Genes for Traits and Psychiatric Disorders Often the Same
New study finds common genetic factors underlying both mild and extreme behavior
Posted Jan 26, 2019
Some kids can be described as active children, while other kids are really active to the point where they may meet criteria for a diagnoses such as ADHD. Some kids are “on the anxious side” while others are so anxious they qualify for terms such as Generalized Anxiety Disorder (GAD) or Social Anxiety Disorder.
Exactly where to draw the line between “trait levels” of various behavior and psychiatric disorders isn’t easy. As I’ve described in previous posts and in my first book, making the diagnosis of ADHD is a little like trying to decide if a person is “tall”—there will be plenty of folks for whom few people would disagree are either tall or not tall, but also lots of people in between where there may be a reasonable difference of opinion.
Psychiatrists and other mental health professionals have increasingly appreciated this more dimensional perspective of behavior in which nearly everything exists along a continuum rather than in more binary form. What has yet to be fully demonstrated, however, is the degree to which the underlying forces that cause traits to develop are the very same forces that can lead to psychiatric disorders.
This new study tries to answer this intriguing and important question at the genetic level. We know already that both personality traits and psychiatric disorders are influenced by genetics, but the question now is whether it is the same genes that influence both trait-level expressions of behavior (like being a little anxious) and more extreme levels too (like generalized anxiety disorder). There already is some evidence for things like ADHD that the answer is probably yes, but there is much less information about other types of behavior.
To get at this question, a group of researchers from Sweden and the United Kingdom examined data from over 20,000 twin pairs between the ages of 9 and 18. The participants completed quantitative rating scales looking at many types of psychiatric symptoms including autism, ADHD, OCD, depression, mania, and psychosis as well as whether or not subjects had actually been diagnosed with specific disorders. They also obtained DNA from a large percentage of them. From there, the researchers decided to try and look at the shared genes question at two different levels—one by using some advanced statistical methods to look at the relations between continuously distributed symptoms and yes/no diagnoses among different types of twin pairs (identical or monozygotic versus fraternal or dizygotic) and also at the molecular level looking at specific versions, or alleles, of genes that have previously been associated with different types of psychiatric symptoms to calculate something called a polygenetic risk score.
This methodology can get pretty technical, but fortunately, the results across different techniques converged at a similar conclusion which is that, across many domains of behaviors, there is strong evidence that many psychiatric disorders share common genetic influences with corresponding traits found in the general population. In other words, the genes that influence someone to have low levels of OCD (or depressive symptoms or paranoia) are many of the very same ones that can lead to someone having full-fledged OCD (or depression, psychosis, etc.).
To be sure, the magnitude of genetic overlap differed depending on what types of behaviors were examined, and nowhere was the overlap even close to complete. Also of interest, the study was able to sort of do the same thing for environmental variables and here they found evidence that there was more independence between people with low levels and extreme levels of a certain trait.
Putting this all together, the results suggest strongly that a continuum view of behavior exists to a significant degree at the level of genes as well. It may be, for example, that people who are a little anxious and extremely anxious share many of the same genes. What makes some people manifest more problematic levels of a particular trait is related to having high numbers of these “risk” genes or the combination of some genetic risk plus exposure to negative environmental experiences such as trauma and abuse.
Ironically, this quite complicated and heavily genetic study also may give us a clue for why it has been so hard to find something on a lab test or brain scan that says definitely that somebody has a psychiatric diagnosis like bipolar disorder or ADHD. Many people have used this lack of identification to argue against the “realness” of psychiatric disorders, but it very may well be that the causes, at least genetically, of many psychiatric disorders aren’t pathological per se but related to the expression of everyday traits. If that sounds like a situation unique to mental health, it actually isn’t: many of the most common medical problems such as high blood pressure, diabetes, and high cholesterol are also dimensional and may have similar stories with regard to genetic and environmental influence.
Taylor MJ, Martin J, et al. Association of Genetic Risk Factors for Psychiatric Disorders and Traits of These Disorders in a Swedish Population Twin Sample. JAMA Psychiatry, published online Dec 19, 2018.