Micah Zarin's Blog

In the Martian language, one of these two figures is a “bouba” and the other is a “kiki”; try to guess which is which:

What I just showed you is a well-known linguistic experiment that demonstrates what is today known as “the bouba-kiki effect.” It turns out that 95% of respondents recognize the sharper shape as Kiki and the rounder form as Bouba even though they had never seen these stimuli before

The effect was first documented in 1929 by German-American psychologist Wolfgang Köhler. In his groundbreaking experiments, Köhler showed participants two abstract shapes—one rounded and amoeba-like, the other pointy and star-shaped. He then asked them to identify which shape was “bouba” and which was “kiki.” An astonishing 95-98% matched bouba with the blobby shape and kiki with the spiky one.

Köhler’s work went largely unnoticed for decades until it was popularized by psychologist Vilayanur Ramachandran in the early 2000s. Ramachandran used the bouba kiki effect to support his theories about synesthesia—the rare neurological phenomenon in which stimulation of one sense involuntarily triggers another. Though the vast majority of us don’t experience synesthesia, the bouba kiki effect suggests we may all have traces of cross-sensory wiring in our brains.

So what exactly explains this phenomenon? Why do certain speech sounds feel intrinsically linked to certain visual shapes? There are a few key hypotheses, but my favorite is mimicry:

Essentially, rounded vowels in “bouba” mimic the rounded contours of the blobby shape. Conversely, the sharp consonants in “kiki” match the jagged shape. This mimicry activates mirror neurons in our brains that detect correspondences between speech articulations and visual forms. Sound symbolism – Certain speech sounds, like rounded vowels, may be more associated with softness, roundness and curvature. Sharper consonants evoke associations with abrupt changes in direction or texture, like points and edges. These intuitive connections arise from shared neurocognitive patterns.

While the mechanisms are still being untangled, this robust cross-sensory match demonstrates that our perceptions of speech and vision are intrinsically intertwined at a fundamental neurological level.

The bouba kiki effect has been replicated across ages, cultures, and languages, suggesting it stems from basic cognitive wiring shared by humans across the globe. For instance, it has been demonstrated in native speakers of Hindi, Tamil, Gujarati, and a variety of non-Indo-European languages with very different phonetic structures from English. Even pre-literate toddlers intuitively match bouba with rounded shapes and kiki with jagged ones, implying we don’t need language experience for these cross-sensory mappings to emerge.

Intriguingly, the association holds true even when the speech sounds have no meaning. Nonsense words like “bouba” and “kiki” exhibit the same effect, though it weakens when real words with known meanings are used. This tells us the effect stems from direct sensory associations rather than semantic information. It also explains why the original nonsense words Köhler devised nearly a century ago remain the standard stimuli used in experiments today.

The effect extends beyond static shapes to dynamic stimuli as well. In one study, participants viewed animated objects moving in either rounded, smooth trajectories or jagged, spiky paths. They consistently matched bouba-like names to the rounded motions and kiki-like names to the spiky motions, showing cross-sensory correspondences even for more complex, abstract visual properties.

Understanding this phenomenon has implications for designing multisensory experiences and interfaces. Companies can leverage cross-modal mappings between shape, motion and sound to create interfaces that feel more intuitive. For example, a study found that touchscreen buttons accompanied by bouba-matching sounds (low amplitude, rounded timbres) were perceived as more organic and appealing. Such insights can inform product and interaction designers.