Below are two made-up shapes. One is called bouba and the other is called kiki. You've never seen either. Which one is bouba?
If you picked the soft, blobby one, you're with almost everybody. When the neuroscientists Vilayanur Ramachandran and Edward Hubbard popularised this test in 2001, they reported that about 95% of people call the round shape bouba and the jagged one kiki. Nobody taught you that rule. The words mean nothing. And yet your brain has a firm opinion.
The idea is older than the names. In 1929 the German psychologist Wolfgang Köhler drew a rounded shape and a spiky one and asked people to match them to two invented words, baluba and takete. In a later edition of his book he swapped baluba for maluma. The results were the same then as now: the round one is maluma, the pointy one is takete.
Try it the other way round in the lab below. Drag the shape to sharpen or soften it and watch what name it "wants". Or switch to the other mode, type any word you like, and see what shape it grows into.
Say "bouba" slowly. Your lips close for the b, then push forward into a round O for the "oo". The whole word flows. Now say "kiki". Your tongue slams up against the roof of your mouth for each k, the air stops dead and then bursts out, and your lips spread wide for the "ee". It's quite literally a sharper movement.
That's the leading explanation: the shapes aren't matched to the letters but to what it feels like, and sounds like, to make the word.
Acoustics play along. Round vowels and nasal sounds like m have their energy concentrated in low frequencies. Sounds like "ee", k and t carry more high-frequency energy and abrupt edges. Low and smooth maps to soft and curved; high and abrupt maps to jagged.
A sceptic might say: the letter K is spiky and the letter B is curvy, so English speakers are just matching the alphabet. It's a fair point, and it was tested directly.
In 2022, a team led by Aleksandra Ćwiek ran the experiment on 917 people speaking 25 languages, from 9 language families and using 10 different writing systems, including scripts where the letters look nothing like B or K. Participants heard the words spoken rather than reading them. Overall, 72% matched bouba to round and kiki to spiky. Speakers of languages written in the Roman alphabet matched a bit more often than others, but the difference was weak, and the effect held across scripts.
It wasn't perfect everywhere. Romanian speakers matched only 36% of the time, and Mandarin, Turkish and Albanian speakers also came in lower. Local words that happen to sound like bouba or kiki may nudge people one way or the other. But in most languages tested, the pull was clear.
Children as young as two and a half have shown the same preference in experiments by Daphne Maurer and colleagues in 2006, long before they can read. A 2013 study reported that even 4-month-old babies look longer when a sound and a shape are "mismatched", as if surprised.
Sight seems to matter, though. People who were born blind show a much weaker version of the effect when they feel shapes instead of seeing them. So this may be a link the brain builds by growing up in a world where you see, hear and feel things at the same time.
For a century linguists have taught that words are arbitrary: there's nothing doggy about the sound "dog", which is why French says chien. That's mostly true. But bouba and kiki show the edges are leaky.
In 2016, Damián Blasi and colleagues checked 100 basic words across nearly two-thirds of the world's languages. Some sounds turned up more often than chance would predict, even in unrelated languages on different continents: words for small tend to contain an "i" sound, words for round an r, nose an n, and tongue an l. Think of teeny, wee, petite, piccolo.
That's the quiet lesson of a silly test with two made-up words. Language is mostly convention, but underneath it runs a thin thread of instinct linking how a word feels in the mouth to what it describes. Next time you name a pet, a product or a monster, listen to it first. A fluffy kitten called Kitzik, anyone?
Sources: Ćwiek et al., "The bouba/kiki effect is robust across cultures and writing systems", Phil. Trans. R. Soc. B (2022); Ramachandran & Hubbard, J. Consciousness Studies (2001); Köhler, Gestalt Psychology (1929, 1947); Maurer, Pathman & Mondloch, Developmental Science (2006); Blasi et al., PNAS (2016).