Mental Illness and Heredity: The Role of Genes in Mental Disorders

You have probably heard that genes play a role in mental illness . If you have a specific mental disorder, your children are also more likely to have that condition. But how much do genes matter? Are genes determinants when it comes to mental illness?

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Researchers talk about the “heritability” of different conditions, which is the part of a condition that can be attributed to genes. One example, which is not a mental illness, is height, which is about 80% hereditary. In other words, the variation in how tall you are is determined by about 80% for your genes and 20% for other factors, such as your environment.

There is still a lot we don’t know about how the genetics of mental illness works , but the researchers have managed to obtain estimates of heritability for mental disorders. A common way of doing this is with  twin studies.

By comparing identical and fraternal twins, researchers can guess how hereditary the different traits and disorders are. Going back to the height example, if identical twins are consistently more similar in height than non-identical twins, this would suggest that genes play a role in height, since identical twins have more genes in common.

It turns out that different mental disorders have different heritabilities.An example of a highly inherited mental illness is  schizophrenia. According to a  study  involving 31,524 pairs of twins (there are many twins!), The inheritance of schizophrenia is about 79%. In other words, genetics seems to strongly influence people’s susceptibility to schizophrenia, although it is not the full story.

Another condition that seems to be in the heritability range of 70% to 80% is  ADHD, the attention deficit hyperactivity disorder. Estimating the inheritance of ADHD proved to be somewhat complicated, because the numbers that the researchers find seem to depend, in part, on whether the symptoms are being reported by people with ADHD or by others. The 70% to 80% estimate comes from a  2015 study  that addressed this problem by combining symptom classifications from various sources.

A little further down the heritability scale are conditions such as  alcohol use disorders  and  obsessive-compulsive disorder,  both estimated to be about 50% hereditary (see  here  and  here  ). But while these conditions seem less hereditary than schizophrenia, for example, it is worth noting that genes play an important role: half of the development of alcoholism or OCD is purely a question of the DNA that a person is born with.

When it comes to understanding the genetics of mental illness , there are still many questions waiting to be answered and discoveries waiting to be asked. However, we know that genes play a key role in determining which mental disorders people are predisposed to .

For most mental health conditions, as well as for most human characteristics in general, we cannot say with certainty  which  genes are important, or how the different relevant genes mix. But we can say that while genes are not the big picture when it comes to mental health, they make a big difference .

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