The study that was conducted by researchers at the University of North Carolina at Chapel Hill found that this happens because when people are trying to intentionally forget information, they need to mentally segregate that information and then block off the information they don't want to retrieve. Emotion undermines both of those steps.
"You make a lot of connections between emotional events and other parts of your life, so it might be difficult to isolate them. As far as blocking retrieval of an unwanted event, emotion makes events very salient and therefore highly accessible," said the study's lead author, Keith Payne, an assistant professor of psychology in the College of Arts and Sciences.
The study's results contrast with previous studies of emotional events and intentional forgetting, but those studies used emotion-laden words as stimuli, like "death" and "sex."
The UNC study took a new approach, asking 218 participants to react to photographs instead of text.
"The word 'murder,' for instance, may or may not make you afraid, but if you see a graphic, violent picture, it may be powerful enough emotionally to change the way you feel," Payne said.
The researchers found that their subjects could not intentionally forget emotional events as easily as mundane ones. They also found that both pleasant and unpleasant emotional memories were resistant to intentional forgetting.
The UNC findings contribute to understanding the ways that emotion constrains mental control and to the question of whether intentional forgetting can be helpful in coping with painful or traumatic experiences.
"Our findings add to accumulating evidence that emotion places limits on the ability to control the contents of the mind," Payne said.
"Our results suggest that even a relatively mild emotional reaction can undermine intentional forgetting. But this doesn't necessarily mean that emotional memories can never be intentionally forgotten. If the motivation to forget is powerful enough, individuals might be able to overcome the effects of emotion by enlisting additional coping strategies," he added.
The study, "Emotional constraints on intentional forgetting," appears in the September 2007 print issue of the Journal of Experimental Social Psychology.