There’s mathematical backing to an age-old fashion rule that clothing trends repeat themselves every 20 years, according to a study from Northwestern University.
“We have all experienced the idea that fashion comes back – that miniskirts are back, or bell-bottom jeans are back,” lead author Dr Emma Zajdela, an applied mathematician at Princeton University, told BBC Science Focus.
“As mathematicians, we were curious to prove or disprove this theory – and recent advances in computer tools and digitised records have made this possible.”
The team of researchers – that included Zajdela as well as other mathematicians, computer scientists, engineers and art historians – compiled a database of approximately 37,000 images of women’s clothes to analyse how fashions changed over 150 years.
The dataset included historical sewing patterns dating from 1869 to 2015, as well as images from Vogue runways, from 1988 to 2023. These images were used to track how key features of womenswear changed over the decades.
“We chose to quantify the dresses along a vertical axis from head to feet: the length of the dress, the waistline and the neckline, using a computer tool our team developed,” explained Zajdela. “This allowed us to have consistent measures that we could compare over time.”
The results revealed that styles seem to cycle in popularity over roughly 20 years. They become popular, fall out of favour, and are eventually rediscovered.
Zajdela said: “The mathematical model we developed uses the concept from psychology of ‘optical distinctiveness’ – or the idea that, to be successful, innovations need to be different but not too different.”
Take skirt length, for instance. Hemlines got shorter in the early 1900s until the flapper styles of the 1920s, then longer around the middle of the century, shorter with 1960s miniskirts, longer with the hippies of the 1970s, and shorter again after that.
However, the researchers found that this 20-year hemline cycle became less clear from the 1980s onwards, as a wider range of skirt lengths appeared at the same time.
“Trends in fashion have been accelerating since the mid-1980s, making the 20-year rule less apparent – although still very present – in the data,” said Zajdela. “Instead, there has been an increase in the variety of styles available.
“Our model suggests that the increase in variety is due to both the acceleration of trends as well as society becoming more accepting of diverse fashion styles.”
The researchers added that the pattern may be indicative of how other trends evolve, including music, art, dog breeds and baby names.
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