The word nostalgia was first coined in 1688 by Swiss physician Johannes Hofer, who defined it as a neurological illness of continually thinking about one's homeland and longing for return.

It wasn't until the 19th century that nostalgia began to be seen as a positive sentiment, rather than a pathological condition. The legendary Swiss psychiatrist Carl Jung, for example, viewed nostalgia as a way of reconnecting with our past in order to understand our present. For Jung, nostalgia was a way to access the "collective unconscious"—the shared history and experiences that we all have as human beings.

And looking at the content of this group, one can definitely see where he was coming from.

Over the past decade or so, some comprehensible literature on the psychology of nostalgia has developed. Research shows that people engage in nostalgic reverie mostly when they're feeling low in an attempt to boost their mood and self-confidence.

Experimental evidence indicates that nostalgia is experienced as an overwhelmingly positive emotion. It not only has the power to boost one's mood but can increase a sense of meaning in life as well. Furthermore, it also raises self-esteem and optimism for the future.

However, some, like University of Southern California psychologist David Newman, argue that these effects may be due more to the experimental setup than to the true nature of nostalgia.

Exploring the relationship between nostalgia and mood, Newman and colleagues performed a time-lag analysis, meaning they correlated people's feelings of nostalgia with mood later that day and the next day.

The results showed that mood tended to remain low throughout that time period. This suggests that nostalgia is either ineffective at boosting mood or that it even causes feelings of depression. Other possible negative effects of nostalgia include:

• A sense of loneliness and isolation;
• Dwelling on the past and becoming unhappy with the present;
• Becoming less likely to take action in the present.

But what is it about these two decades that people remember? Well, in the 1980s, many embraced a new conservatism in social, economic, and political life, characterized by the policies of U.S. President Ronald Reagan and, in the United Kingdom, Prime Minister Margaret Thatcher.

For some, the 1960s and 1970s had been a troubling time: the countercultural movements of the era, the Watergate scandal, the Vietnam War, the energy crisis, rising crime, and inflation had undermined Americans' confidence in their fellow citizens and in their government. But the 1980s are often regarded for the opposite, namely its materialism and consumerism.

This period also saw the rise of the "yuppie," an explosion of blockbuster movies and the emergence of cable networks like MTV, which introduced the music video and launched the careers of many iconic artists, and the emergence of the AIDS crisis, which would go on to kill more than 700,000 people in the U.S. alone. 

Similarly, the 1990s is also often remembered as a decade of relative peace and prosperity. The Soviet Union fell, ending the decades-long Cold War, and the rise of the Internet ushered in a radical new era of communication, business, and entertainment.

Canadian novelist, designer, and visual artist Douglas Coupland thinks that for North Americans and Europeans, the 1990s possessed a sense of happiness that seems long vanished.

"Money still generated money. Computers were becoming fast easy and cheap, and with them came a sense of equality for everyone," he wrote. "Things were palpably getting better everywhere. History was over and it felt great. I also remember working at Wired magazine, though, in 1993, and having a discussion about the internet with one of the editors, Kevin Kelly. The thrust was that there was an internet, sure, but there was nowhere to go. Kevin said, 'Nonsense,' and took me to a website showing a slowly downloading weather map of Northern California and southern Oregon."

So what can we do if we think about those times and find ourselves in a bad place instead of a good one? Experts suggest to:

• Think about the present moment. What are you doing right now that you enjoy?
• Make an effort to connect with others in the present. Spend time with people you care about. Talk to them about your positive memories.
• Do things that make you happy. Listen to music, go for walks, and watch your favorite movie.
• Talk to a therapist. If you're feeling particularly down, talking to a therapist can help.
• Be mindful. Be aware of how much time you spend dwelling on the past.