The first rule of fashion – that vertical stripes flatter the figure – is a myth, according to a study conducted by scientists at York University.
Women who avoid wearing clothes with horizontal stripes in the belief that it makes their bum look larger should be reassured by a scientific study showing quite the opposite. It found that horizontal stripes actually make people look slimmer.
It is a common misconception that dresses or tops with horizontal stripes appear to broaden a person's figure, according to Peter Thompson, a psychologist at York University, who is probably the first scientist to investigate a well-known optical illusion in relation to high street fashion.
The illusion was described in the 19th century by the great German physicist and physiologist, Hermann von Helmholtz. Dr Thompson applied the principle to women's clothes and found that a dress with vertical stripes made a women appear about 6 per cent wider than an equivalent-sized dress with horizontal stripes. "Helmholtz said that the horizontal stripes look taller and narrower than the vertical lines. That made me think that horizontal stripes are going to make things look taller and narrower according to this illusion," Dr Thompson said.
"That seemed to fly in the face of another well-known belief that we have, that horizontal stripes make us look fat. So I decided to see if there was truth in that," he told the British Association's science festival at Liverpool University.
He carried out a study on about 20 people, who were asked to assess the relative size of different dresses in either horizontal or vertical stripes. The supremacy of horizontal stripes in the "big bum" debate was clear-cut, he said.
"Horizontal stripes don't make you look fat. It's a subtle effect, but in fact people wearing vertical stripes look wider than the ones who are wearing horizontal stripes," Dr Thompson said.
"Indeed, horizontal stripes, if anything, actually make you look thinner. Helmholtz actually said that women wear horizontal stripes to make themselves look taller. So, in the 19th century, wearing horizontal stripes had a completely different belief attached to it than it does now," he said.
Dr Thompson said that he did not know when the idea that horizontal stripes made women look fat came to prominence, and he also cannot explain why vertical stripes should make someone look less slim and shorter than someone wearing horizontal stripes.
"I don't know why the effect works, and I don't know whether there has been any good explanation for the [optical] illusion in the first place," he told the science festival.
He originally became interested in the subject when he was visiting some ancient Greek temples on holiday, which had columns that bulged out slightly in the middle. This is described in guide books as "entasis", which is widely understood as a way of countering the optical effect of columns with parallel sides – which are supposed to appear to be thinner in the middle.
However, Dr Thompson investigated this idea and found no evidence to support it. "I carried out some experiments to see if there is an illusion that columns appear to be wasted in the middle if the sides are parallel, and the answer is no, they don't. Parallel-sided columns look parallel, or straight," he said.
This led him to look at the other well-known visual illusion of horizontal stripes, and he again found that there was little or no evidence to support the idea that horizontal stripes make people look fat.
"So these are just two illusions from everyday life where I think we've got it wrong," Dr Thompson said.
But are there any clothes that do help people to look slim? "Wearing black is a good thing. We know that works because we know that a black circle on a white background looks smaller than a white circle on a black background," he said.
"Wearing plain black is what you want to do – with a few horizontal stripes," he added.