Mmm...I'd hedge my bets a bit. I think a better generalization is that many psychiatric disorders are caused by genetic abnormalities (or clusters of abnormalities, or complemented with epigenetics), but that the potential pool of candidates is vast. It only takes a few rotten eggs in the whole batch to ruin things.

It's like a rocket. You need all the parts of a rocket to work or you won't get off the ground. If you are really unlucky, it may even explode

With psychiatric disorders, it's similar. Break a few choice genes and the whole system goes to shit. But unlike a rocket, it keeps working, just not in the fashion that other brains works. So you get some kind of disorder like schizophrenia or bipolar.

I was involved in a few projects where they were mapping schizophrenia with SNPs. Looking at the data, it's clear that some individuals cluster around certain SNPs...but most of them are scattered everywhere. Lots of things can cause schizophrenia, because it really isn't a single disease but a set of common symptoms.