Former Bolingbrook, Ill., police Sgt. Drew Peterson, seen here May 8, 2009, was found guilty Thursday of killing his third wife. M. Spencer Green/AP hide caption

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Many in the crowd at Sunday evening's memorial service, held outside of the Aurora Municipal Center, held up handmade signs, honoring the shooting victims and the city of Aurora, Colo. Sam Sanders/NPR hide caption

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Police gathered at the east entrance of Columbine High School in Littleton, Colo., on April 20, 1999, after two teen students went on a shooting rampage before turning the weapons on themselves. Mark Leffingwell/AFP/Getty Images hide caption

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In a video by Our Mexico of the Future, child actors play police, criminals and other adult roles, with the goal of highlighting the challenges facing Mexico. Nuestro Mexico Del Futuro hide caption

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A photo provided by the California Department of Corrections shows killer Charles Manson, 77, on April 4, 2012. AP hide caption

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The arrest of an Occupy DC protester earlier this month in Washington — one type of arrest that would show up in the researchers' numbers. Nicholas Kamm /AFP/Getty Images hide caption

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