That clear line disappears, though, in the life histories of these former prisoners. Most of them were raised in poverty, in chaotic environments where routines and adult supervision were rare, and where their families were under stress.

Violence was common, not because poor people are more prone to it, but because poverty shapes social interaction in a way that makes violence more likely, research suggests. Anyone dropped into the same environment, Mr. Western said, could be swept up by violence, too.

Studies show that children who experience trauma are more likely later in life to suffer from asthma, depression, unemployment, and they’re also more likely to use drugs. The more trauma a child faces, the higher these odds. Children perform less well on standardized tests right after violent crimes have occurred in their neighborhood — even if they didn’t personally witness the violence.

The typical measures of trauma — “adverse childhood experiences” that include growing up in a household with physical or substance abuse — don’t gauge anything quite like witnessing lethal violence. That distress alters the picture of the population the Boston Reentry Study followed: These adults in the criminal justice system were once children exposed to awful things.

What, then, is to be done with the knowledge that four in 10 prisoners typical to the Massachusetts state prison system saw someone killed as a child?

Mr. Western argues that this should force us to reconsider the simplified model of offenders-and-victims, and to allow more second chances to people we peg in the first category.

“The whole ethical foundation of our system of punishment I think is threatened once you take into account the reality of people’s lives,” he said. In the study, the people who had experienced the most extreme childhood trauma and violence also struggled the most in adulthood with drug addiction and mental and health problems. The line between the two is not straightforward. But it’s also not irrelevant.