The Hemline Index:

It is an economic principle that posits a correlation between the length of women’s skirts and economic health. The fundamental idea is that in times of economic boom, skirts are shorter and in times of economic recession, skirts are longer. This concept, introduced by economist George Taylor in 1926, believes that skirt length is an outward sign of consumer confidence and prosperity.
Here’s a more in-depth analysis:
Economic Boom:
People tend to be in better moods and more inclined to spend money on luxury and fashion when the economy is booming. Skirts get shorter, deemed to be more fashionable and risky.
Economic Downturn:
In times of economic downturn or uncertainty, people become more conservative when they spend their money and dress. Skirts get longer, viewed as more modest or sensible.
Examples:
The 1920s, an era of prosperity economically, was when shorter skirts such as the flapper dress emerged.
The 1930s, after the Great Depression, were when skirts returned to being longer.
The 1960s, which were a time of economic prosperity, were marked by mini-skirts, while the economic recession of the early 1970s had longer hemlines.