Several studies on autobiographical memory and happiness have determined that human memory is biased toward happiness and that mild depression can disrupt this bias for good over bad.

There are two causes for recollections of the past to be positively biased, according to a research team headed by W. Richard Walker, Ph.D., of Winston-Salem State University. The first cause is that pleasant events outnumber unpleasant events because people seek out positive experiences and try to avoid negative encounters, the researchers found.

Twelve studies conducted by five different research teams found that people of different racial and ethnic backgrounds and participants ranging in age from their late teens to early fifties consistently reported that positive events outnumbered the negative experiences in their lives, Walker said.

The other process involves a memory system that treats pleasant emotions differently from unpleasant feelings. Seven studies reviewed by the team supported a "fading affect" for negative emotions.

"Pleasant emotions have been found to fade more slowly from our memory that unpleasant emotions," the researchers said. "One mechanism for this uneven fading may involve a process known as minimization. In order to return to our normal level of happiness, we try to minimize the impact of life events."

This minimization process, which occurs biologically, cognitively and socially, is usually stronger for negative events than for positive events, the team noted.

"This implies there is a tendency to deaden the emotional impact of negative events relative tothe impact of positive events," Walker said. "Such deadening occurs directly because people are motivated to view their life events in a relatively positive light."

The research should be viewed as "evidence of healthy coping processes operating in memory and suggests that although people do remember negative events, they just remember them less negatively," according to the team. These "healthy coping processes" also help reduce stressors and ensuing stress levels, team members added.

Of course, life is not pleasant for everyone; some study respondents reported more unpleasant than pleasant events, which indicates that the fading affect does not work for everybody, the research team found. In addition, unpleasant and pleasant emotions tend to fade evenly among those with mild depression, according to the findings.

"But for those not suffering from depression, the bias suggests that autobiographical memory represents an important exception to the theoretical claim that bad is stronger than good and allows people to cope with tragedies, celebrate joyful moments and look forward to tomorrow," Walker concluded.