Since 2011, the #TBT Instagram trend has blown up -- and its popularity has nostalgia's restorative powers to thank.

FORTUNE -- I archive dive almost every Thursday, searching for the perfect photograph: a shot from one of college's many Ugly Sweater parties; my best friend and I, 20 pounds lighter, grinning at prom; my sisters and I huddled together in 1996, our matching bowl cuts perfectly aligned. With a little help from the slight aging powers of the Valencia filter, my picture-perfect memories are posted to Instagram -- never without the beloved #tbt hashtag.

I'm far from Instagram's only wistful user. To date, more than 228 million photos have been tagged with a "Throwback Thursday" hashtag -- either #tbt or #throwbackthursday -- indicating the use of a crowd-pleasing photo from days gone by. (#Love, which is Instagram's most popular tag, is used in a total of 523.6 million photos.) Even celebrities are in on the act: Michele Obama, Beyoncé, and Ariana Grande had some of the most-liked #tbt photos in 2013, but lost to Niall Horan of the pop group One Direction; his photo raked in over 798,000 insta-hearts.

Despite its popularity, Throwback Thursday's origins are murky. According to an Instagram spokesperson, the earliest existing use of #throwbackthursday can be found on a February 10, 2011 photo by @bobbysanders22. And @ashro posted the earliest existing #tbt post on September 30, 2011. KnowYourMeme.com, a website that tracks Internet culture fads, dug deeper, tracing the phenomenon's first Internet mention to illustrator Saxton Moore. In January 2006, he posted several retro cartoons on his website with the title "Throwback Thursdays 10." The following July, the sneaker blog Nice Kicks launched a Throwback Thursday-themed series. The blogosphere helped the trend snowball -- and so did Kim Kardashian. According to Digital Trends, reality TV's biggest star tweeted a Throwback Thursday Instagram (sans hashtag) in February 2012. Around the same time, the popularity of the term for web searches spiked, as measured by Google Trends.

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Despite Kardashian's significant social influence, the Instagram hashtag's success roots itself in deeper territory: Nostalgia. The rise of shareable platforms -- blogs, Facebook, and all manner of social media in between, with photo-focused Instagram at the forefront -- mixed with improved smartphone cameras and a universal longing for the past created the perfect #tbt storm.

Sharing old photos is a fun way to remember the good ol' days, but it may also help people counteract feeling alone. A 2008 study published in Psychological Science found that nostalgia alleviates feelings of social exclusion. Loneliness decreases perceived social support, but it also increases nostalgia, which increases perceived social support. Resilient participants -- the term used for those who experienced and rebounded from past trauma like a terrorist attack, divorce or death of a spouse -- were found to greatly rely on sentimentality to curb emotional isolation. "Nostalgia is a psychological resources that protects and fosters mental health. Nostalgia strengthens social connectedness and belongingness, partially ameliorating the harmful repercussions of loneliness," the study's authors write. "The past, when appropriately harnessed, can strengthen psychological resistance to the vicissitudes of life."

Last fall, the University of Southampton released a study that dove deeper into nostalgia's restorative powers. It found that nostalgia "fostered social connectedness, which subsequently lifted self-esteem, which then heightened optimism." The research adds that longing for the past is often sparked by moments of loneliness, sadness, boredom, or existential doubt. Nostalgia helps quiet those moments of lost, helping individuals cope with psychological adversity. (With that said, more research must be done on nostalgia in clinically depressed people.)

One of the study's co-authors, Wing-Yee Cheung, says #tbt's popularity makes sense. "When you're not connected to people who are close to you, you think about a picture of your family and friends. It compensates for what you don't have in that moment." She adds that nostalgia provides a sense of social continuity, helping individuals make better sense of their lives.

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TimeHop CEO Jonathan Wegener hopes to bank on #tbt's popularity -- and society's love of reminiscing. The company has raised just over $4 million, thanks to Spark Capital and angel investors like Foursquare co-founders Dennis Crowley, Naveen Selvadurai and Alex Rainert and GroupMe's Steve Martocci. "Throwback Thursday is the most mainstream expression of what we're trying to do at Timehop," he says. "It's about framing the conversation about the past and giving [people] an opportunity to share the past." Content gains value with time, he adds, and platform like Instagram give people an excuse to reflect.

Life is full of transitions. They can leave people feeling overwhelmed, confused, socially disconnected, and desiring to tap into some really great times in the past. (Who doesn't miss being a kid with hardly any responsibilities?) Social networks like Instagram provide a platform to calm those anxieties. "Look how cute I was!" is one reason to post a #tbt. Another: It may end up simply making you feel good.