Dopamine deficiency more frequent in the addiction prone?
It seems that genetic plays significant roles in dopamine addiction:
http://www.ncbi.nlm.nih.gov/pubmed/22350797

And also a study of "born-addicts":
http://www.addictscience.com/genetics-d2/

But also it is connected to ADHD studies. It is well known that people with ADHD are vulnerable to addiction. They think it is because reward deficiency system:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2626918/

When ADHD patients get medicine based on dopamine or amphetamine their attention improves.