Determining the genetic causes of mental illness isn't as simple as a blood test. Because there are so many mental disorders, attempting to find a genetic link can be tricky. Or is it? A 2013 study in the Lancet showed that five mental and developmental disorders -- schizophrenia, bipolar disorder, autism, major depression and attention deficit hyperactivity disorder -- share several genetic variations [source: Kolata].
Of course, genetics and environment aren't mutually exclusive. It might seem superficially that if you and your identical twin both had bipolar disorder, a strong genetic link would be likely. But if you and your twin both experienced extremely similar environmental influences -- from the air quality to the food you ate to the medicines you took when you had a headache, to list a very, very few -- the results wouldn't necessarily mean as much. It would be extraordinarily difficult to point a finger at one or the other (not to mention impossible to raise two kids with the exact same environmental influences).
That being said, if we're looking for a strong case for genetics causing mental illness, schizophrenia is a good candidate. When a parent is diagnosed schizophrenic, the probability of that person's children developing schizophrenia is 10 times higher than that of the general population [source: Encyclopædia Britannica]. (Only about 1 percent of the population is generally at risk to develop the disorder.) If both parents have the disorder, the child's risk takes a stunning jump, increasing the probability of a diagnosis by anywhere from 35-65 percent [source: Encyclopædia Britannica].
Moreover, there's a good example of twin genetics for schizophrenia. If one fraternal twin is schizophrenic, the likelihood of their twin being diagnosed is 12 percent; if one identical twin has the disorder, the number spikes to 40 or 50 percent [source: Encyclopædia Britannica].
And schizophrenia isn't the only example of familial studies that link mental illnesses with genetics. Schizophrenia, manic depressive illness, major depression, attention deficit hyperactivity disorder, panic disorder (to name a few) have all been shown to carry a genetic influence [source: Hyman].
But because you're a smart cookie, you might also recognize that the twin schizophrenic study, for one, is also a strong indicator that genetics aren't just to blame for mental disorders. After all, if schizophrenia were purely a genetic condition, both twins would get it. So what other factors contribute to mental illness?