Bell-bottoms today, miniskirts tomorrow: Math reveals fashion’s 20-year cycle

Fashion insiders and beauty magazines have long cited the “20-year-rule” — the idea that clothing trends often resurface every two decades. 

According to Northwestern University scientists, that observation isn’t just anecdotal. It’s a mathematical reality.

In a new study, the Northwestern team developed a new mathematical model showing that fashion trends tend to cycle roughly every 20 years. By analyzing roughly 37,000 images of women’s clothing spanning from 1869 to today, the team found that styles rise in popularity, fall out of favor and then eventually experience renewal.

Along with supporting common perceptions about the life cycles of fads, the researchers say these results could help explain how new ideas spread in society.

The study’s lead author Emma Zajdela will present these findings at 4:30 p.m. MDT on Tuesday, March 17 at the American Physical Society (APS) Global Physics Summit in Denver. Her talk, “Back in Fashion: Modeling the Cyclical Dynamics of Trends,” is part of the session “Statistical Physics of Networks and Complex Society Systems.”

Complimentary registration is available for members of the media. Press should contact the APS media teamfor more information.

“To our knowledge, this is the first time that someone developed such an extensive and precise database of fashion measures across more than a century,” Zajdela said. “We have some very interesting results, including that the cycle we uncovered in the data (20 years) matches industry knowledge. Historically, the lack of data posed a barrier to explicit quantitative study of this system.”

When this work was conducted, Zajdela was a Ph.D. candidate at Northwestern’s McCormick School of Engineering, where she was advised by Daniel Abrams, a professor of engineering sciences and applied mathematics at McCormick and co-director of the Northwestern Institute on Complex Systems. Now, Zajdela is a postdoctoral fellow at Princeton University and a research fellow at the Santa Fe Institute.

Zajdela and Abrams coauthored the study with Alicia Caticha, an assistant professor of art history at Northwestern’s Weinberg College of Arts and Sciences, and Jeremy White and Emily Kohlberg, who were both members of Abrams’ research group.

To conduct the study, the researchers compiled one of the most comprehensive quantitative datasets of fashion ever assembled. Drawing from historical sewing patterns from the Commercial Pattern Archive at the University of Rhode Island and runway collections, the team analyzed tens of thousands of garments dating back to the late 1800s.

Using custom tools, they measured key features of dresses — hemline, neckline and waistline positions — turning clothing designs into numerical data that could be measured and tracked across decades. To analyze the data, researchers built a mathematical model based on a simple idea: the tension between wanting to stand out while still fitting in. Once a style becomes too common, designers move away from it — but not so far that the clothes become unwearable.

“Over time, this constant push to be different from the recent past causes styles to swing back and forth,” Abrams said. “The system intrinsically wants to oscillate, and we see those cycles in the data.”

The results revealed a striking pattern. While fashion evolves gradually over time, the rise and fall of styles follows a repeating wave that peaks roughly every two decades. One of the clearest patterns involves hemline length. Over the past century, skirt lengths have repeatedly shortened and lengthened — from shorter flapper dresses in the 1920s to longer, more conservative styles in the 1950s and then to miniskirts of the late 1960s.

But this pattern loses its clarity in recent decades. Starting in the 1980s, the data show a wider range of skirt lengths appearing at the same time, suggesting that fashion trends are becoming more fragmented. Rather than one dominant trend, niches emerge, reflecting more diversity in fashion.

“In the past, there were two options — short dresses and long dresses,” Zajdela said. “In more recent years, there are more options: really short dresses, floor-length dresses and midi dresses. There is an increase in variance over time and less conformity.”

END