According to the study, 20 percent of prisoners were lifers last year.  That's compared to just eight percent in 1990.  Robert Weisberg of Stanford University's Criminal Justice Center co-authored the study.  He says such inmates are granted parole by the Board of Parole Hearings about 18 percent of the time.  However, Governors often reverse the Board's decisions.  Weisberg says it's worth noting that lifers are an increasingly geriatric population, which makes them less likely to re-offend when paroled:

Weisberg: "For at least violent crime, the age curve of criminals is like the age curve of professional athletes and as far as I know there are no 48-year- old running backs in the National Football League."