In 1977, when Gerald L. Hessell was sentenced to life in prison for second-degree murder, a gallon of gas cost 62 cents and a life term in Michigan meant about 15 years.

But Michigan changed its parole system in 1992, replacing a parole board made up of civil servants with political appointees. The new board summarized its thinking in a presentation to the state’s judges in 2001: “A life sentence means life in prison.”

Mr. Hessell was released the other day, thanks in part to a decision from a federal judge in Detroit last month ruling that the state had violated the ex post facto clause of the Constitution when it changed the parole rules. The clause says the government cannot increase punishments retroactively.

Mr. Hessell was 19 when he was sentenced, and he is 50 now. “I was told by my attorney at the time that I could look to do about 12 to 15 years provided I had a clean record,” he said last week. “It was a 31-year fight to get out.”