As temperatures rise, clothes get thinner and body shape is more likely to stand out. In such cases, people often look for clothes with vertical stripes to appear taller and slimmer, but in fact, clothes with horizontal stripes are the best choice. Since the 19th century, scientists have identified this as the optimal fashion formula.
Li-Hsun Peng, a professor at the Graduate School of Design at National Yunlin University of Science and Technology in Taiwan, and researcher Tzu-Yu Chen said in i-Perception on Mar. 16 that "contrary to common belief, experiments with an actual model found that stripes divided horizontally into thin bands create the slimmest-looking effect."
◇Optimal at 1:2 width ratio of horizontal stripes to whitespace
German scientist Hermann von Helmholtz (1821–1894) reported in 1867 that horizontal lines make space or a person appear longer and thinner than vertical lines. The researchers demonstrated this visual perception effect, known as the "Helmholtz illusion," using clothes worn by an actual model.
They first photographed a female model wearing a form-fitting, short-sleeved striped top. The model's body mass index (BMI, weight divided by height squared) was 20.8, slightly above the average for Taiwanese women. The clothes the model wore had stripes arranged horizontally and vertically, with different spacings.
In three designs, the widths of the black lines and white whitespace were 1 cm, 2 cm, and 5 cm, respectively, and matched each other. Two designs were pencil stripes consisting of 1 cm black lines with either 2 cm or 5 cm white whitespace. They repeat thin stripes as if drawn with a pencil.
When 241 college students were shown photos of a woman wearing various striped tops and asked how she looked, respondents generally said the model in horizontal stripes appeared slimmer. Among five horizontal-stripe tops, the one with 1 cm black lines and 2 cm white whitespace (1X2h) was overwhelmingly rated as the slimmest in both the front view (56.1%) and the back view (61.2%).
Even when comparing stripe direction within the same pencil-stripe ratio, respondents rated horizontal stripes as slimmer than vertical stripes (1X2v). They said it contours the figure well and is visually appealing.
That does not mean horizontal stripes always create a slimmer effect than vertical stripes. Peng said, "If the line and whitespace widths are both 1 cm, 75.2% answered that the vertical-stripe outfit (1X1V) looks slimmer from the back," noting, "This clearly shows how human perception changes with even a slight shift in viewpoint."
◇Explained by an illusion theory over 160 years old
The principle by which horizontal stripes make clothes look slimmer than vertical stripes stems from the 19th-century Oppel-Kundt illusion. German physicist Johann Josef Oppel noted in 1860 that spaces filled with lines or dots appear visually larger than empty spaces, and August Kundt studied this systematically beginning in 1863.
The Helmholtz illusion applies the Oppel-Kundt illusion to rectangles. A rectangle filled with horizontal lines appears partitioned up and down, making it seem taller and narrower than it is. Conversely, a rectangle filled with vertical lines appears partitioned left and right, making it seem wider than it is.
The Helmholtz illusion also applies to the three-dimensional human body. By densely filling vertical space, horizontal-stripe clothing visually extends body height and makes width appear narrower. Peter Thompson, a psychology professor at the University of York in the United Kingdom, reported in 2011 in the same journal that the slimming effect of horizontal stripes is as much as 10.7%. For an adult weighing 60 kg, that equates to a visible reduction of more than 6 kg.
At the time, the York team tested a three-dimensional version of the Helmholtz illusion by dressing mannequins. The Taiwan team advanced the research by experimenting with an actual model. The study also examined how the Helmholtz effect changes with stripe spacing. In particular, it identified an exceptional case in which, with equidistant stripes where the line and whitespace widths are equal, the vertical direction can look slimmer.
Peng said, "We wanted to move beyond the traditional dichotomy of 'horizontal stripes versus vertical stripes' in design psychology," adding, "We also found that women respond more sensitively than men to stripe patterns when evaluating body image."
References
i-Perception (2026), DOI: https://doi.org/10.1177/20416695261441454
i-Perception (2011), DOI: https://doi.org/10.1068/i0405