Vertical stripes make you look fat

If science teaches us anything, it’s that you should look at the evidence and not just believe what you’re told. Even when it’s common-sense wisdom like ‘horizontal stripes make you look fat.’ Really, the opposite is true, and it’s due to an optical illusion described in 1867 by Hermann von Helmholtz.

Ah, the 19th century, when a scientist could still be an expert in everything. Helmholtz was like that. Born in 1821, his first major work, when he was 26 years old, was to identify conservation of energy in the context of muscle movement.

Helmholtz was far from the first to ‘discover’ the principle of conservation of energy. But his work was significant because it went against what most German natural philosophers believed at the time, which was that there’s some sort of vital force needed to move muscles. Helmholtz realised that the energy in muscles, which he called a ‘force’, was no different to that found in mechanics, heat, light, electricity and magnetism.

He went on to measure the speed at which nerve signals travel (he got between 24 and 38 metres per second), he invented an acoustic resonator that could mimic vowel sounds by combining multiple frequencies, he studied electric oscillations, and the Helmholtz coil with its uniform magnetic field is named after him.

In optics, he became famous for inventing an opthalmoscope, which was used to examine the inside of the human eye. But in his book on optics, published in 1867, he also mentioned some optical illusions.

The one that is named after him is the Helmoltz illusion, in which a square filled with horizontal lines appears taller than one filled with vertical lines. This is a particular version of what’s also known as the Oppel Kundt illusion, in which a space that’s filled looks bigger than one that’s empty.

Two optical illusions: the Helmholtz illusion, where a square of horizontal lines appears to be taller and narrower than an identical square of vertical lines, and the Oppel–Kundt illusion, where filled region appears larger than an unfilled one
Two optical illusions: on the left, the Helmholtz illusion, where the square of horizontal lines appears to be taller and narrower than the identical square of vertical lines. On the right, the Oppel–Kundt illusion: point B appears closer to A than to C, indicating that the filled region B–C appears larger than the unfilled A–B (Image from Thompson and Mikellidou, i-Perception)

In his book, Helmholtz wrote about the practical consequences of these illusions:

“There are numerous illustrations of the same effect in everyday life. An empty room looks smaller than one that is furnished; and a wall covered with a paper pattern looks larger than one painted uniformly in one colour. Ladies’ frocks with cross stripes on them make the figure look taller.”
– Helmholtz 1867, Handbuch der physiologischen Optik, vol. 3 (translation by J P C Southall 1925)

This is the exact opposite of modern fashion advice. So which is right?

Enter Peter Thompson and Kyriaki Mikellidou from the University of York. Dr Thompson first presented some work on this topic at a conference in 2008, and last year the pair published a complete paper (Thompson P & Mikellidou K 2011, “Applying the Helmholtz illusion to fashion: horizontal stripes won’t make you look fatter”, i-Perception, vol. 2, no. 1, pp. 69–76, DOI: 10.1068/i0405).

To begin with, they tested the basic Helmholtz illusion by flashing up on a screen images of rectangles of horizontal and vertical lines, and testing whether volunteers thought they were the same height and width. The results varied with the thickness of the lines, but a pattern of vertical lines had to be 4.1-10.1% taller than an equivalent pattern of horizontal lines to be perceived as being the same height.

Similarly, a pattern of horizontal lines had to be 1.3-6.5% wider to appear the same as a vertical pattern. Or to put it another way, in squares of the same size, horizontal patterns looked taller and vertical patterns looked wider. It seems Helmholtz was right.

Naturally, this doesn’t necessarily generalise to human figures or 3-dimensional shapes. So next they tried it on both a cartoon figure of a woman and on pictures of cylinders, and got similar results.

Finally, they used 3-D images of mannequins with vertical or horizontal stripes on their torsos. And again, they found that the mannequin in horizontal stripes had to be 10.7% wider to be seen as the same as the one wearing vertical stripes – consistent with what Helmholtz said.

Stereoscopic image of the mannequins used in the experiment, one with vertical stripes the other with horizontal - the vertically-striped one looks broader
An example of the mannequins used in the experiment, where both have the same outline but the one with vertical stripes appears broader. This is a stereoscopic image, so try crossing your eyes to make it look 3-D! (Image from Thompson and Mikellidou, i-Perception)

Thompson and Mikellidou claim that another way to observe this effect yourself is by stacking coins: people asked to stack coins to a height equal to their diameter will typically fall short by about 30% – try it and see!

So considering that this optical illusion has been known for so long, why has fashion gotten it wrong? Is it one of those ideas that sounds like it makes sense, so we think it must be true?

If so, then this demonstrates that common sense isn’t always sensible, and you should question everything. And the scientific method, employing careful experiments, is the best way to find the answers.

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