Horizontal stripes made people appear slimmer, the research found
Wearing horizontal stripes can actually make a person appear thinner, a university research project has found.
Flying in the face of orthodox fashion advice, horizontal stripes are in fact more slimming than vertical ones, said Peter Thompson, a perception expert.
The University of York's Psychology Department asked people to decide which women wearing striped dresses looked slimmer in 200 pairs of pictures.
In women of the same size, horizontal stripes made the model appear thinner.
Further research on the subject showed that to make the women appear to be the same size, the ones wearing the horizontal stripes had to be 6% wider.
Dr Thompson's study was based on the Helmholtz square illusion, created by 19th-Century scientist Hermann von Helmholtz who drew two identically-sized squares and put vertical stripes on one and horizontal stripes on the other.
That experiment showed the square with the horizontal stripes appeared taller and thinner than the other square, prompting Helmholtz - correctly, according to Dr Thompson - to recommend women to wear horizontal stripes to make them look taller.
It is not clear when the idea that horizontal stripes are more fattening than vertical ones took hold.
Dr Thompson said: "We carried out a number of experiments both with squares and oblongs and pictures of women wearing horizontal and vertical stripes.
"Horizontal stripes don't make you look fat. The one wearing the vertical stripes looks wider than the one wearing the horizontal stripes.
"Horizontal stripes, if anything, make you look thinner."
Dr Thompson said it was not clear why the visual illusion existed, although it could be that horizontal stripes made an image appear more three-dimensional, with the introduction of depth reducing width.
But he explained that another long-held belief about how to look thinner was indeed accurate - black really is slimming.
"Wearing black is a good thing. We know that a black circle on a white background looks smaller than a white circle on a black background."