When someone is sent to prison or jail, their family and friends suffer, too, says Daniel Kruger, PhD, a psychologist at the Prevention Research Center at University of Michigan's School of Public Health.

"We see much higher levels of stress and much higher levels of depression symptoms amongst those who know somebody who's incarcerated," he says, adding that conclusion holds true even when researchers controlled for risk factors such as smoking, drinking, exercise and education level.

The finding is particularly significant because the United States has one of the highest incarceration rates in the world, with about one out of 100 adults in prison or jail at any given time, according to a report released by the Pew Center on the States One in 100: Behind Bars in America 2008.

The idea of looking into how incarceration affects inmates' friends and family originated from community members who belong to the research center's advisory panel, who argued that incarceration rates were adversely and disproportionately affecting the African-American community.

The study confirmed their hunch, finding that 49 percent of African-American participants reported knowing someone incarcerated, compared with 20 percent for whites.

The research showed the closer a person felt, the worse the outcome in physical and mental health. The link to poorer health might arise from the missing financial and social support, and the emotional toll caused by worrying about an incarcerated friend or family member, Kruger says.

"That could be extremely stressful, going to sleep at night and not knowing whether a loved one is safe," he says.

In Kruger's view, the results bolster the case for reforming the criminal justice system, and focusing on rehabilitation, not punishment, for non-violent offenders.

—C. Munsey