Dutch child rights organisation Terre des Hommes put a fake picture online in 2013 of a 10 year-old girl purporting to be available for sexual activity. Known as “operation Sweetie”, potential abusers were asked to make a $20 (£14) online transfer.
In just 10 weeks, more than 1,000 paedophiles in 71 countries were caught by the sting. While few arrests were made because the operation could be deemed to be entrapment, the experiment showed how online payments could track paedophiles.
http://www.theguardian.com/sustainable-business/2016/jan/28/bitcoin-darknet-online-child-abuse-financial-sector
