If you have more than a passing interest in fashion or beauty, or fashion and beauty, it is highly likely you will spend hours scoring magazines and websites – looking for what is hot and what is not. Fashion is always changing.

But who decides what you should wear, or what is ‘in’? It certainly isn’t the man or woman on the street that says that this summer, you have to get a trendy playsuit. Who dictates that people are mainly going to be wearing tartan jeggings – or whatever may be the hot thing to do?

Is it celebrities? As Eva Wiseman says: “When Alexa Chung gets dressed, the high street cashes in.” As a nation we are obsessed with celebrities – the millions of pounds brands throws behind them in sponsorship and the like is testament to that. There are theories that say such endorsement can ‘alter brain activity’.

In 2010, a group of experts from Holland scanned the brains of women as they looked at pictures of celebrities and attractive non-famous contemporaries sporting certain shoes. The study saw heightened activity in a certain part of the brain – the medial oribitofrontal cortex – when shown a picture of someone famous.

“The enhanced memory performance for items that were encoded in the context of a famous individual can neither be explained by increased attractiveness of the celebrity nor by a higher level of perceived expertise, but only by the persuasiveness of fame itself,” wrote lead author Mirre Stallen of Erasmus University.

But at the same time, celebrities have to look good. For a lot of them it is in the job description. So it might be better to describe them as up with what’s hot rather than dictating it.

The obvious answer to this question would be designers. It is the fashion houses that want celebrities to wear their clothes. And it is the celebrities that can afford them before watered down versions make their way to the high street rails. So the trends the great and the good choose are decided by designers for them, in a way.

But, what inspires a designer to introduce a load of plaid or tartan to their newest line? It could be social and economic factors. If you go all the way back to the 1960s, a lot was dictated by the flower power and carefree attitude of the time – something which was a response to factors like the Vietnam War. Similarly, the birth of the miniskirt was the result of a liberated younger generation breaking away from the shadow of World War Two, which for the last 20-or-so years had been looming over everything.

So that leaves us with you, the consumer. When asked, clothing manufacturers have admitted themselves that the person on the street can have a direct influence on the items they produce.

Linda DeFranco recalls a meeting with her boss, the head of trend forecasting at Cotton Inc. while strolling the streets of Stockholm she had noticed teens wearing their jeans rolled up a few inches to make them tighter around the legs. She and her colleagues predicted a coming trend — skinny jeans — and relayed that to their retail clients.

“At the time a lot of ’80s fashion was coming back into style, and this look clearly fit with that,” she said. By 2006, skinny jeans were all the rage.

The industry has used trend forecasts for years – but their role has changed since the 60s and 70s. Then forecasters, as trend spotters, would report back on the interesting things they saw on the street and in clubs. Nowadays, about 1,000 to 1,500 people work as fashion trend forecasters. They take photos of everything and anything they think might influence fashion.

“You become a translator, looking at cultural signposts and connecting things that appear to be disparate, but aren’t,” says Helen Job, who teaches trend-spotting at Parsons the New School for Design, New York.

So, the answer to what makes things trendy, if there is one, is a lot of things. Which isn’t very helpful.

This piece was cotributed by Jane Foulds, the trendiest girl in town

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