A lot of professionals believe that many disorders are the result of heredity and environment. A person might have a genetic predisposition toward a certain mental disorder, but not develop it because the environment is not in keeping with it.

I do believe that environment is a major factor in many disorders. A stressful one can bring out problems or even cause them. I was told that if I had not had a sister to talk to about our parents, etc, then I would have become psychotic. I developed bipolar, BPD, and anxiety, but was able to keep in touch with reality.

Who in their right mind would mentally stay in some really, really abusive environment without developing DID, schizophrenia, or some other disorder?!

It's interesting that schizophrenia also often occurs in early adulthood when major brain developmental changes are occurring. But also early adulthood is a time when lots of environmental changes are happening, too, such as leaving home for the first time, starting a first full-time job, going to college, being considered an adult and being given many more responsbilities, etc.

Identical twins tend to both develop the same disorders, but then they have similar experiences, too. The key is if they are separated and grow up in different environments, but still develop ths same disorders. If so, then that gives credence to heredity as the cause.