Well,
At 3 months old I was thrown by my dad and ended up in a coma. After my dad got out of prison for this, my parents didn't divorce because of my mother's severe health issues and inability to work. For the same reasons I basically grew up taking care of my mom instead of it being the other way around.

My dad always preferred to sit in the corner and drink beer instead of actually getting to know his kids or help with my mom. He was also physically and emotionally abusive, particularly when I was younger. It was either that or basically be non-existent as a parent.

I was homeschooled almost my entire life because my mom even included my (and my brother's) education in her need for excessive control. That and the fact that I moved 14 times by the age of 12 made it generally impossible to make or keep any friends. By the time I was a teenager I just decided to totally isolate myself and escape in music. My mom was also emotionally abusive in my early teens, though with her the problem was mainly the over-protection and having to take care of her (which I know is NOT her fault, but it still affects me).

I have one brother who is only a year and a half older. When we were young his main hobby was kicking me into furniture. Once I started martial arts, he switched to just making fun of me. Sometimes he's joking, sometimes not. I don't know if this affects me much, but I know it doesn't help things.

I was never sexually abused that I remember, though there are some mainly blurry memories from very early childhood or parts of stories I've heard that I sometimes question, but realize I really don't want to know.

I'll stop ranting now, but do I believe childhoods play a part in mental health problems? Yes. If it's not the cause, it definitely contributes.