. Understanding how these conditions spill into each other and how the same genes can play a part in different conditions could be a key to improving treatment in the future.