Mara Salvatrucha (MS) gang members show off their weapons in the Las Victorias district of San Salvador. In March 2012, the two largest gangs in El Salvador - the Mara Salvatrucha (MS) and the Barrio 18 (M18) - agreed on a truce following secret negotiations between gang leaders in prison which were mediated by a bishop and a former rebel leader. It is unclear whether the decision was the idea of the gangs themselves or whether they were encouraged by the government. (Photo by Adam Hinton)

The anthropometric record card of Sarah Giles, a servant who was convicted of stealing in 1897, on display making up part of a mosaic of cards on a table top during a press preview for the Crime Museum Uncovered exhibition at the Museum of London in the City of London, Wednesday, October 7, 2015. Drawn from Scotland Yard's private collection, the show charts more than a century of violence and suffering, from the murders of Jack the Ripper to IRA and al-Qaida bombings. But it also celebrates the brains, bravery and scientific advances that helped catch perpetrators and solve crimes. (Photo by Alastair Grant/AP Photo)

An inmate (L) tends to a fellow prisoner while performing ear candling during an alternative therapy session as part of the ACUDA programme, at a complex of ten prisons in Porto Velho, Rondonia State, Brazil, August 27, 2015. Ear candling, which involves inserting a hollow cone-shaped device into the ear canal and lighting the exposed end, is believed by practitioners to draw out earwax. According to ACUDA the therapy is beneficial for the inmates' emotional health. (Photo by Nacho Doce/Reuters)

A young man, who says he is part of a local criminal gang, poses for a picture holding a gun in the neighbourhood of Korogcho in Nairobi, Kenya, March 19, 2015. Around 2 million people live in the shantytowns packed in around Kenya's capital. Crime is high amid chronic unemployment levels, while basic services and sanitation are scarce. Residents try to make the best of things, eking out a living and picking up work where they can. (Photo by Siegfried Modola/Reuters)

Vintage Newton, a pop-up exhibition of Helmut Newton prints from 1974-1984, opens in west London at the ONGallery. These prints, which feature Charlotte Rampling and Elsa Peretti, were produced from a series of transparencies that he considered his most provocative and important. Here: Saddle II. (Photo by Helmut Newton)

Actress Athene Seyler attempts to contort her body, in a scene from the show, “Skin Deep”. (Photo by Sasha/Getty Images). Circa 1928

Old Heidelberg, Dog with Pipe, 1905. (Photo by Detroit Publishing Company)

A new monorail service in Houston, Texas, ready for its inaugural service. (Photo by Evans/Three Lions/Getty Images). Circa 1950