Is memory biased towards positive or negative memories? I am trying to understand the relationship between positive/negative memories and the congruence of memory to existing beliefs.  These quotes are from Wikipedia, presented in alphabetical order:


Choice-supportive bias: "positive aspects tend to be remembered as part of the chosen option, whether or not they originally were part of that option, and negative aspects tend to be remembered as part of rejected options."
Egocentric bias: "This bias suggests that people remember the past as
they want it to be rather than the way it was"
Fading affect bias: "a psychological phenomenon in which information
regarding negative emotions tends to be forgotten more quickly than
that associated with pleasant emotions."
Mood congruent memory bias: "the improved recall of information
congruent with one's current mood."
Mnemic neglect: "a pattern of selective forgetting in which certain
autobiographical memories tend to be recalled more easily if they are
consistent with positive self-concept."
Negativity bias: "the psychological phenomenon by which humans have a
greater recall of unpleasant memories compared with positive
memories."
Placement bias: "tendency to remember ourselves to be better than
others at tasks at which we rate ourselves above average and tendency
to remember ourselves to be worse than others at tasks at which we
rate ourselves below average."
Pollyanna principle: "the tendency for people to remember pleasant items more accurately than unpleasant ones."
Positivity effect: "older adults' memories are more likely to consist
of positive than negative information and more likely to be distorted
in a positive direction."
Rosy retrospection: "refers to the finding that subjects later rate
past events more positively than they had actually rated them when
the event occurred"
Self-verification: "self-views bias memory recall to favor
self-confirming material over non-confirming elements."


Most of these suggest congruence with existing beliefs rather than a strict positive bias, but since "on the balance, most people tend to view themselves positively," and most people are happy, it makes sense that there would be a positivity bias overall.

The oddball in the above list is Negativity bias, that appears to contradict other findings.  Another exception is Mnemic neglect: "mnemic neglect is based on self-protection rather than self-verification" - ie, there is a strict positive bias, rather than a congruence with existing beliefs, in direct contradiction with research in Self-verification and others.

I think the resolution of these apparent contradictions is in the details: Is the memory about personal experience or just memorizing items?  Is the memory self-referent or otherwise?  Is the bias referring to memory encoding or memory recall?  Is it an issue of missing or distorted memories?

So my question is, is anyone aware of any studies that look at the circumstances under which memory bias is strictly positive, congruent with existing beliefs, strictly negative, or unaffected by mood or belief?  Or failing that, a comprehensive review of memory bias that might clarify this?
The short answer is: it depends on age. For younger adults, negative memories last longer than positive memories. The opposite is true for older adults (above 60 years old).

This paper is a good review of some of the above effects: Choice-supportive bias, Mood congruent memory bias, Positivity effect, and Negativity bias. It suggests that negative bias only exists among younger adults, while positive memories last longer than negative memories among older adults. The paper also provided several explanations. One of them is that older adults tend to exhibit auto-activation of emotional regulation ability, thus cognitive control comes into play during the memory recall process.

Mather, M. (2006). Why memories may become more positive as people age. Memory and emotion: Interdisciplinary perspectives, 135-158.