Anonymous terrorism -- i.e. terrorist attacks that no group or person has been publicly claimed responsibility for -- constitutes most terrorist attacks in the world at least since 1998. This proportion has been called "surprisingly high" and is in conflict with the conventional wisdom that terrorists "mount an operation to call attention to their grievances" and to "the costs of ignoring" those grievances, which can't happen if the perpetrators don't make public the "cost", i.e. announce that an event of killing and/or destruction was their work. A number of theories have been advanced as to why terror groups sometimes claim an attack and sometimes do not.