Why has the US prison population has grown for decades, surpassing two million? We’ve put more people in jail, but new research shows it’s not just how many people go to prison. What counts, for prison growth, is how long they stay. Ryan King, Senior Fellow at the Urban Institute’s Justice Policy Center, has created a ground-breaking study of how the exponential growth in prisons has really been driven by the growth in long sentences. Even as some states have reformed incarceration around low level offenses, long sentences remain stubbornly in place, and receive almost no attention.