In a growing trend which some experts are questioning, a number of states are using mental institutions to keep sex offenders locked up -- after they have served their prison sentences and although they show no signs of mental illness.

Critics say the answer to the need for additional confinement lies in longer prison sentences, not turning psychiatric units into prisons.

• Eight states have passed laws allowing sexually violent predators to be sent to mental institutions after they have completed their jail terms.

• Thirteen other states, including Texas and New York, reportedly have such legislation on the fast track.

• Under some of the state statutes, in Kansas for example, a convict about to be released need only be certified as having a "personality disorder" or "a mental abnormality" -- a term foreign to psychiatry and defined as a condition "which predisposes the person to commit sexually violent offenses."

• Although the Kansas Supreme Court declared the law unconstitutional, the U.S. Supreme Court last year reversed that decision -- as well as its own previous decisions which had held mental illness must be a precondition for confinement and a person could not be held simply for being a potential danger to society.

Legal scholars say this opens the way for civil commitment of many types of violent felons who have trouble controlling their impulses.

California's sexually violent predator law has resulted in 178 felons being transferred to a state mental hospital. To make way for them, mental patients are reportedly being moved from hospitals to penal institutions.