Scientists have found that in substance addiction, environmental cues such as being in a place where drugs are taken or meeting another person who takes drugs cause sharp surges of dopamine in crucial reward and habit regions of the brain, specifically, the nucleus accumbens and dorsal striatum. This triggers cravings in anticipation of experiencing pleasure and relief through intoxication. Recent studies show that similarly, cues such as experiencing or being reminded of a perceived wrong or injustice — a grievance — activate these same reward and habit regions of the brain, triggering cravings in anticipation of experiencing pleasure and relief through retaliation. To be clear, the retaliation doesn’t need to be physically violent—an unkind word, or tweet, can also be very gratifying.

Although these are new findings and the research in this area is not yet settled, what this suggests is that similar to the way people become addicted to drugs or gambling, people may also become addicted to seeking retribution against their enemies—revenge addiction. This may help explain why some people just can’t let go of their grievances long after others feel they should have moved on—and why some people resort to violence.

It’s worth asking whether this helps explain Trump’s fixation on his grievances and ways of exacting retribution for them.