Do prisons make us safer? By taking would-be offenders off the streets, prisons clearly have reduced crime in the short run. In the long run, though, imprisonment erodes the bonds of work, family, and community that help preserve public safety.

Three effects are fundamental. First, former prisoners do worse economically than if they had never been incarcerated. We can see some evidence in a study I conducted in 2004 with the Princeton sociologist Devah Pager. We ran an audit experiment that sent trained testers to apply for more than 1,000 entry-level jobs throughout New York City. The fake job applicants were dressed similarly, gave similar answers, and provided résumés with identical education and work experience. At each job interview, however, one randomly chosen tester would tick the application box indicating a criminal record and submit a résumé that mentioned a prison and provided a parole officer as a reference.

White testers who were assigned a criminal record received call-backs or job offers from employers only half as often as testers with clean records. For African Americans, a criminal record reduced employment opportunities by two-thirds. Labor force data from the National Longitudinal Survey of Youth paint a similar picture of incarceration's negative effects: Wages fall by about 15 percent after prison, yearly earnings are reduced by about 40 percent, and the pay of former prisoners (unlike compensation for the rest of the labor force) remains stagnant as they get older.

The second important effect of imprisonment falls not on ex-inmates but on their families. About half of all prison and jail inmates are parents with children under 18. By 2008 about 2.6 million children had a parent in prison or jail. By age 17, one in four African-American youth has a father who has been sent to prison.

Because of their poor job prospects, formerly incarcerated fathers are less able to contribute financially to their families. Because incarceration strains marital relations, those fathers are also less involved as parents. Compared to otherwise similar kids whose parents haven't been behind bars, the children of incarcerated parents are more likely to be depressed, behave aggressively, and drop out of high school. These problems appear to be more common for boys than girls. Incarceration, it seems, is weakening the bonds between fathers and sons.

The third important effect of incarceration is cultural, shaping how the institutions of law and order are viewed in high-crime/high-incarceration neighborhoods. The prison population is drawn overwhelmingly from low-income inner-city areas whose residents come to associate police and the courts with the surrounding social problems of violence and poverty. Police are viewed as unhelpful, and often unaccountable, contributing to what the Harvard sociologist Robert Sampson calls "legal cynicism" in troubled, crime-ridden neighborhoods.

Part of the power of punishment as a deterrent to crime is the shame and stigma of a criminal record. Where incarceration has become commonplace, as it has in poor African-American communities, the righteousness of the police is no longer assumed and a prison record is not distinctive. The authority of the criminal justice system has been turned upside down, and the institutions charged with maintaining safety become objects of suspicion.

The negative effects of incarceration reduce the penal system's capacity to control crime. Drug dealing and other illegal activities are more attractive to people with prison records, who have few legitimate prospects. Children of incarcerated parents, without a secure and predictable home life, are at risk of delinquency and school failure. And a community, soured on a capricious and unaccountable police force, is less likely to call for help or assist in investigations.

Because of the mounting social costs of incarceration, the benefits of prison have reached a point of diminishing returns. Sixty percent of state inmates are re-arrested within three years of being released from prison. Recidivism rates have not fallen despite a fourfold increase in incarceration rates since the 1970s.

We may care little about the job prospects of ex-cons. We may not even care much about their children or neighborhoods. But if the social costs of imprisonment grow without limit along with the prison population, mass incarceration becomes a self-defeating strategy for crime control.

Reducing these social costs is an urgent priority. Successful programs now offer transitional jobs to released prisoners, support the children of incarcerated parents, cultivate police-community relations, and send fewer people to prison in the first place. Measures like these reduce the social damage of mass incarceration and promise a sustainable public safety.

Bruce Western (Bruce_Western@harvard.edu) is a professor of sociology and director of the Program in Inequality and Social Policy at Harvard University.