Across the country, at least 31,000 state and federal inmates are serving terms of natural life.

The rest were inmates serving terms of less than a year.

The state is required to hold inmates serving sentences of a year or more.

The detention center holds inmates serving time of 18 months or less.

Though the 155 inmates in question generally served at least a decade on death row, none of them killed again.

Of the 1,349 inmates serving life sentences, 208 have not been tested.

The federal prison system requires all inmates to serve at least 85 percent of their sentences.

Generally, though, an inmate must have served at least 10 percent of a sentence.

The report did not suggest why inmates serving life terms have a lower recidivism rate.

It also requires inmates to serve at least 85 percent of the fixed sentence they receive.