There is no single cause of mental illness. Each mental illness is different and even the same symptoms can have different causes in different people.

What’s common across all mental illnesses is that there are always some causes that are due to biology (the brain and genetics) AND some factors that come from the environment (situations, stressors and triggers that come up in the course of life).

One way to think about the biology factor is to look at the “heritability” of the disorder — that means the likelihood that someone with identical genes (like an identical twin) would get the same illness even if they have totally different lives.

For bipolar disorder for example, genetics account for about 60% to 80% of the cause of the illness. Rates are similar in schizophrenia at anywhere from 40% to 79%. Remember that these rates are for identical DNA -- not just simple genetic relatedness. These statistics do NOT mean that if someone in your family has a disorder like this that you will get it too. Most people even with a genetic link will NOT develop the mental illness.