There are few things that will more deeply and continuously effect a human being than childhood physical/sexual abuse.
The correlation between it and developing an alcohol or drug dependence is variously reported in professional journals to be from 75-90 percent.
When trauma occurs, fundamental developmental changes take place. Trust, the cornerstone of every relationship, is broken.
The belief in the world as a friendly, supportive place is shattered and replaced by hypervigilance to try to prevent the same from happening again. Attachment, the developmental process that teaches us how to engage with, and be mutually supportive of, other human beings, becomes a condition to be feared because vulnerability must be avoided at all cost.
What is left is a recipe for a life of pain.
Their view of the world is as a dangerous place. They avoid relationship intimacy because people are not to be trusted.
Vulnerability is not possible because of the awful, out-of-control feelings associated with it. Hypervigilance and constant scanning of the environment replaces the ability pay attention over a significant length of time.
Feelings become expressed in extremes, or totally internalized, with little ability to moderate them.
In short, a promising, prosocial human being becomes doubtful of self, introverted except for outbursts and lonely, even in a crowd, because they fundamentally avoid connection to avoid experiencing more pain and fear.