Researchers have pointed out at a "trend towards centralization". Although bitcoin can be sent directly from user to user, in practice intermediaries are widely used. :220–222 Bitcoin miners join large mining pools to minimize the variance of their income. :215, 219–222:3 Because transactions on the network are confirmed by miners, decentralization of the network requires that no single miner or mining pool obtains 51% of the hashing power, which would allow them to double-spend coins, prevent certain transactions from being verified and prevent other miners from earning income.  As of 2013[update] just six mining pools controlled 75% of overall bitcoin hashing power.  In 2014 mining pool Ghash. io obtained 51% hashing power which raised significant controversies about the safety of the network. The pool has voluntarily capped their hashing power at 39. 99% and requested other pools to act responsibly for the benefit of the whole network. c. 2017 over 70% of the hashing power and 90% of transactions were operating from China.