Less than half of the 28 states affected by a 2012 U.S. Supreme Court decision banning mandatory sentences of life in prison without parole for juvenile murderers have reformed their laws.

And of the 13 states that have made legislative changes in response to Miller v. Alabama, only four—Delaware, North Carolina, Washington and Wyoming—allow resentencing for their existing juvenile life-without-parole populations, according to a study by The Sentencing Project in Washington, which does sentencing policy research and reform advocacy.