How Heritable Is Mental Illness?

You’ve probably already heard that genes play a role in mental illness. If you have a particular psychiatric condition, your children are also more likely to have that condition. But how much do genes matter? Are genes destiny when it comes to mental illness?

Researchers talk about the “heritability” of different conditions, which is what portion of a condition can be attributed to genes. An example that’s not a mental illness is height, which is about 80 percent heritable. In other words, the variation in how tall you are is determined about 80 percent by your genes and 20 percent by other factors like your environment.

There’s still plenty we don’t know about how the genetics of mental illness work, but researchers have been able to derive heritability estimates for psychiatric disorders. A common way of doing this is with twin studies.

By comparing identical and fraternal twins, researchers can take a pretty good guess at how heritable different traits and disorders are. Going back to the example of height, if identical twins are consistently more similar in height than fraternal twins, that would suggest that genes play a role in height, since identical twins have more genes in common.

As it turns out, different mental illnesses have different heritabilities.

An example of a mental illness that’s highly heritable is schizophrenia. According to a study involving 31,524 pairs of twins (that’s a lot of twins!), the heritability of schizophrenia is about 79 percent. In other words, genetics seems to strongly influence people’s susceptibility to schizophrenia although it isn’t the full story.

Another condition that seems to be in the 70-80 percent heritability range is ADHD. Estimating the heritability of ADHD has proved a little tricky because the numbers researchers come up with seem to depend partly on whether symptoms are being reported by the people with ADHD themselves or by others. The 70-80 percent estimate comes from a 2015 study that addressed this problem by combining symptom ratings from multiple sources.

A little lower down on the scale of heritability are conditions like alcohol use disorders and obsessive-compulsive disorder, both of which have been estimated to be about 50 percent heritable (see here and here). But even though these conditions appear to be less heritable than schizophrenia, for example, it’s worth noting that genes are playing an important role: half of whether someone develops alcoholism or OCD is purely a matter of the DNA they’re born with.

When it comes to understanding the genetics of mental illness, there are still a lot of questions waiting to be answered and discoveries waiting to be made. However, we do know that genes play a key role in determining what mental illnesses people are predisposed to.

For most mental health conditions, just like for most human traits in general, we can’t definitively say which genes matter, or how the many different genes that are relevant intermingle with each other. But we can say that while genes aren’t the complete picture when it comes to mental health, they do make a big difference.

Image: Flickr/Andy Leppard