All In The Family: Is Mental Illness Hereditary?

"What if the baby gets one too?," a pregnant friend recently pondered worriedly; nearly everyone in her immediate family has some kind of mental health diagnosis, though the labels they were given are as varied as bipolar disorder, alcoholism, borderline personality disorder, and anxiety. Given the fact that my diagnosis-free friend is the odd one out in her family of origin, it isn't surprising that she is concerned. Even in the absence of official statistics, her lived experience has already taught her that mental illness runs in families — or, at the very least, in hers.

So how likely is it, exactly, that her child will get a mental illness too? Is mental illness really hereditary?

Mental Illness: Nature, Nurture, Or Beyond?

What exactly is "mental illness"? Though we all experience mental health challenges on occasion, a mental illness is a condition that affects someone's thought patterns, behavior, and emotions in a way that adversely impacts their daily functioning — and frequently that of those in their lives — over a long period of time. That's a rather generic definition, and mental illnesses come in all sorts of shapes and forms, with over 200 diagnosable conditions currently recorded. In order to be labeled with a particular mental illness, someone needs to meet a certain number of diagnostic criteria for that illness.

Mental illness, we are aware, "runs in families". Why, though? Even before the complete human genome was mapped, and before we became able to really delve into possible genetic origins of mental illnesses, several possible explanations were being discussed. 

READ Pot Smoking Linked to Increased Risk of Mental Illness

• Nature: Indeed, genes that determine whether or not someone will develop a particular mental illness may exist. Research indicates that people whose parents have mental illness are up to 10 times more likely to develop mental illness as well, so the hypothesis that genes could be responsible is not far-fetched.
• Nurture: Another possibility is that mental illness is, instead, the result of nurture. Being raised in an environment where other people have mental illness could mold a person's thinking and behavioral patterns to the point that they, too, qualify for a diagnosis. If mental illness is the result of environmental exposure (and some, like Post Traumatic Stress Disorder, certainly are), then growing up in similarly sub-optimal and stressful circumstances could be the cause of mental illness as well.
• Beyond: Exposure to environmental toxins, including in the womb, may alter a person's brain chemistry to the point that they develop mental illness. 
• And yet more: Epigenetics is a relatively new field that investigates how existing genes can be "switched on or turned off" depending on the person's life circumstances, often starting in utero. 

These causes aren't necessarily mutually exclusive, and they probably all play a role.

So, Is Mental Illness Hereditary?

Back to my friend — does the fact that a large number of her relatives, including both her parents, several uncles and aunts, and a sibling, had very different mental illnesses mean that her unborn child is at an above-average risk of developing a generic "mental illness" too? How can very different mental illnesses be related? Does it make any sense that someone whose mother had schizophrenia has higher odds of developing bipolar disorder, for instance? Science is only now inching towards half-answers in that general direction. 

In 2013, researchers funded by the National Institutes of Health (NIH) published some fascinating findings that shed at least a little light on exactly that. They found that five mental disorders that were previously deemed unrelated, namely autism, ADHD, bipolar disorder, major depression and schizophrenia, shared some surprising genetic overlap. These disorders are "more likely to have suspect genetic variation at the same four chromosomal sites," the NIH shared, adding: These included risk versions of two genes that regulate the flow of calcium into cells."

The ground-breaking and large-scale research found that a variation in a gene called CACNA1C, already associated with bipolar disorder, major depression, and schizophrenia before, was strongly associated with a person's risk of developing either of the five disorders. The gene was identified as being related to emotion, memory, attention, and thinking patterns. A variation of the gene CACNB2 was also linked to these mental disorders, as well as variations of certain chromosomes.

This research, published in The Lancet, is the closest thing we have so far to pinpointing specific gene variations as being responsible for mental illness. 

Is that the full story, though? Jordan Smoller MD from Massachusetts General Hospital, Boston, a coordinator of the study, pointed out that it isn't, saying: “Although statistically significant, each of these genetic associations individually can account for only a small amount of risk for mental illness, making them insufficient for predictive or diagnostic usefulness by themselves.”

In other words, though human genome sequencing has brought us the possibility of finding out more about the genetic causes of mental illness, no definite answers have been discovered yet. Beyond this large study, other research reveals that more than 1000 differentially expressed genes are linked to autism, as well as hundreds to schizophrenia and 80 to alcoholism. Yet, genetic testing for these disorders and other mental health conditions doesn't exist yet — because, at the end of the day, it is not clear which genetic variations can be taken as a "positive test" for any mental disorders right now. 

READ Dealing With Mental Illness In A Loved One

Despite increased knowledge of human genes, then, we are pretty much back to the drawing board: like before, we can reliably suspect that genetics play a role in the development of mental illness, but we aren't yet sure how or why, and can't yet use the newly-gained knowledge as a diagnostic tool. Is mental illness hereditary, then? Yes, probably, but it takes more than genes to develop a mental illness. Having a family history of mental illness increases your risk of ending up with a diagnosis yourself, but that is only part of the story.