Have you ever wondered why designers gravitate toward particular decades, resurrecting specific relics of the past each season? Apparently, basic math holds the answer: The algorithm for figuring out why specific eras begin trending again is the current year, minus 25, according to Vogue. So, that would explain the resurgence of the '90s in Fall 2015 collections, and in other recent seasons. Referencing the look and feel of a time period 25 years earlier isn’t arbitrary. Designers digging deep into those late-20th century aesthetics are mostly in their mid-40s, and therefore were twenty somethings living it up circa the '90s, Vogue says. Younger designers also reference the same decade, but in a romanticized, wish-I-was-there manner. They were likely in preschool when grunge was going strong and deconstructed minimalism was flourishing, after all. Who says you can't be sentimental about something you never actually witnessed or experienced? That equation doesn’t quite explain why other eras, like a strong '70s story or a hint of the '80s, has made a comeback. But at least you now know why labels like Rag & Bone, Céline, Phillip Lim, and Marques’Almeida are bringing back the slip dress, flirting with plaid, and/or throwing frocks over pants.