Depending on your definition of millennials, they are either just starting to or may have already passed that dreaded age, three zero. Even for generations not accused of being entirely self-absorbed -- as all generations have been at some point by their crotchety elders -- milestones like that are points of self-reflection; a chance to look back and ponder one's accomplishments. Millennials are thinking about how grown-up they've become, even as a record number of them still live with their helicopter parents. Maybe the reality of aging is creeping in as they realize that the baby photos their friends are posting on Facebook are of their own kids, not some "change your icon to your baby picture to raise awareness of..." meme.

In short, millennials, all 71.3 million of them, are now just old enough to get nostalgic.

Kate Loveland, a PhD candidate at Arizona State University's W.P. Carey School of Business, thought it was intriguing that her friends were buying the same toys for their kids that they themselves had grown up with. So she spent the last few years studying nostalgia and how it can be a trigger for purchasing decisions. The premise that buying a product associated with your childhood would make you feel nostalgic seems pretty on the mark. How else can you explain toy rotary phones on the market today -- what current toddler is ever going to be faced with a phone that has a cord, let alone a dial? Ms. Loveland says it "creates a symbolic, shared experience between the mom and the child."

Her research found some even more compelling ties. There is a bond between a sense of nostalgia and a social need to belong. If you can engage the "social self" in someone, according to her experiments, it provokes a "super-strong need for nostalgic products." What's more, if the subject then consumed a nostalgic product (e.g., an Oreo cookie) it actually satiated their need to belong. Test subjects were also inclined to watch shows such as "Saved by the Bell" or "The Fresh Prince of Bel Air" when the "social self" was triggered. When thinking of themselves or the future, they preferred contemporary shows such as "Grey's Anatomy" or "The Office."

This is especially true during social times of year (read: holidays) and periods of life transition like turning 30, graduating or getting married. In other words, when there is a heightened need to belong to a group, the desire for products that remind us of more comfortable times is greater.

To put that in practical terms, think about a setting that connects you to your past. Facebook is a pretty made-to-order example. Ms. Loveland's research would indicate that it's an ideal platform to advertise any product that provokes a sense of nostalgia. That might help explain how Oreo is the top brand on Facebook.

And it might mean Gen-Xers like myself will see a lot more Facebook ads with B. A. Baracus and Huey Lewis pitching us on the all-new 2012 K-cars.