The Mari are a telepathic, humanoid species in the Delta Quadrant.
Until as late as the mid-24th century, the Mari society was violent and crime was a major issue on their world. This was primarily caused by violent instincts being passed from one person to another via telepathy.
To solve this problem, the Mari outlawed violent thoughts and developed a memory adjustment technique to remove the offending engrams from a criminal's mind. By their system, any crime was caused by the person who originally had the violent thought and it was their mind that was altered.
Although violent thoughts had apparently been eradicated, a black market arose in which people sold violent thoughts to allow others to experience dark emotions. This remained a relatively harmless practice until 2374 when a thought harvested from B'Elanna Torres of the USS Voyager proved so violent that its hosts became unstable. As a result, one Mari individual was badly beating and another murdered before the culprits were caught and the black market and outlawed thoughts was uncovered. (VOY: "Random Thoughts")