LOS ANGELES (KABC) -- A new UCLA study reveals mental illness and substance abuse are key causes of homelessness among unsheltered people living on the streets.Researchers at the California Policy Lab, a new research unit at UCLA, analyzed just over 64,000 national surveys given to both sheltered and unsheltered homeless individuals.Among their findings: much higher rates of mental health and substance abuse in the unsheltered homeless population compared to those who are sheltered."They are also reporting these as the cause of their homelessness at much higher rates than homeless individuals who are accessing shelters," says California Policy Lab's Janey Rountree.The numbers are striking as 84% of unsheltered homeless individuals self-report having physical health conditions compared to 19% of sheltered homeless, and 78% of unsheltered homeless report mental health conditions versus 50% of those living in shelters.And 75% of the unsheltered homeless report substance abuse conditions compared to just 13% of those living in shelters.Unsheltered homeless women in particular appear to be much more vulnerable than even unsheltered men, or men and women who are sheltered.These women report trauma and abuse as the cause of their homelessness at 80%, twice as high as unsheltered men who are surveyed."...The significance of the finding about women is that we really need to make sure the housing solutions that we're designing are appropriate for them," Rountree said.Researchers say the study results confirm common sense theories, but they caution against the suggestion that mental illness and substance abuse are the only causes of homelessness.