Remember how you felt in the not-so-good times? While we certainly remember what happened, our emotional recall dims with time. Researchers have found that the mind is biased toward positive emotional memories—as negative emotions fade faster. This isn't universal, however, as mild depression can skew emotional memory toward the negative.

"People have an inherent bias to view their experiences in a positive light," says study author Richard Walker, Ph.D., assistant professor of psychology at Winston-Salem State University in North Carolina. For one, the positive bias can be explained simply because there are more positive events than negative ones, explains Walker.

In his research, he found that happier emotions have a longer shelf life in our memories. In a review of 12 studies, he found that people consistently report experiencing more positive events. Negative emotions tended to fade faster than positive ones.

This "fading effect" works differently for those with depression. The researcher tracked the emotional recollection of 330 participants and found that their positive and negative emotions fade at the same rate. "The folks who are mildly depressed tend to report more negative life events," says Walker. "And they tend to have more difficulty dealing with negative emotions. This is likely to be one of the contributing factors [to depression]."