A research paper published by the University of Sydney, the University of Technology Sydney and the Stockholm School of Economics showed that every fourth uses bitcoin for illegal or criminal activity.
According to the research, about $8 billion is stored on accounts involved in crime. The account holders have made 36 million transactions during the year with a total of $72 billion.
Read the details in the article of Coinidol dot com, the world blockchain news outlet: https://coinidol.com/crime-scene-over-half-of-bitcoin-transactions-used-for-illegal-purposes/