Prison is not a healthy place. It’s pretty well established that many who wind up there aren’t in great health to begin with, and their sentences can exacerbate underlying issues. Solitary confinement destroys already fragile minds. Incarceration robs men and women of their youth, regurgitating aged shadows back into the streets. Beatings and abuse at the hands of officers lead to injury and even death, and violence between inmates is seen as common enough to pass as normal.

But the underlying reasons for the vulnerabilities of the incarcerated are poorly addressed by policymakers, and there is little understanding of what that vulnerability means en masse, in the society that incarcerates more people than any other.

Harvard University researcher Bruce Western’s new book, Homeward: Life in the Year After Prison, could add significantly to that understanding, illuminating the role prisons play for the poor and highlighting the contours of infirmity that mark the lives of incarcerated people, often from birth to death. While Homeward is a gripping study of the totality of the lives of people reentering society, it also uncovers the role of the carceral system in breaking bodies and minds.

At the heart of Homeward is the Boston Reentry Study, a longitudinal survey of 122 people released from Massachusetts Department of Corrections facilities around Boston between 2012 and 2014. Western and his co-researchers followed people over their first year of reentry, maintaining contact with a notoriously difficult population of people to track. They followed the cohort through homeless shelters, joblessness, and shifting addresses; through psychiatric hospitals; and, for some participants, through downward spirals into drugs, violence, and additional stints in prison.

The book is unsparing, providing windows into the experiences of even some hardened and violent offenders in order to “bear witness to the lives of those held captive in America’s experiment with mass incarceration,” as Western writes.

There’s Jerry, a sex offender who lived in a homeless shelter because his conviction made him ineligible for public housing. There’s Eddie, a middle-aged black veteran who first fights his crack addiction but later succumbs, and eventually resorts to street scams to support his habit before he’s incarcerated again. There’s the tragedy of Aman, a young Afro-Caribbean man diagnosed with schizophrenia who says his sole reason for participating in the study is to have “friends like yourselves to come by.” There’s Maria, who manages to banish her heroin addiction and pick up carpentry. Western’s academic prose belies the fact that he tells dozens of Sisyphean tragedies, with even the personal victories accompanied by setbacks.

These narratives, buttressed with reams of data, serve as valuable observations on the difficult nature of reentry and the range of likely poor outcomes, of which recidivism is only a part. In the six months after release, most of the Boston Reentry Study participants were in poverty, leaning on government benefits and family support to survive. Respondents rarely found full-time employment, although 43 percent found some kind of work—mostly temp jobs—within two months. Women respondents were especially vulnerable to violence. And there were significant racial disparities in support and economic access. Even black and Latino participants with strong support networks found themselves less likely to land jobs than white men with few contacts or no connections at all.

But perhaps the most intriguing findings of Western’s study are those related to what he calls “human frailty.” When I asked Western to define the term, he said that it involves “the mental infirmity and the physical problems that accumulate with people under poverty.”

As Homeward describes, the Boston Reentry Study found that, along with poverty, human frailty is perhaps the defining feature of incarceration. Fifty-four percent of the reentry population reported a history of problems with drugs or alcohol. Two-thirds had a history of mental illness or addiction. One-third reported serious back pain, arthritis, or some other disability. Oftentimes, many people entering prison have chronic diseases stemming from drug use, along with other unmet physical and mental needs—needs that immediately become unmet again upon reentry, when their institutionalized care ends.

“The general finding is that physical health tends to improve in prison and then dramatically declines after people leave prison,” Western told me. “This is mostly because they’re better able to manage chronic conditions while they’re in prison, and they’re able to stay on their meds and so on.”

Mental health is another realm altogether, and Homeward illuminates the staggering burden of mental-health and addiction issues that the violence and isolation of prison can exacerbate. “We know far less about people’s mental-health status and the management of mental illness on the inside, and certainly we heard reports of depression, PTSD, anxiety, and such on the inside,” Western told me.

As grim a picture as Homeward paints, the case study actually describes a best-case scenario in the United States: Massachusetts has a relatively robust state Medicaid program that automatically enrolls people upon reentry, and can provide low- or no-cost services, including mental-health services. But the book shows, in detail, how people leaving prison don’t often seek care before major problems manifest.

“So much happens in those first 30 days, and there’s so many other kinds of stress that they’re dealing with,” Western said. People have to adjust to the minutiae of everyday life that was previously under tight control or unavailable to them—think walking in crowds, using telephones, and taking public transit. Sudden exposure to the hustle and bustle of Boston almost seems to be a trauma unto itself for people still coping with the long-term effects of prison.

The picture Western paints is bleakest for people dealing with substance abuse. While the enforced prohibition of drugs in prisons forces sobriety on inmates, Homeward shows how difficult it can be to remain sober upon reentry. Heavy substance abuse is known for straining familial and friend networks, and imprisonment often involves the intentional severing of those networks.

Homeward chronicles the lives of several people with addictions who leave prison with threadbare social networks. Almost invariably they wind up using again, end up back in prison, or both. The point of failure is often one that many outsiders might sneer at: personal willpower. But the willpower needed to overcome addictions without social support can be easily eroded, Western argues. “Much of the agency—the will to change—that even our most humane rehabilitative programs ask of people in prison is compromised by precisely the physical and mental difficulties that places them at risk of incarceration in the first place,” Western writes. “The people we ask to make the largest changes in their lives often have the least capacity to do so.”

This seems to be true of the entire project of incarceration. Especially in the realm of physical and mental health, poor people and people of color have the least resources and the least capacity to treat illness and promote wellness. That much is known—those groups are prone to higher rates of obesity, diabetes, kidney failure, asthma, and almost every other mortality indicator, including undiagnosed or poorly managed mental-health issues. Instead of extending the safety net to deal with these factors, it’s prison time that’s extended: People with drug-use and mental-health issues are incarcerated rather than funneled toward treatment and preventative carea punishment that worsens both physical and mental-health outcomes for them and their families.

For Western, this is the fundamental problem uncovered in his study: Instead of finding root causes for chaos, violence, and instability, every wrong is attributed to individual agencyeven when agency is eroded by health problems. “The fundamental thing about the criminal-justice system is that it’s a blaming system,” Western told me. “But the health-care system attends to causes and physical and mental vulnerability. If we want to do that, then we can’t be involved in a blaming system.”

As Western notes in Homeward, the very premise of incarceration is built on a contradiction. It’s increasingly clear that many people who go to prison are incarcerated because they are ill, and also that poverty erodes agency and wellness in a way that creates conditions for crime, violence, and addiction. Instead of investing in the necessary services to heal those people, society instead punishes them in a prison system lacking in rehabilitative services. The end result is not only generations of frail people, but frail communities that can’t do much more than repeat the cycle.