The long manifesto left behind by Elliot Rodger seems to be evidence of some very disturbed thinking.

Rodger has not been confirmed with any specific mental illness diagnosis. But the 22-year old had been in therapy and Rodger said he was prescribed an anti-psychotic medication, which he was not taking.

What happened in Santa Barbara has put the spotlight on mental health issues in this age group, which research has shown is particularly vulnerable to the onset of mental disorders.

Dr. Carrie Bearden, a professor in UCLA's departments of Psychology and Psychiatry cautioned that there is no correlation between mental illness and acts of violence like Rodger perpetrated, but young people in his age range are at a higher risk for manifestation of mental illness like depression and schizophrenia.

Changes in the brain, hormones and social pressures as the result of moving away from home and navigating independence can exacerbate underlying predisposition to mental disorders. Additionally, use of drugs and alcohol can also be a trigger  in at-risk individuals.