Inside America's prisons

Inmates stand in a gymnasium where they are housed due to overcrowding at the California Institution for Men state prison in Chino, California, June 3, 2011. REUTERS/Lucy Nicholson

Jail cells are seen in the Enhanced Supervision Housing Unit at the Rikers Island Correctional facility in New York March 12, 2015. REUTERS/Brendan McDermid

Chris Willis, 34, works out in the exercise yard at San Quentin state prison in San Quentin, California, June 8, 2012. REUTERS/Lucy Nicholson

Inmates serving a jail sentence make a phone call at Maricopa County's Tent City jail in Phoenix July 30, 2010. REUTERS/Joshua Lott

Marvin Caldwell, 63, who said he was imprisoned for 20 years under the three strikes law for possession and sale of methamphetamine, looks out of his cell at San Quentin state prison in San Quentin, California, June 8, 2012. REUTERS/Lucy Nicholson

Cori Walters, 32, (R) hugs her daughter Hannah Walters, 6, at California Institute for Women state prison in Chino, California May 5, 2012. REUTERS/Lucy Nicholson

Administrative segregation prisoners take part in a group therapy session at San Quentin state prison in San Quentin, California, June 8, 2012. REUTERS/Lucy Nicholson

Inmates walk around a gymnasium where they are housed due to overcrowding at the California Institution for Men state prison in Chino, California, June 3, 2011. REUTERS/Lucy Nicholson

Inmates exercise at the California Institution for Men state prison in Chino, California, June 3, 2011. REUTERS/Lucy Nicholson

Corrections officers work in the Enhanced Supervision Housing Unit at the Rikers Island Correctional facility in New York March 12, 2015. REUTERS/Brendan McDermid

A jail cell is seen in the Enhanced Supervision Housing Unit at the Rikers Island Correctional facility in New York March 12, 2015. REUTERS/Brendan McDermid

Inmates walk in San Quentin state prison in San Quentin, California, June 8, 2012. REUTERS/Lucy Nicholson

Women chat as they lie in beds placed in the communal area outside cells, due to overcrowding at the Los Angeles County Women's jail in Lynwood, California April 26, 2013. REUTERS/Lucy Nicholson

An inmate looks out from his cell in the Security Housing Unit (SHU) at Corcoran State Prison in Corcoran, California October 1, 2013. REUTERS/Robert Galbraith

Tylan Gregory plays with his daughter Tyla, 5, during a "Get On the Bus" visiting day to Folsom State Prison arranged by the California Department of Corrections and Rehabilitation (CDCR) and the Center for Restorative Justice Works (CRJW) in Folsom, California July 26, 2014. REUTERS/Elijah Nouvelage

Inmates participate in the workshop "Commedia Dell'Arte", part of the The Actors' Gang Prison Project program at the California Rehabilitation Center in Norco, California September 30, 2014. REUTERS/Mario Anzuon

A woman sits in a cell at the Los Angeles County Women's jail in Lynwood, California April 26, 2013. REUTERS/Lucy Nicholson

Inmate Donnell Moore touches the head of his son Donnell Moore during a special Father's Day visit part of the "Get On The Bus" program at California Men's Colony in San Luis Obispo, California May 30, 2015. REUTERS/Mario Anzuoni

An inmate visits a dentist in the medical facility at San Quentin state prison in San Quentin, California, June 8, 2012. REUTERS/Lucy Nicholson

Prisoners work at computers following a graduation ceremony from a computer coding program at San Quentin State Prison in San Quentin, California April 20, 2015. REUTERS/Robert Galbraith

Inmate Jamar Allah flips his tassel as a graduate before receiving his High School GED (General Equivalency Diploma) along with 26 others at a graduation ceremony for inmates at the George Motchan Detention Center at New York City's Rikers Island correctional facility June 26, 2012. REUTERS/Mike Segar

A prison inmate works out at Oak Glen Conservation Fire Camp #35 in Yucaipa, California November 6, 2014. Thousands of convicted felons form the backbone of California's wildfire protection force under a unique and little-known prison labor program. REUTERS/Lucy Nicholson

An inmate patient is shown in his cell at the California Medical Facility (CMF) in Vacaville, California, March 17, 2010. CMF was established to provide a centrally located medical and psychiatric institution for the health care needs of California's male felon population. REUTERS/Robert Galbraith

San Quentin post office is seen outside San Quentin state prison in San Quentin, California, June 8, 2012. REUTERS/Lucy Nicholson

An inmate waits for a visitor at the California Institution for Men state prison in Chino, California, June 3, 2011. REUTERS/Lucy Nicholson

Albert Ruiz, 50, who is on death row for murder, is led down a corridor at San Quentin state prison in San Quentin, California, June 8, 2012. REUTERS/Lucy Nicholson

Martell Collins, 51, does push-ups in the exercise yard at San Quentin state prison in San Quentin, California, June 8, 2012. REUTERS/Lucy Nicholson

Inmates sort laundry at the Orange County jail in Santa Ana, California, May 24, 2011. REUTER

Prisoners walk in the exercise yard at San Quentin State Prison in San Quentin, California April 20, 2015. REUTERS/Robert Galbraith

The entrance to death row is seen at San Quentin state prison in San Quentin, California, June 8, 2012. REUTERS/Lucy Nicholson