Why do we remember some things well, while other memories fade? Researchers suggest it could be that good memories persist longer than bad - helping to keep the human race happy and resilient.
Psychologists say that holding onto our good memories - and leaving the bad ones behind - helps us to deal with unpleasant situations and retain a positive outlook on life.
It was 80 years ago that the idea of negative memories fading faster was first proposed.
Back in the 1930s psychologists collected recollections about life events like people's holidays - marking them as pleasant or unpleasant.
Weeks later an unannounced request came from the researchers to recall their memories.
Of the unpleasant experiences nearly 60% were forgotten - but only 42% of the pleasant memories had faded.
This is something which many of us recognise - after a holiday we might reminisce about the pleasant days out and people we met - but forget about how terrible the flight delays were.
Flight delays before a happy holiday are likely to be forgotten
Later studies of this so-called Fading Affect Bias or FAB were more rigorous.
In the 1970s instead of asking people to recall random memories - where people might be biased towards recalling just positive ones - the participants were asked to keep diaries, recording the emotional intensity of their memories.
But because around 80% of all psychological research is carried out on American students, it wasn't clear whether the bias would exist in other cultures too.
To see if it was universal, Timothy Ritchie from the University of Limerick in Ireland decided to analyse data from samples collected by academics at six universities around the world.
The fall of the Berlin Wall in 1989
These researchers had access to participants from many different English-speaking ethnic groups including African-Americans, Ghanaians, Germans, Native Americans and New Zealanders of both European descent and Maori/Pasifica descent.
In all, 2,400 autobiographical memories were included, from 562 individuals in 10 countries.
The memories in the different studies were selected in various ways but many involved asking subjects to recall a few positive and negative events including details like the time and location as well as sensory information.
The data from New Zealand and Ghana included just men and women under the age of 30 but others like the German and Irish samples included older participants.
Most were asked to recall random events from their own lives - both positive and negative.
But the German subjects were asked about their emotional response to a single momentous event: the fall of the Berlin Wall in 1989.
Those recalling their emotional responses to memories were asked to remember them again later after various time lapses - and rate how they felt about them.
These are known as the initial effect and the current effect and the difference between them was measured.
The researchers found that the FAB occurred in each study, regardless of the cultural background of the participants.
The authors believe this study shows that the faster fading of unpleasant memories is a pan-cultural phenomenon and this helps individuals to process negativity and adapt to changes in their environment whilst retaining a positive outlook on life.
One group of people which has real problems recalling positive memories is those with severe depression.
Dr Tim Dalgleish, a clinical psychologist from the University of Cambridge, tries to help those with serious depression to access positive memories.
The technique he has used is known as the method of loci. This method is thousands of years old and uses visual imagery which you imagine along a route or in a location such as your home.
Participants in Dr Dalgleish's study all had severe depression. Because they find it so difficult to retrieve their memories a researcher helped to flesh them out - with details like sensory information such as any smells, colours or sounds.
One of the people who took part in the study, Emma Brinkley, found it hard to remember any positive memories.
She said: "Even now when I'm feeling low it's difficult to to call anything positive to mind to cheer you up. It's almost like your brain refuses to allow you to recall it into your consciousness."
Good memories are more durable than unpleasant recollections
Once decided upon, the memories are "placed" along a route such as a journey to work or college - or even inside your own home.
Dr Dalgleish says this is a vital part of the process "You set up say 10 points on the route - so the front door, the porch, the kitchen and lounge if it's around your house - and then you choose the memories you'd want to put in your suitcase - the sort of things you'd like to bring to mind when times are tough.
"And you create a bizarre and memorable image that links that memory to each point along the route."
"You could picture your living room full of sand, with the TV on and the screen having the sea on - the sound of seagulls and the sea.
"And the fact that the sound was in the living room makes it more bizarre and makes it easier to remember than if you just remembered sand on a beach."
It is the kind of technique that memory champions use successfully when performing feats like remembering the sequence of a whole deck of cards.
The researchers found that creating this mental map or "memory palace" enhanced the recall of the participants - when compared with another group which used a different technique - that of chunking memories into different sets and rehearsing them.
The method was found to have lasting benefits, with effects still seen when people were re-tested a week later.
Emma Brinkley has been surprised how long-lasting those memories are. "I find myself on certain days feeling a bit low and so I simply have just put myself through that familiar route and just try to think of some happy memories to try and cheer me up.
"Some days it is more effort than others. But I have found that there has been a real profound lifting of my mood."
Courtesy: Paula McGrath (http://www.bbc.com/news/health-27193607