At the request of a good friend of mine, I'm copying and pasting this from another thread over to here.

The Diathesis-Stress Model:

The Diathesis Stress Model basically explains that the development of psychological issues is very complex and it can't be attributed to biology NOR to experience, but instead is influenced by both.

We can inherit a tendency toward developing a particular trait, but that trait may not develop without certain stressors triggering it. Our biology isn't the one answer to understanding, and neither is experience.

Biology influences our experiences, our experiences influence our biology, both influence our mental/emotional/spiritual/physical well-being, and vice versa.

Basically the idea is that there isn't a single "cause" nor is there a simple explanation for psychological issues, but a lot of contributing factors that can be difficult (if even possible) to fully sort out and understand.