It’s one of the cardinal rules of dressing: horizontal stripes make you look fat; vertical ones slim you down.

But new research suggests that the nation’s women have been sorely misled and that the opposite is the case – vertical stripes are a no-no.

Peter Thompson, who conducted the study at the University of York, is mystified as to how women ever got it so wrong. Scientists have, he says, been aware of the unflattering properties of vertical stripes for nearly 150 years.

In a study involving more than 200 pairs of images of women in a variety of stripy combinations, he and his colleagues at York asked volunteers to decide which looked slimmer. Horizontal stripes were judged more slimming by a margin of six percentage points, which he described as significant. Dr Thompson also pointed to the efforts of Hermann von Helmholtz, a German physiologist who created the Helmholtz squares illusion in the 1860s. This consisted of two sets of parallel lines, one vertical and the other horizontal, that fitted perfectly inside a square. The squares were exactly the same size, yet the vertical lines appeared to cover a wider area.

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Helmholtz applied his finding to women’s fashion in his Handbook of Physiological Optics, published in 1867, in which he noted that “ladies’ frocks with cross stripes on them make the figure look taller”.

Helmholtz’s wisdom was lost in the 20th century when, Dr Thompson explained, the erroneous idea that a fuller figure would benefit from vertical stripes became commonplace. “I think we’ve got it wrong as a society and I’ve no idea how we got the idea that vertical stripes are slimming,” he told the British Association for the Advancement of Science in Liverpool.

He added that the popular concept of black being slimming was, however, correct. “Wearing black is a good thing,” he said. “That one works. We looked at a black circle on a white background and a white circle on a black background. The black circle looked smaller than the white one."

Dr Thompson’s findings may be fascinating from a scientific point of view (he is also interested in the illusions created by patterned wallpaper and bulging Greek columns) but they are unlikely to rock the foundations of the fashion world assembled in the US for New York Fashion Week. No self-respecting fashionista believes in the horizontal/vertical stripe thing and they all know about black – why else do fashion editors wear it all the time?

Here’s the unscientific reason why vertical stripes make you look fat: women’s bodies are, by their very nature, curvy things. Stripes are straight. If you put a straight vertical stripe on a curvy bottom, the line of the stripe will be distorted by the body beneath – which will serve only to accentuate the bulge.

The same is not nearly so true of horizontal stripes, which is why hooped tights occasionally make a comeback, whereas vertically striped ones, as favoured by Mary Quant in the Sixties, are consigned to the history books.

In truth, stripes in general are not particularly flattering to the fuller figure. Geometric patterns and organic shapes, on the other hand, work very well, breaking up the surface area covered and confusing the eye into believing it smaller. But the awful truth remains: being fat makes you look fat, and no amount of fabric, can ever truly conceal it.

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