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The killings in San Bernadino, California, bring to 353 the number of mass shootings in the US this year alone. Yet perhaps curiously, it's thought there's been a sharp decline in gun violence over the past 20 years, with fewer Americans dying in this way. Here's Professor Franklin Zimring, a criminologist at the University of California. (Photo: : People hold candles as they attend a vigil to remember those injured and killed during the shooting on December 3, 2015 in San Bernardino, California, that left at least 14 people dead and another 17 injured on December 2nd.) (Credit: Joe Raedle/Getty Images)