My DBIL is the Director of the homeless coalition for the county, and writes most/all of the grants for their funding. Because of this he is exposed to a massive amount of recent/current/ongoing research.
His understanding is that there are two very distinct types of homelessness; situational or circumstantial, meaning their lives have worsened and they have no funds for housing or food. They want help, accept help, want a better life and will work for it. They may use drugs to feel better about themselves, but would rather create a more stable life. These homeless can be helped with providing minimal/low cost housing and supplimented but a highly controlled food source. They sign a contract that is behavioral and time limited, and almost 100% do well. He calls them the invisible homeless, and they make up about 65% of the homeless population in the country.
The number one cause of homelessness are single women having children they cannot afford.
The second group are the chronic homeless, and he says from research, they are almost 100% mentally ill. They are unable to help themselves, although they may appear socially capable. There is not much funding available for them, even through homeless organizations. Shelters are about it, and some are closing because of lack of funding. This issue goes back to the 1970's when the USA closed the facilities that housed the mentally ill, and sent them out them to the streets.